Merge pull request #1857 from rouault/rfc4_doc_improvementsrfc4_dev

[RFC4_dev] Doc improvements

23 files changed, 849 insertions, 690 deletions

diff --git a/docs/source/apps/cct.rst b/docs/source/apps/cct.rst
index e06f11f8..96b5f13b 100644
--- a/docs/source/apps/cct.rst
+++ b/docs/source/apps/cct.rst
@@ -100,6 +100,8 @@ If the :envvar:`PROJ_NETWORK` environment variable is set to ``ON``,
 :program:`cct` will attempt to use remote grids stored on CDN (Content
 Delivery Network) storage, when they are not available locally.
 
+More details are available in the :ref:`network` section.
+
 Examples
 ********
 
diff --git a/docs/source/apps/cs2cs.rst b/docs/source/apps/cs2cs.rst
index 1003940d..c95bbaed 100644
--- a/docs/source/apps/cs2cs.rst
+++ b/docs/source/apps/cs2cs.rst
@@ -180,6 +180,8 @@ If the :envvar:`PROJ_NETWORK` environment variable is set to ``ON``,
 :program:`cs2cs` will attempt to use remote grids stored on CDN (Content
 Delivery Network) storage, when they are not available locally.
 
+More details are available in the :ref:`network` section.
+
 Examples
 ********
 
diff --git a/docs/source/community/rfc/rfc-4.rst b/docs/source/community/rfc/rfc-4.rst
index 6cd9a82f..5019a5f6 100644
--- a/docs/source/community/rfc/rfc-4.rst
+++ b/docs/source/community/rfc/rfc-4.rst
@@ -695,613 +695,11 @@ choice.
 
 .. _description_geotiff_format:
 
-Description of the PROJ GeoTIFF format
-++++++++++++++++++++++++++++++++++++++
-
-The general principles that guide the following requirements and recommendations
-are such that files will be properly recognized by PROJ, and also by GDAL which
-is an easy way to inspect such grid files:
-
-- `TIFF 6.0 <https://www.awaresystems.be/imaging/tiff/specification/TIFF6.pdf>`_
-  based (could possibly be BigTIFF without code changes, if we ever
-  need some day to handle grids larger than 4GB)
-
-- `GeoTIFF 1.1 <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html>`_ for the georeferencing.
-  GeoTIFF 1.1 is a recent standard, compared to the original GeoTIFF 1.0 version,
-  but its backward compatibility is excellent, so that should not cause much trouble
-  to readers that are not official GeoTIFF 1.1 compliant.
-
-- Files hosted on the CDN will use a Geographic 2D CRS for the GeoTIFF GeoKeys.
-  That CRS is intended to be the interpolation CRS as defined in
-  `OGC Abstract Specification Topic 2 <http://docs.opengeospatial.org/as/18-005r4/18-005r4.html>`_,
-  that is the CRS to which grid values are refered to.
-
-  Given that they will nominally be related to the EPSG dataset, the `GeodeticCRSGeoKey
-  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_geodeticcrsgeokey>`_
-  will be used to store the EPSG code of the CRS. If the CRS cannot be reliably
-  encoded through that key or other geokeys, the ``interpolation_crs_wkt`` metadata
-  item detailed afterwards should be used.
-
-  This CRS will be generally the source CRS (for geographic to
-  geographic horizontal shift grids, or geographic to vertical shift grids), but
-  for vertical to vertical CRS adjustment, this will be the geographic CRS to
-  which the grid is referenced. In some very rare cases of geographic to vertical
-  shift grids, the interpolation CRS might be a geographic CRS that is not the
-  same as the source CRS (into which ellipsoidal height are expressed). The only
-  instance we have in mind is for the EPSG:7001 "ETRS89 to NAP height (1)" transformation
-  using the naptrans2008 VDatum-grid which is referenced to Amersfoort EPSG:4289
-  instead of ETRS89...
-
-  On the reading side, PROJ will ignore that information:
-  the CRS is already stored in the source_crs or interpolation_crs column of the
-  grid_transformation table.
-
-  For geographic to vertical shift files (geoid models), the GeoTIFF 1.1
-  convention will be used to store the value of the `VerticalGeoKey
-  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_verticalgeokey>`_
-  So a geoid model that apply to WGS 84 EPSG:4979 will have GeodeticCRSGeoKey = 4326
-  and VerticalGeoKey = 4979.
-
-- Files hosted on the CDN will use the GeoTIFF defined `ModelTiepointTag and ModelPixelScaleTag
-  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_raster_to_model_coordinate_transformation_requirements>`_ TIFF tags
-  to store the coordinates of the upper-left pixel and the resolution of the pixels.
-  On the reading side, they will be required and ModelTransformationTag will be ignored.
-
-  .. note::
-
-    Regarding anti-meridian handling, a variety of possibilities exist.
-    We do not attempt to standardize this and filesh hosted on the CDN will use
-    a georeferencing close to the original data producer.
-    For example, NOAA vertical grids that apply to Conterminous USA might even have a top-left
-    longitude beyond 180 (for consistency with Alaska grids, whose origin is < 180)
-    Anti-meridian handling in PROJ has probably issues. This RFC does not attempt
-    to address them in particular, as they are believed to be orthogonal to the
-    topics it covers, and being mostly implementation issues.
-
-- Files hosted on the CDN will use the `GTRasterTypeGeoKey
-  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_gtrastertypegeokey>`_
-  = PixelIsPoint convention.
-  This is the convention used by most existing grid formats currently. Note that GDAL
-  typically use a PixelIsArea convention (but can handle both conventions), so the
-  georeferencing it displays when opening a .gsb or .gtx file appears to have a 
-  half-pixel shift regarding to the coordinates stored in the original grid file. On
-  the reading side, PROJ will accept both conventions (for equivalent georeferencing,
-  the value of the origin in a PixelIsArea convention is shifted by a half-pixel
-  towards the upper-left direction). Unspecified behaviour if this GeoKey is absent.
-
-- Files hosted on the CDN will be tiled, presumably with 256x256 tiles (small
-  grids that are smaller than 256x256 will use a single strip). On the reading
-  side, PROJ will accept TIFF files with any strip or tile organization.
-  Tiling is expressed by specifying the TileWidth, TileHeight, TileOffsets
-  and TileByteCounts tags. Strip organization is expressed by specifying the
-  RowsPerStrip, StripByteCounts and StripOffsets tags.
-
-- Files hosted on the CDN will use `Compression
-  <https://www.awaresystems.be/imaging/tiff/tifftags/compression.html>`_ = DEFLATE
-  or LZW (to be determined, possibly with
-  `Predictor <https://www.awaresystems.be/imaging/tiff/tifftags/predictor.html>`_ = 2
-  or 3)
-  On the reading side, PROJ will accept TIFF files with any compression method
-  (appropriate for the data types and PhotometricInterpretation considered)
-  supported by the libtiff build used by PROJ. Of course uncompressed files will be supported.
-
-- Files hosted on the CDN will use little-endian byte ordering. On the reading
-  side, libtiff will transparently handle both little-endian and big-endian
-  ordering.
-
-- Files hosted on the CDN will use PlanarConfiguration=Separate.
-  The tools described in a later section will order blocks so that blocks needed
-  for a given location are close to each other.
-  On the reading side, PROJ will handle also PlanarConfiguration=Contig.
-
-- Files hosted on the CDN will generally use Float32 (BitsPerSample=32 and SampleFormat=IEEEFP)
-  Files may be created using Signed Int 16 (
-  `BitsPerSample <https://www.awaresystems.be/imaging/tiff/tifftags/bitspersample.html>`_ =16 and
-  `SampleFormat <https://www.awaresystems.be/imaging/tiff/tifftags/sampleformat.html>`_ = INT),
-  Unsigned Int 16 (BitsPerSample=16 and SampleFormat=UINT), Signed Int 32 or Unsigned Int 32 generally with an
-  associate scale/offset.
-  On the reading side, only those three data types will be supported as well.
-
-- Files hosted on the CDN will have a `PhotometricInterpretation
-  <https://www.awaresystems.be/imaging/tiff/tifftags/photometricinterpretation.html>`_ = MinIsBlack.
-  It will be assumed, and ignored on the reading side.
-
-- Files hosted on the CDN will nominally have:
-
-    * `SamplesPerPixel <https://www.awaresystems.be/imaging/tiff/tifftags/samplesperpixel.html>`_ = 2
-      for horizontal shift grid, with the first sample being the longitude offset
-      and the second sample being the latitude offset.
-
-    * SamplesPerPixel = 1 for vertical shift grids.
-
-   In the future, different values of SamplesPerPixel may be used to accomodate
-   for other needs. For example for deformation models, SamplesPerPixel = 3 to combine
-   horizontal and vertical adjustments.
-   And even for the current identified needs of horizontal or vertical shifts,
-   more samples may be present (to indicate for example uncertainties), but
-   will be ignored by PROJ.
-
-   The `ExtraSamples <https://www.awaresystems.be/imaging/tiff/tifftags/extrasamples.html>`_
-   tag should be set to a value of SamplesPerPixel - 1 (given the rules that
-   apply for PhotometricInterpretation = MinIsBlack)
-
-- The `ImageDescription <https://www.awaresystems.be/imaging/tiff/tifftags/imagedescription.html>`_
-  tag may be used to convey extra information about the name, provenance, version
-  and last updated date of the grid.
-  Will be set when possible fo files hosted on the CDN.
-  Ignored by PROJ.
-
-- The `Copyright <https://www.awaresystems.be/imaging/tiff/tifftags/copyright.html>`_
-  tag may be used to convey extra information about the copyright and license of the grid.
-  Will be set when possible fo files hosted on the CDN.
-  Ignored by PROJ.
-
-- The `DateTime <https://www.awaresystems.be/imaging/tiff/tifftags/datetime.html>`_
-  tag may be used to convey the date at which the file has been created or
-  converted. In case of a file conversion, for example from NTv2, this will be
-  the date at which the conversion has been performed. The ``ImageDescription``
-  tag however will contain the latest of the CREATED or UPDATED fields from the NTv2 file.
-  Will be set when possible fo files hosted on the CDN.
-  Ignored by PROJ.
-
-- Files hosted on the CDN will use the `GDAL_NODATA
-  <https://www.awaresystems.be/imaging/tiff/tifftags/gdal_nodata.html>`_ tag to encode
-  the value of the nodata / missing value, when it applies to the grid.
-
-  If offset and/or scaling is used, the nodata value corresponds to the raw value,
-  before applying offset and scaling.
-  The value found in this tag, if present, will be honoured (to the extent to
-  which current PROJ code makes use of nodata).
-  For floating point data, writers are strongly discouraged to use non-finite values
-  (+/- infinity, NaN) of nodata to maximimize interoperability.
-  The GDAL_NODATA value applies to all samples of a given TIFF IFD.
-
-- Files hosted on the CDN will use the `GDAL_METADATA
-  <https://www.awaresystems.be/imaging/tiff/tifftags/gdal_metadata.html>`_ tag to encode extra
-  metadata not supported by baseline or extended TIFF.
-
-  * The root XML node should be ``GDALMetadata``
-
-  * Zero, one or several child XML nodes ``Item`` may be present.
-
-  * A Item should have a ``name`` attribute, and a child text node with its value.
-    ``role`` and ``sample`` attributes may be present for attributes that have
-    a special semantics (recognized by GDAL). The value of `sample` should be
-    a integer value between 0 and number_of_samples - 1.
-
-  * Scale and offset to convert integer raw values to floating point values
-    may be expressed with XML `Item` elements whose name attribute is respectively
-    ``SCALE`` and ``OFFSET``, and their ``role`` attribute is respectively ``scale``
-    and ``offset``. The decoded value will be: {offset} + {scale} * raw_value_from_geotiff_file
-
-    For a offset value of 1 and scaling of 2, the following payload should be
-    stored:
-
-    .. code-block:: xml
-
-        <GDALMetadata>
-            <Item name="OFFSET" sample="0" role="offset">1</Item>
-            <Item name="SCALE" sample="0" role="scale">2</Item>
-        </GDALMetadata>
-
-  * The type of the grid must be specified with a `Item` whose ``name`` is set
-    to ``TYPE``.
-
-    Values recognized by PROJ currently are:
-
-    - ``HORIZONTAL_OFFSET``: implies the presence of at least two samples.
-      The first sample must contain the latitude offset and the second
-      sample must contain the longitude offset.
-      Corresponds to PROJ ``hgridshift`` method.
-
-    - ``VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL``: implies the presence of at least one sample.
-      The first sample must contain the vertical adjustment. Must be used when
-      the source/interpolation CRS is a Geographic CRS and the target CRS a Vertical CRS.
-      Corresponds to PROJ ``vgridshift`` method.
-
-    - ``VERTICAL_OFFSET_VERTICAL_TO_VERTICAL``: implies the presence of at least one sample.
-      The first sample must contain the vertical adjustment. Must be used when
-      the source and target CRS are Vertical CRS.
-      Corresponds to PROJ ``vgridshift`` method.
-
-    - ``GEOCENTRIC_TRANSLATION``: implies the presence of at least 3 samples.
-      The first 3 samples must be respectively the geocentric adjustments along
-      the X, Y and Z axis. Must be used when the source and target CRS are
-      geocentric CRS. The interpolation CRS must be a geographic CRS.
-      Corresponds to PROJ ``xyzgridshift`` method.
-
-    - ``VELOCITY``: implies the presence of at least 3 samples.
-      The first 3 samples must be respectively the velocities along
-      the E(ast), N(orth), U(p) axis in the local topocentric coordinate system.
-      Corresponds to PROJ ``deformation`` method.
-
-    For example:
-
-    .. code-block:: xml
-
-        <Item name="TYPE">HORIZONTAL_OFFSET</Item>
-
-  * The description of each sample must be specified with a Item whose ``name``
-    attribute is set to ``DESCRIPTION`` and ``role`` attribute to ``description``.
-
-    Values recognized by PROJ for this Item are currently:
-
-    + ``latitude_offset``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
-      the value to add a latitude expressed in the CRS encoded in the GeoKeys
-      to obtain a latitude value expressed in the target CRS.
-
-    + ``longitude_offset``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
-      the value to add a longitude expressed in the CRS encoded in the GeoKeys
-      to obtain a longitude value expressed in the target CRS.
-
-    + ``geoid_undulation``: valid for TYPE=VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL.
-      For a source CRS being a geographic CRS and a target CRS being a vertical CRS,
-      sample values should be the value to add to a geoid-related height (that
-      is expressed in the target CRS) to
-      get an ellipsoidal height (that is expressed in the source CRS), also
-      called the geoid undulation.
-      Note the possible confusion related to what is the source CRS and target CRS and
-      the semantics of the value stored (to convert from the source to the target,
-      one must subtract the value contained in the grid). This is the convention
-      used by the `EPSG:9665 <https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:method:EPSG::9665>`_
-      operation method.
-
-    + ``vertical_offset``: valid for TYPE=VERTICAL_OFFSET_VERTICAL_TO_VERTICAL.
-      For a source and target CRS being vertical CRS,
-      sample values should be the value to add to an elevation expressed in the
-      source CRS to obtain a longitude value expressed in the target CRS.
-
-    + ``x_translation`` / ``y_translation`` / ``z_translation``: valid for
-      TYPE=GEOCENTRIC_TRANSLATION.
-      Sample values should be the value to add to the input geocentric coordinates
-      expressed in the source CRS to geocentric coordinates expressed in the target CRS.
-
-    + ``east_velocity`` / ``north_velocity`` / ``up_velocity``: valid for
-      TYPE=VELOCITY.
-      Sample values should be the velocity in a linear/time unit in a ENU local
-      topocentric coordinate system.
-
-    For example:
-
-    .. code-block:: xml
-
-        <Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item>
-        <Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item>
-
-    Other values may be used (not used by PROJ):
-
-    + ``latitude_offset_accuracy``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
-      the accuracy of corresponding latitude_offset samples. Generally in metre (if converted from NTv2)
-
-    + ``longitude_offset_accuracy``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
-      the accuracy of corresponding longitude_offset samples. Generally in metre (if converted from NTv2)
-
-  * The sign convention for the values of the ``longitude_offset`` channel
-    should be indicated with an Item named ``positive_value`` whose value
-    can be ``west`` or ``east``. NTv2 products originally use a ``west``
-    convention, but when converting from them to GeoTIFF, the sign of those
-    samples will be inverted so they use a more natural ``east`` convention.
-    If this item is absent, the default value is ``east``.
-
-  * The unit of the values stored in the grid must be specified for each
-    sample through an Item of name ``UNITTYPE`` and role ``unittype``
-    Valid values should be the name of entries from the EPSG ``unitofmeasure``
-    table. To maximize interoperability, writers are strongly encouraged to
-    limit themselves to the following values:
-
-    For linear units:
-
-    - ``metre`` (default value assumed if absent for vertical shift grid files, and value used for files stored on PROJ CDN)
-    - ``US survey foot``
-
-    For angular units:
-
-    - ``degree``
-    - ``arc-second`` (default value assumed if absent for longitude and latitude offset samples of horizontal shift grid files, and value used for files stored on PROJ CDN)
-
-    For velocity units:
-
-    - ``millimetres per year``
-
-    The longitude and latitude offset samples should use the same unit.
-    The geocentric translation samples should use the same unit.
-    The velocity samples should use the same unit.
-
-    Example:
-
-    .. code-block:: xml
-
-        <Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item>
-        <Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item>
-
-  * The ``target_crs_epsg_code`` metadata item should be present.
-    For a horizontal shift grid, this is the EPSG
-    code of the target geographic CRS. For a vertical shift grid, this is the
-    EPSG code of a the target vertical CRS.
-    If the target CRS has no associated EPSG code, ``target_crs_wkt`` must be
-    used.
-    Ignored by PROJ currently.
-
-  * The ``target_crs_wkt`` metadata item must be present if the
-    ``target_crs_epsg_code`` cannot be used.
-    Its value should be a valid WKT string according to
-    `WKT:2015 <http://docs.opengeospatial.org/is/12-063r5/12-063r5.html>`_
-    or `WKT:2019 <hhttp://docs.opengeospatial.org/is/18-010r7/18-010r7.html>`_
-    Ignored by PROJ currently.
-
-  * The ``source_crs_epsg_code`` metadata item must be present if the source
-    and interpolation CRS are not the same (typical use case is vertical CRS to vertical CRS
-    transformation), because the GeoKeys encode the interpolation CRS and not the source CRS.
-    If the source CRS has no associated EPSG code, ``source_crs_wkt`` must be
-    used.
-    Ignored by PROJ currently.
-
-  * The ``source_crs_wkt`` metadata item must be present if the
-    ``source_crs_epsg_code`` cannot be used.
-    Its value should be a valid WKT string according to WKT:2015 or WKT:2019.
-    Ignored by PROJ currently.
-
-  * The ``interpolation_crs_wkt`` metadata item may be present if the GeoKeys
-    cannot be used to express reliably the interpolation CRS.
-    Its value should be a valid WKT string according to WKT:2015 or WKT:2019.
-    Ignored by PROJ currently.
-
-  * The ``recommended_interpolation_method`` metadata item may be present to
-    describe the method to use to interpolation values at locations not
-    coincident with nodes stored in the grid file. Potential values: ``bilinear``,
-    ``bicubic``.
-    Ignored by PROJ currently.
-
-  * The ``area_of_use`` metadata item can be used to indicate plain text information
-    about the area of use of the grid (like "USA - Wisconsin"). In case of multiple
-    subgrids, it should be set only on the first one, but applies to the whole
-    set of grids, not just the first one.
-
-  * The ``grid_name`` metadata item should be present if there are
-    subgrids for this grid (that is grids whose extent is contained in the extent
-    of this grid), or if this is a subgrid.
-    It is intended to be a relatively short identifier
-    Will be ignored by PROJ (this information can be inferred by the grids extent)
-
-  * The ``parent_grid_name`` metadata item should be present if this is a
-    subgrid and its value should be equal to the paren's ``grid_name``
-    Will be ignored by PROJ (this information can be inferred by the grids extent)
-
-  * The ``number_of_nested_grids`` metadata item should be present if there are
-    subgrids for this grid (that is grids whose extent is contained in the extent
-    of this grid).
-    Will be ignored by PROJ (this information can be inferred by the grids extent)
-
-Example
-+++++++
-
-https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntf_r93.tif has
-been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/ntf_r93.gsb
-with https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_to_gtiff.py
-
-::
-
-    $ tiffinfo ntf_r93.tif 
-
-    TIFF Directory at offset 0x4e (78)
-    Image Width: 156 Image Length: 111
-    Bits/Sample: 32
-    Sample Format: IEEE floating point
-    Compression Scheme: AdobeDeflate
-    Photometric Interpretation: min-is-black
-    Extra Samples: 3<unspecified, unspecified, unspecified>
-    Samples/Pixel: 4
-    Rows/Strip: 111
-    Planar Configuration: separate image planes
-    ImageDescription: NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31)
-    DateTime: 2019:12:09 00:00:00
-    Copyright: Derived from work by IGN France. Open License https://www.etalab.gouv.fr/wp-content/uploads/2014/05/Open_Licence.pdf
-    Tag 33550: 0.100000,0.100000,0.000000
-    Tag 33922: 0.000000,0.000000,0.000000,-5.500000,52.000000,0.000000
-    Tag 34735: 1,1,1,3,1024,0,1,2,1025,0,1,2,2048,0,1,4275
-    Tag 42112: <GDALMetadata>
-    <Item name="grid_name">FRANCE</Item>
-    <Item name="target_crs_epsg_code">4171</Item>
-    <Item name="TYPE">HORIZONTAL_OFFSET</Item>
-    <Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item>
-    <Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item>
-    <Item name="positive_value" sample="1">east</Item>
-    <Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item>
-    <Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item>
-    <Item name="UNITTYPE" sample="2" role="unittype">arc-second</Item>
-    <Item name="DESCRIPTION" sample="2" role="description">latitude_offset_accuracy</Item>
-    <Item name="UNITTYPE" sample="3" role="unittype">arc-second</Item>
-    <Item name="DESCRIPTION" sample="3" role="description">longitude_offset_accuracy</Item>
-    </GDALMetadata>
-
-    Predictor: floating point predictor 3 (0x3)
-
-
-::
-
-    $ listgeo ntf_r93.tif 
-
-    Geotiff_Information:
-        Version: 1
-        Key_Revision: 1.1
-        Tagged_Information:
-            ModelTiepointTag (2,3):
-                0                 0                 0                
-                -5.5              52                0                
-            ModelPixelScaleTag (1,3):
-                0.1               0.1               0                
-            End_Of_Tags.
-        Keyed_Information:
-            GTModelTypeGeoKey (Short,1): ModelTypeGeographic
-            GTRasterTypeGeoKey (Short,1): RasterPixelIsPoint
-            GeodeticCRSGeoKey (Short,1): Code-4275 (NTF)
-            End_Of_Keys.
-        End_Of_Geotiff.
-
-    GCS: 4275/NTF
-    Datum: 6275/Nouvelle Triangulation Francaise
-    Ellipsoid: 7011/Clarke 1880 (IGN) (6378249.20,6356515.00)
-    Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
-    Projection Linear Units: User-Defined (1.000000m)
-
-    Corner Coordinates:
-    Upper Left    (  5d30' 0.00"W, 52d 0' 0.00"N)
-    Lower Left    (  5d30' 0.00"W, 40d54' 0.00"N)
-    Upper Right   ( 10d 6' 0.00"E, 52d 0' 0.00"N)
-    Lower Right   ( 10d 6' 0.00"E, 40d54' 0.00"N)
-    Center        (  2d18' 0.00"E, 46d27' 0.00"N)
-
-::
-
-    $ gdalinfo ntf_r93.tif
-
-    Driver: GTiff/GeoTIFF
-    Files: ntf_r93.tif
-    Size is 156, 111
-    Coordinate System is:
-    GEOGCRS["NTF",
-        DATUM["Nouvelle Triangulation Francaise",
-            ELLIPSOID["Clarke 1880 (IGN)",6378249.2,293.466021293627,
-                LENGTHUNIT["metre",1]]],
-        PRIMEM["Greenwich",0,
-            ANGLEUNIT["degree",0.0174532925199433]],
-        CS[ellipsoidal,2],
-            AXIS["geodetic latitude (Lat)",north,
-                ORDER[1],
-                ANGLEUNIT["degree",0.0174532925199433]],
-            AXIS["geodetic longitude (Lon)",east,
-                ORDER[2],
-                ANGLEUNIT["degree",0.0174532925199433]],
-        ID["EPSG",4275]]
-    Data axis to CRS axis mapping: 2,1
-    Origin = (-5.550000000000000,52.049999999999997)
-    Pixel Size = (0.100000000000000,-0.100000000000000)
-    Metadata:
-      AREA_OR_POINT=Point
-      grid_name=FRANCE
-      target_crs_epsg_code=4171
-      TIFFTAG_DATETIME=2019:12:09 00:00:00
-      TIFFTAG_IMAGEDESCRIPTION=NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31)
-      TYPE=HORIZONTAL_OFFSET
-    Image Structure Metadata:
-      COMPRESSION=DEFLATE
-      INTERLEAVE=BAND
-    Corner Coordinates:
-    Upper Left  (  -5.5500000,  52.0500000) (  5d33' 0.00"W, 52d 3' 0.00"N)
-    Lower Left  (  -5.5500000,  40.9500000) (  5d33' 0.00"W, 40d57' 0.00"N)
-    Upper Right (  10.0500000,  52.0500000) ( 10d 3' 0.00"E, 52d 3' 0.00"N)
-    Lower Right (  10.0500000,  40.9500000) ( 10d 3' 0.00"E, 40d57' 0.00"N)
-    Center      (   2.2500000,  46.5000000) (  2d15' 0.00"E, 46d30' 0.00"N)
-    Band 1 Block=156x111 Type=Float32, ColorInterp=Gray
-      Description = latitude_offset
-      Unit Type: arc-second
-    Band 2 Block=156x111 Type=Float32, ColorInterp=Undefined
-      Description = longitude_offset
-      Unit Type: arc-second
-      Metadata:
-        positive_value=east
-    Band 3 Block=156x111 Type=Float32, ColorInterp=Undefined
-      Description = latitude_offset_accuracy
-      Unit Type: arc-second
-    Band 4 Block=156x111 Type=Float32, ColorInterp=Undefined
-      Description = longitude_offset_accuracy
-      Unit Type: arc-second
-
-Multi-grid storage
+Format description
 ++++++++++++++++++
 
-Formats like NTv2 can contain multiple subgrids. This can be transposed to
-TIFF by using several IFD chained together with the last 4 bytes (or 8 bytes
-for BigTIFF) of an IFD pointing to the offset of the next one.
-
-The first IFD should have a full description according to the
-:ref:`Description of the PROJ GeoTIFF format <description_geotiff_format>`.
-Subsequent IFD might have a more compact description, omitting for example, CRS
-information if it is identical to the main IFD (which should be the case for
-the currently envisionned use cases), or Copyright / ImageDescription metadata
-items.
-
-Each IFD will have its
-`NewSubfileType <https://www.awaresystems.be/imaging/tiff/tifftags/newsubfiletype.html>`_
-tag set to 0.
-
-If a low-resolution grid is available, it should be put before subgrids of
-higher-resolution in the chain of IFD linking. On reading, PROJ will use the
-value from the highest-resoluted grid that contains the point of interest.
-
-For efficient reading from the network, files hosted on the CDN will use
-a layout similar to the one described in the `low level paragraph of the Cloud Optimized GeoTIFF
-GDAL driver page <https://gdal.org/drivers/raster/cog.html#low-level>`_
-
-The layout for a file converted from NTv2 will for example be:
-
-- TIFF/BigTIFF header/signature and pointer to first IFD (Image File Directory)
-- "ghost area" indicating the generated process
-- IFD of the first grid, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays
-- ...
-- IFD of the last grid, followed by TIFF tags values, excluding the GDAL_METADATA tag, TileOffsets and TileByteCounts arrays
-- TileOffsets and TileByteCounts arrays for first IFD
-- ...
-- TileOffsets and TileByteCounts arrays for last IFD
-- Value of GDAL_METADATA tag for IFDs following the first IFD
-- First IFD: Data corresponding to latitude offset of Block_0_0
-- First IFD: Data corresponding to longitude offset of Block_0_0
-- First IFD: Data corresponding to latitude offset of Block_0_1
-- First IFD: Data corresponding to longitude offset of Block_0_1
-- ...
-- First IFD: Data corresponding to latitude offset of Block_n_m
-- First IFD: Data corresponding to longitude offset of Block_n_m
-- ...
-- Last IFD: Data corresponding to latitude offset of Block_0_0
-- Last IFD: Data corresponding to longitude offset of Block_0_0
-- Last IFD: Data corresponding to latitude offset of Block_0_1
-- Last IFD: Data corresponding to longitude offset of Block_0_1
-- ...
-- Last IFD: Data corresponding to latitude offset of Block_n_m
-- Last IFD: Data corresponding to longitude offset of Block_n_m
-
-If longitude_offset_accuracy and latitude_offset_accuracy are present, this
-will be followed by:
-
-- First IFD: Data corresponding to latitude offset accuracy of Block_0_0
-- First IFD: Data corresponding to longitude offset accuracy of Block_0_0
-- ...
-- First IFD: Data corresponding to latitude offset accuracy of Block_n_m
-- First IFD: Data corresponding to longitude offset accuracy of Block_n_m
-- ...
-- Last IFD: Data corresponding to latitude offset accuracy of Block_0_0
-- Last IFD: Data corresponding to longitude offset accuracy of Block_0_0
-- ...
-- Last IFD: Data corresponding to latitude offset accuracy of Block_n_m
-- Last IFD: Data corresponding to longitude offset accuracy of Block_n_m
-
-.. note::
-
-    TIFF has another mechanism to link IFDs, the SubIFD tag. This potentially
-    enables to define a hiearchy of IFDs (similar to HDF5 groups). There is no
-    support for that in most TIFF-using software, notably GDAL, and no compelling
-    need to have a nested hiearchy, so "flat" organization with the standard IFD chaining
-    mechanism is adopted.
-
-Examples of multi-grid dataset
-++++++++++++++++++++++++++++++
-
-https://github.com/rouault/sample_proj_gtiff_grids/blob/master/GDA94_GDA2020_conformal.tif has
-been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/oceania/GDA94_GDA2020_conformal.gsb
-with https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_to_gtiff.py
-
-It contains 5 subgrids. All essential metadata to list the subgrids and their
-georeferencing is contained within the first 3 KB of the file.
-
-The file size is 4.8 MB using DEFLATE compression and floating-point predictor.
-To be compared with the 83 MB of the original .gsb file.
-
-https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_0.tif  has
-been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/north-america/ntv2_0.gsb
-
-It contains 114 subgrids. All essential metadata to list the subgrids and their
-georeferencing is contained within the first 40 KB of the file.
-
+The format description is available in a dedicated :ref:`geodetictiffgrids`
+document.
 
 Tooling
 +++++++
diff --git a/docs/source/index.rst b/docs/source/index.rst
index e9590d99..3a2df4b4 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -17,6 +17,7 @@ PROJ
    resource_files
    geodesic
    development/index
+   specifications/index
    community/index
    faq
    glossary
diff --git a/docs/source/resource_files.rst b/docs/source/resource_files.rst
index f268e070..28628d30 100644
--- a/docs/source/resource_files.rst
+++ b/docs/source/resource_files.rst
@@ -14,6 +14,8 @@ In addition to the bundled init-files the PROJ project also distributes a number
 of packages containing transformation grids and additional init-files not included
 in the main PROJ package.
 
+.. _resource_file_paths:
+
 Where are PROJ resource files looked for ?
 -------------------------------------------------------------------------------
 
@@ -31,10 +33,12 @@ The following paths are checked in order:
 - Path(s) set with the :c:func:`proj_context_set_search_paths`. If set, the
   next tests will not be run.
 
+.. _user_writable_directory:
+
 - The PROJ user writable directory, which is :
 
     * on Windows, ${LOCALAPPDATA}/proj
-    * on MacOSX, ${HOME}/Library/Logs/proj
+    * on MacOSX, ${HOME}/Library/Application Support/proj
     * on other platforms (Linux), ${XDG_DATA_HOME}/proj if :envvar:`XDG_DATA_HOME`
       is defined. Else ${HOME}/.local/share/proj
 
diff --git a/docs/source/specifications/geodetictiffgrids.rst b/docs/source/specifications/geodetictiffgrids.rst
new file mode 100644
index 00000000..98dc8408
--- /dev/null
+++ b/docs/source/specifications/geodetictiffgrids.rst
@@ -0,0 +1,626 @@
+.. _geodetictiffgrids:
+
+================================================================================
+Geodetic TIFF grids (GTG)
+================================================================================
+
+.. versionadded:: 7.0
+
+Introduction
+++++++++++++
+
+The Geodetic TIFF grid format has been introduced per :ref:`rfc4`. It is a
+profile of the TIFF and GeoTIFF formats that addresses the specific requirements
+of geodetic grids: horizontal shifts, vertical shifts, velocity grids, etc...
+It also follows the `Cloud Optimized GeoTIFF <http://cogeo.org>`_ principles.
+
+Such grids are available on a :ref:`CDN of GeoTIFF grids <cdn_grids>`.
+
+.. _gtg_general_description:
+
+General description
++++++++++++++++++++
+
+The general principles that guide the following requirements and recommendations
+are such that files will be properly recognized by PROJ, and also by GDAL which
+is an easy way to inspect such grid files:
+
+- `TIFF 6.0 <https://www.awaresystems.be/imaging/tiff/specification/TIFF6.pdf>`_
+  based (could possibly be BigTIFF without code changes, if we ever
+  need some day to handle grids larger than 4GB)
+
+- `GeoTIFF 1.1 <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html>`_ for the georeferencing.
+  GeoTIFF 1.1 is a recent standard, compared to the original GeoTIFF 1.0 version,
+  but its backward compatibility is excellent, so that should not cause much trouble
+  to readers that are not official GeoTIFF 1.1 compliant.
+
+- Files hosted on the CDN will use a Geographic 2D CRS for the GeoTIFF GeoKeys.
+  That CRS is intended to be the interpolation CRS as defined in
+  `OGC Abstract Specification Topic 2 <http://docs.opengeospatial.org/as/18-005r4/18-005r4.html>`_,
+  that is the CRS to which grid values are refered to.
+
+  Given that they will nominally be related to the EPSG dataset, the `GeodeticCRSGeoKey
+  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_geodeticcrsgeokey>`_
+  will be used to store the EPSG code of the CRS. If the CRS cannot be reliably
+  encoded through that key or other geokeys, the ``interpolation_crs_wkt`` metadata
+  item detailed afterwards should be used.
+
+  This CRS will be generally the source CRS (for geographic to
+  geographic horizontal shift grids, or geographic to vertical shift grids), but
+  for vertical to vertical CRS adjustment, this will be the geographic CRS to
+  which the grid is referenced. In some very rare cases of geographic to vertical
+  shift grids, the interpolation CRS might be a geographic CRS that is not the
+  same as the source CRS (into which ellipsoidal height are expressed). The only
+  instance we have in mind is for the EPSG:7001 "ETRS89 to NAP height (1)" transformation
+  using the naptrans2008 VDatum-grid which is referenced to Amersfoort EPSG:4289
+  instead of ETRS89...
+
+  On the reading side, PROJ will ignore that information:
+  the CRS is already stored in the source_crs or interpolation_crs column of the
+  grid_transformation table.
+
+  For geographic to vertical shift files (geoid models), the GeoTIFF 1.1
+  convention will be used to store the value of the `VerticalGeoKey
+  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_verticalgeokey>`_
+  So a geoid model that apply to WGS 84 EPSG:4979 will have GeodeticCRSGeoKey = 4326
+  and VerticalGeoKey = 4979.
+
+- Files hosted on the CDN will use the GeoTIFF defined `ModelTiepointTag and ModelPixelScaleTag
+  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_raster_to_model_coordinate_transformation_requirements>`_ TIFF tags
+  to store the coordinates of the upper-left pixel and the resolution of the pixels.
+  On the reading side, they will be required and ModelTransformationTag will be ignored.
+
+  .. note::
+
+    Regarding anti-meridian handling, a variety of possibilities exist.
+    We do not attempt to standardize this and filesh hosted on the CDN will use
+    a georeferencing close to the original data producer.
+    For example, NOAA vertical grids that apply to Conterminous USA might even have a top-left
+    longitude beyond 180 (for consistency with Alaska grids, whose origin is < 180)
+    Anti-meridian handling in PROJ has probably issues. This RFC does not attempt
+    to address them in particular, as they are believed to be orthogonal to the
+    topics it covers, and being mostly implementation issues.
+
+- Files hosted on the CDN will use the `GTRasterTypeGeoKey
+  <http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_gtrastertypegeokey>`_
+  = PixelIsPoint convention.
+  This is the convention used by most existing grid formats currently. Note that GDAL
+  typically use a PixelIsArea convention (but can handle both conventions), so the
+  georeferencing it displays when opening a .gsb or .gtx file appears to have a 
+  half-pixel shift regarding to the coordinates stored in the original grid file. On
+  the reading side, PROJ will accept both conventions (for equivalent georeferencing,
+  the value of the origin in a PixelIsArea convention is shifted by a half-pixel
+  towards the upper-left direction). Unspecified behaviour if this GeoKey is absent.
+
+- Files hosted on the CDN will be tiled, presumably with 256x256 tiles (small
+  grids that are smaller than 256x256 will use a single strip). On the reading
+  side, PROJ will accept TIFF files with any strip or tile organization.
+  Tiling is expressed by specifying the TileWidth, TileHeight, TileOffsets
+  and TileByteCounts tags. Strip organization is expressed by specifying the
+  RowsPerStrip, StripByteCounts and StripOffsets tags.
+
+- Files hosted on the CDN will use `Compression
+  <https://www.awaresystems.be/imaging/tiff/tifftags/compression.html>`_ = DEFLATE
+  or LZW (to be determined, possibly with
+  `Predictor <https://www.awaresystems.be/imaging/tiff/tifftags/predictor.html>`_ = 2
+  or 3)
+  On the reading side, PROJ will accept TIFF files with any compression method
+  (appropriate for the data types and PhotometricInterpretation considered)
+  supported by the libtiff build used by PROJ. Of course uncompressed files will be supported.
+
+- Files hosted on the CDN will use little-endian byte ordering. On the reading
+  side, libtiff will transparently handle both little-endian and big-endian
+  ordering.
+
+- Files hosted on the CDN will use PlanarConfiguration=Separate.
+  The tools described in a later section will order blocks so that blocks needed
+  for a given location are close to each other.
+  On the reading side, PROJ will handle also PlanarConfiguration=Contig.
+
+- Files hosted on the CDN will generally use Float32 (BitsPerSample=32 and SampleFormat=IEEEFP)
+  Files may be created using Signed Int 16 (
+  `BitsPerSample <https://www.awaresystems.be/imaging/tiff/tifftags/bitspersample.html>`_ =16 and
+  `SampleFormat <https://www.awaresystems.be/imaging/tiff/tifftags/sampleformat.html>`_ = INT),
+  Unsigned Int 16 (BitsPerSample=16 and SampleFormat=UINT), Signed Int 32 or Unsigned Int 32 generally with an
+  associate scale/offset.
+  On the reading side, only those three data types will be supported as well.
+
+- Files hosted on the CDN will have a `PhotometricInterpretation
+  <https://www.awaresystems.be/imaging/tiff/tifftags/photometricinterpretation.html>`_ = MinIsBlack.
+  It will be assumed, and ignored on the reading side.
+
+- Files hosted on the CDN will nominally have:
+
+    * `SamplesPerPixel <https://www.awaresystems.be/imaging/tiff/tifftags/samplesperpixel.html>`_ = 2
+      for horizontal shift grid, with the first sample being the longitude offset
+      and the second sample being the latitude offset.
+
+    * SamplesPerPixel = 1 for vertical shift grids.
+
+   In the future, different values of SamplesPerPixel may be used to accomodate
+   for other needs. For example for deformation models, SamplesPerPixel = 3 to combine
+   horizontal and vertical adjustments.
+   And even for the current identified needs of horizontal or vertical shifts,
+   more samples may be present (to indicate for example uncertainties), but
+   will be ignored by PROJ.
+
+   The `ExtraSamples <https://www.awaresystems.be/imaging/tiff/tifftags/extrasamples.html>`_
+   tag should be set to a value of SamplesPerPixel - 1 (given the rules that
+   apply for PhotometricInterpretation = MinIsBlack)
+
+- The `ImageDescription <https://www.awaresystems.be/imaging/tiff/tifftags/imagedescription.html>`_
+  tag may be used to convey extra information about the name, provenance, version
+  and last updated date of the grid.
+  Will be set when possible fo files hosted on the CDN.
+  Ignored by PROJ.
+
+- The `Copyright <https://www.awaresystems.be/imaging/tiff/tifftags/copyright.html>`_
+  tag may be used to convey extra information about the copyright and license of the grid.
+  Will be set when possible fo files hosted on the CDN.
+  Ignored by PROJ.
+
+- The `DateTime <https://www.awaresystems.be/imaging/tiff/tifftags/datetime.html>`_
+  tag may be used to convey the date at which the file has been created or
+  converted. In case of a file conversion, for example from NTv2, this will be
+  the date at which the conversion has been performed. The ``ImageDescription``
+  tag however will contain the latest of the CREATED or UPDATED fields from the NTv2 file.
+  Will be set when possible fo files hosted on the CDN.
+  Ignored by PROJ.
+
+- Files hosted on the CDN will use the `GDAL_NODATA
+  <https://www.awaresystems.be/imaging/tiff/tifftags/gdal_nodata.html>`_ tag to encode
+  the value of the nodata / missing value, when it applies to the grid.
+
+  If offset and/or scaling is used, the nodata value corresponds to the raw value,
+  before applying offset and scaling.
+  The value found in this tag, if present, will be honoured (to the extent to
+  which current PROJ code makes use of nodata).
+  For floating point data, writers are strongly discouraged to use non-finite values
+  (+/- infinity, NaN) of nodata to maximimize interoperability.
+  The GDAL_NODATA value applies to all samples of a given TIFF IFD.
+
+- Files hosted on the CDN will use the `GDAL_METADATA
+  <https://www.awaresystems.be/imaging/tiff/tifftags/gdal_metadata.html>`_ tag to encode extra
+  metadata not supported by baseline or extended TIFF.
+
+  * The root XML node should be ``GDALMetadata``
+
+  * Zero, one or several child XML nodes ``Item`` may be present.
+
+  * A Item should have a ``name`` attribute, and a child text node with its value.
+    ``role`` and ``sample`` attributes may be present for attributes that have
+    a special semantics (recognized by GDAL). The value of `sample` should be
+    a integer value between 0 and number_of_samples - 1.
+
+  * Scale and offset to convert integer raw values to floating point values
+    may be expressed with XML `Item` elements whose name attribute is respectively
+    ``SCALE`` and ``OFFSET``, and their ``role`` attribute is respectively ``scale``
+    and ``offset``. The decoded value will be: {offset} + {scale} * raw_value_from_geotiff_file
+
+    For a offset value of 1 and scaling of 2, the following payload should be
+    stored:
+
+    .. code-block:: xml
+
+        <GDALMetadata>
+            <Item name="OFFSET" sample="0" role="offset">1</Item>
+            <Item name="SCALE" sample="0" role="scale">2</Item>
+        </GDALMetadata>
+
+  * The type of the grid must be specified with a `Item` whose ``name`` is set
+    to ``TYPE``.
+
+    Values recognized by PROJ currently are:
+
+    - ``HORIZONTAL_OFFSET``: implies the presence of at least two samples.
+      The first sample must contain the latitude offset and the second
+      sample must contain the longitude offset.
+      Corresponds to PROJ ``hgridshift`` method.
+
+    - ``VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL``: implies the presence of at least one sample.
+      The first sample must contain the vertical adjustment. Must be used when
+      the source/interpolation CRS is a Geographic CRS and the target CRS a Vertical CRS.
+      Corresponds to PROJ ``vgridshift`` method.
+
+    - ``VERTICAL_OFFSET_VERTICAL_TO_VERTICAL``: implies the presence of at least one sample.
+      The first sample must contain the vertical adjustment. Must be used when
+      the source and target CRS are Vertical CRS.
+      Corresponds to PROJ ``vgridshift`` method.
+
+    - ``GEOCENTRIC_TRANSLATION``: implies the presence of at least 3 samples.
+      The first 3 samples must be respectively the geocentric adjustments along
+      the X, Y and Z axis. Must be used when the source and target CRS are
+      geocentric CRS. The interpolation CRS must be a geographic CRS.
+      Corresponds to PROJ ``xyzgridshift`` method.
+
+    - ``VELOCITY``: implies the presence of at least 3 samples.
+      The first 3 samples must be respectively the velocities along
+      the E(ast), N(orth), U(p) axis in the local topocentric coordinate system.
+      Corresponds to PROJ ``deformation`` method.
+
+    For example:
+
+    .. code-block:: xml
+
+        <Item name="TYPE">HORIZONTAL_OFFSET</Item>
+
+  * The description of each sample must be specified with a Item whose ``name``
+    attribute is set to ``DESCRIPTION`` and ``role`` attribute to ``description``.
+
+    Values recognized by PROJ for this Item are currently:
+
+    + ``latitude_offset``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
+      the value to add a latitude expressed in the CRS encoded in the GeoKeys
+      to obtain a latitude value expressed in the target CRS.
+
+    + ``longitude_offset``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
+      the value to add a longitude expressed in the CRS encoded in the GeoKeys
+      to obtain a longitude value expressed in the target CRS.
+
+    + ``geoid_undulation``: valid for TYPE=VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL.
+      For a source CRS being a geographic CRS and a target CRS being a vertical CRS,
+      sample values should be the value to add to a geoid-related height (that
+      is expressed in the target CRS) to
+      get an ellipsoidal height (that is expressed in the source CRS), also
+      called the geoid undulation.
+      Note the possible confusion related to what is the source CRS and target CRS and
+      the semantics of the value stored (to convert from the source to the target,
+      one must subtract the value contained in the grid). This is the convention
+      used by the `EPSG:9665 <https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:method:EPSG::9665>`_
+      operation method.
+
+    + ``vertical_offset``: valid for TYPE=VERTICAL_OFFSET_VERTICAL_TO_VERTICAL.
+      For a source and target CRS being vertical CRS,
+      sample values should be the value to add to an elevation expressed in the
+      source CRS to obtain a longitude value expressed in the target CRS.
+
+    + ``x_translation`` / ``y_translation`` / ``z_translation``: valid for
+      TYPE=GEOCENTRIC_TRANSLATION.
+      Sample values should be the value to add to the input geocentric coordinates
+      expressed in the source CRS to geocentric coordinates expressed in the target CRS.
+
+    + ``east_velocity`` / ``north_velocity`` / ``up_velocity``: valid for
+      TYPE=VELOCITY.
+      Sample values should be the velocity in a linear/time unit in a ENU local
+      topocentric coordinate system.
+
+    For example:
+
+    .. code-block:: xml
+
+        <Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item>
+        <Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item>
+
+    Other values may be used (not used by PROJ):
+
+    + ``latitude_offset_accuracy``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
+      the accuracy of corresponding latitude_offset samples. Generally in metre (if converted from NTv2)
+
+    + ``longitude_offset_accuracy``: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be
+      the accuracy of corresponding longitude_offset samples. Generally in metre (if converted from NTv2)
+
+  * The sign convention for the values of the ``longitude_offset`` channel
+    should be indicated with an Item named ``positive_value`` whose value
+    can be ``west`` or ``east``. NTv2 products originally use a ``west``
+    convention, but when converting from them to GeoTIFF, the sign of those
+    samples will be inverted so they use a more natural ``east`` convention.
+    If this item is absent, the default value is ``east``.
+
+  * The unit of the values stored in the grid must be specified for each
+    sample through an Item of name ``UNITTYPE`` and role ``unittype``
+    Valid values should be the name of entries from the EPSG ``unitofmeasure``
+    table. To maximize interoperability, writers are strongly encouraged to
+    limit themselves to the following values:
+
+    For linear units:
+
+    - ``metre`` (default value assumed if absent for vertical shift grid files, and value used for files stored on PROJ CDN)
+    - ``US survey foot``
+
+    For angular units:
+
+    - ``degree``
+    - ``arc-second`` (default value assumed if absent for longitude and latitude offset samples of horizontal shift grid files, and value used for files stored on PROJ CDN)
+
+    For velocity units:
+
+    - ``millimetres per year``
+
+    The longitude and latitude offset samples should use the same unit.
+    The geocentric translation samples should use the same unit.
+    The velocity samples should use the same unit.
+
+    Example:
+
+    .. code-block:: xml
+
+        <Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item>
+        <Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item>
+
+  * The ``target_crs_epsg_code`` metadata item should be present.
+    For a horizontal shift grid, this is the EPSG
+    code of the target geographic CRS. For a vertical shift grid, this is the
+    EPSG code of a the target vertical CRS.
+    If the target CRS has no associated EPSG code, ``target_crs_wkt`` must be
+    used.
+    Ignored by PROJ currently.
+
+  * The ``target_crs_wkt`` metadata item must be present if the
+    ``target_crs_epsg_code`` cannot be used.
+    Its value should be a valid WKT string according to
+    `WKT:2015 <http://docs.opengeospatial.org/is/12-063r5/12-063r5.html>`_
+    or `WKT:2019 <hhttp://docs.opengeospatial.org/is/18-010r7/18-010r7.html>`_
+    Ignored by PROJ currently.
+
+  * The ``source_crs_epsg_code`` metadata item must be present if the source
+    and interpolation CRS are not the same (typical use case is vertical CRS to vertical CRS
+    transformation), because the GeoKeys encode the interpolation CRS and not the source CRS.
+    If the source CRS has no associated EPSG code, ``source_crs_wkt`` must be
+    used.
+    Ignored by PROJ currently.
+
+  * The ``source_crs_wkt`` metadata item must be present if the
+    ``source_crs_epsg_code`` cannot be used.
+    Its value should be a valid WKT string according to WKT:2015 or WKT:2019.
+    Ignored by PROJ currently.
+
+  * The ``interpolation_crs_wkt`` metadata item may be present if the GeoKeys
+    cannot be used to express reliably the interpolation CRS.
+    Its value should be a valid WKT string according to WKT:2015 or WKT:2019.
+    Ignored by PROJ currently.
+
+  * The ``recommended_interpolation_method`` metadata item may be present to
+    describe the method to use to interpolation values at locations not
+    coincident with nodes stored in the grid file. Potential values: ``bilinear``,
+    ``bicubic``.
+    Ignored by PROJ currently.
+
+  * The ``area_of_use`` metadata item can be used to indicate plain text information
+    about the area of use of the grid (like "USA - Wisconsin"). In case of multiple
+    subgrids, it should be set only on the first one, but applies to the whole
+    set of grids, not just the first one.
+
+  * The ``grid_name`` metadata item should be present if there are
+    subgrids for this grid (that is grids whose extent is contained in the extent
+    of this grid), or if this is a subgrid.
+    It is intended to be a relatively short identifier
+    Will be ignored by PROJ (this information can be inferred by the grids extent)
+
+  * The ``parent_grid_name`` metadata item should be present if this is a
+    subgrid and its value should be equal to the paren's ``grid_name``
+    Will be ignored by PROJ (this information can be inferred by the grids extent)
+
+  * The ``number_of_nested_grids`` metadata item should be present if there are
+    subgrids for this grid (that is grids whose extent is contained in the extent
+    of this grid).
+    Will be ignored by PROJ (this information can be inferred by the grids extent)
+
+Example
++++++++
+
+https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntf_r93.tif has
+been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/ntf_r93.gsb
+with https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_to_gtiff.py
+
+::
+
+    $ tiffinfo ntf_r93.tif 
+
+    TIFF Directory at offset 0x4e (78)
+    Image Width: 156 Image Length: 111
+    Bits/Sample: 32
+    Sample Format: IEEE floating point
+    Compression Scheme: AdobeDeflate
+    Photometric Interpretation: min-is-black
+    Extra Samples: 3<unspecified, unspecified, unspecified>
+    Samples/Pixel: 4
+    Rows/Strip: 111
+    Planar Configuration: separate image planes
+    ImageDescription: NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31)
+    DateTime: 2019:12:09 00:00:00
+    Copyright: Derived from work by IGN France. Open License https://www.etalab.gouv.fr/wp-content/uploads/2014/05/Open_Licence.pdf
+    Tag 33550: 0.100000,0.100000,0.000000
+    Tag 33922: 0.000000,0.000000,0.000000,-5.500000,52.000000,0.000000
+    Tag 34735: 1,1,1,3,1024,0,1,2,1025,0,1,2,2048,0,1,4275
+    Tag 42112: <GDALMetadata>
+    <Item name="grid_name">FRANCE</Item>
+    <Item name="target_crs_epsg_code">4171</Item>
+    <Item name="TYPE">HORIZONTAL_OFFSET</Item>
+    <Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item>
+    <Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item>
+    <Item name="positive_value" sample="1">east</Item>
+    <Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item>
+    <Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item>
+    <Item name="UNITTYPE" sample="2" role="unittype">arc-second</Item>
+    <Item name="DESCRIPTION" sample="2" role="description">latitude_offset_accuracy</Item>
+    <Item name="UNITTYPE" sample="3" role="unittype">arc-second</Item>
+    <Item name="DESCRIPTION" sample="3" role="description">longitude_offset_accuracy</Item>
+    </GDALMetadata>
+
+    Predictor: floating point predictor 3 (0x3)
+
+
+::
+
+    $ listgeo ntf_r93.tif 
+
+    Geotiff_Information:
+        Version: 1
+        Key_Revision: 1.1
+        Tagged_Information:
+            ModelTiepointTag (2,3):
+                0                 0                 0                
+                -5.5              52                0                
+            ModelPixelScaleTag (1,3):
+                0.1               0.1               0                
+            End_Of_Tags.
+        Keyed_Information:
+            GTModelTypeGeoKey (Short,1): ModelTypeGeographic
+            GTRasterTypeGeoKey (Short,1): RasterPixelIsPoint
+            GeodeticCRSGeoKey (Short,1): Code-4275 (NTF)
+            End_Of_Keys.
+        End_Of_Geotiff.
+
+    GCS: 4275/NTF
+    Datum: 6275/Nouvelle Triangulation Francaise
+    Ellipsoid: 7011/Clarke 1880 (IGN) (6378249.20,6356515.00)
+    Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
+    Projection Linear Units: User-Defined (1.000000m)
+
+    Corner Coordinates:
+    Upper Left    (  5d30' 0.00"W, 52d 0' 0.00"N)
+    Lower Left    (  5d30' 0.00"W, 40d54' 0.00"N)
+    Upper Right   ( 10d 6' 0.00"E, 52d 0' 0.00"N)
+    Lower Right   ( 10d 6' 0.00"E, 40d54' 0.00"N)
+    Center        (  2d18' 0.00"E, 46d27' 0.00"N)
+
+::
+
+    $ gdalinfo ntf_r93.tif
+
+    Driver: GTiff/GeoTIFF
+    Files: ntf_r93.tif
+    Size is 156, 111
+    Coordinate System is:
+    GEOGCRS["NTF",
+        DATUM["Nouvelle Triangulation Francaise",
+            ELLIPSOID["Clarke 1880 (IGN)",6378249.2,293.466021293627,
+                LENGTHUNIT["metre",1]]],
+        PRIMEM["Greenwich",0,
+            ANGLEUNIT["degree",0.0174532925199433]],
+        CS[ellipsoidal,2],
+            AXIS["geodetic latitude (Lat)",north,
+                ORDER[1],
+                ANGLEUNIT["degree",0.0174532925199433]],
+            AXIS["geodetic longitude (Lon)",east,
+                ORDER[2],
+                ANGLEUNIT["degree",0.0174532925199433]],
+        ID["EPSG",4275]]
+    Data axis to CRS axis mapping: 2,1
+    Origin = (-5.550000000000000,52.049999999999997)
+    Pixel Size = (0.100000000000000,-0.100000000000000)
+    Metadata:
+      AREA_OR_POINT=Point
+      grid_name=FRANCE
+      target_crs_epsg_code=4171
+      TIFFTAG_DATETIME=2019:12:09 00:00:00
+      TIFFTAG_IMAGEDESCRIPTION=NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31)
+      TYPE=HORIZONTAL_OFFSET
+    Image Structure Metadata:
+      COMPRESSION=DEFLATE
+      INTERLEAVE=BAND
+    Corner Coordinates:
+    Upper Left  (  -5.5500000,  52.0500000) (  5d33' 0.00"W, 52d 3' 0.00"N)
+    Lower Left  (  -5.5500000,  40.9500000) (  5d33' 0.00"W, 40d57' 0.00"N)
+    Upper Right (  10.0500000,  52.0500000) ( 10d 3' 0.00"E, 52d 3' 0.00"N)
+    Lower Right (  10.0500000,  40.9500000) ( 10d 3' 0.00"E, 40d57' 0.00"N)
+    Center      (   2.2500000,  46.5000000) (  2d15' 0.00"E, 46d30' 0.00"N)
+    Band 1 Block=156x111 Type=Float32, ColorInterp=Gray
+      Description = latitude_offset
+      Unit Type: arc-second
+    Band 2 Block=156x111 Type=Float32, ColorInterp=Undefined
+      Description = longitude_offset
+      Unit Type: arc-second
+      Metadata:
+        positive_value=east
+    Band 3 Block=156x111 Type=Float32, ColorInterp=Undefined
+      Description = latitude_offset_accuracy
+      Unit Type: arc-second
+    Band 4 Block=156x111 Type=Float32, ColorInterp=Undefined
+      Description = longitude_offset_accuracy
+      Unit Type: arc-second
+
+Multi-grid storage
+++++++++++++++++++
+
+Formats like NTv2 can contain multiple subgrids. This can be transposed to
+TIFF by using several IFD chained together with the last 4 bytes (or 8 bytes
+for BigTIFF) of an IFD pointing to the offset of the next one.
+
+The first IFD should have a full description according to the
+:ref:`General description <gtg_general_description>`.
+Subsequent IFD might have a more compact description, omitting for example, CRS
+information if it is identical to the main IFD (which should be the case for
+the currently envisionned use cases), or Copyright / ImageDescription metadata
+items.
+
+Each IFD will have its
+`NewSubfileType <https://www.awaresystems.be/imaging/tiff/tifftags/newsubfiletype.html>`_
+tag set to 0.
+
+If a low-resolution grid is available, it should be put before subgrids of
+higher-resolution in the chain of IFD linking. On reading, PROJ will use the
+value from the highest-resoluted grid that contains the point of interest.
+
+For efficient reading from the network, files hosted on the CDN will use
+a layout similar to the one described in the `low level paragraph of the Cloud Optimized GeoTIFF
+GDAL driver page <https://gdal.org/drivers/raster/cog.html#low-level>`_
+
+The layout for a file converted from NTv2 will for example be:
+
+- TIFF/BigTIFF header/signature and pointer to first IFD (Image File Directory)
+- "ghost area" indicating the generated process
+- IFD of the first grid, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays
+- ...
+- IFD of the last grid, followed by TIFF tags values, excluding the GDAL_METADATA tag, TileOffsets and TileByteCounts arrays
+- TileOffsets and TileByteCounts arrays for first IFD
+- ...
+- TileOffsets and TileByteCounts arrays for last IFD
+- Value of GDAL_METADATA tag for IFDs following the first IFD
+- First IFD: Data corresponding to latitude offset of Block_0_0
+- First IFD: Data corresponding to longitude offset of Block_0_0
+- First IFD: Data corresponding to latitude offset of Block_0_1
+- First IFD: Data corresponding to longitude offset of Block_0_1
+- ...
+- First IFD: Data corresponding to latitude offset of Block_n_m
+- First IFD: Data corresponding to longitude offset of Block_n_m
+- ...
+- Last IFD: Data corresponding to latitude offset of Block_0_0
+- Last IFD: Data corresponding to longitude offset of Block_0_0
+- Last IFD: Data corresponding to latitude offset of Block_0_1
+- Last IFD: Data corresponding to longitude offset of Block_0_1
+- ...
+- Last IFD: Data corresponding to latitude offset of Block_n_m
+- Last IFD: Data corresponding to longitude offset of Block_n_m
+
+If longitude_offset_accuracy and latitude_offset_accuracy are present, this
+will be followed by:
+
+- First IFD: Data corresponding to latitude offset accuracy of Block_0_0
+- First IFD: Data corresponding to longitude offset accuracy of Block_0_0
+- ...
+- First IFD: Data corresponding to latitude offset accuracy of Block_n_m
+- First IFD: Data corresponding to longitude offset accuracy of Block_n_m
+- ...
+- Last IFD: Data corresponding to latitude offset accuracy of Block_0_0
+- Last IFD: Data corresponding to longitude offset accuracy of Block_0_0
+- ...
+- Last IFD: Data corresponding to latitude offset accuracy of Block_n_m
+- Last IFD: Data corresponding to longitude offset accuracy of Block_n_m
+
+.. note::
+
+    TIFF has another mechanism to link IFDs, the SubIFD tag. This potentially
+    enables to define a hiearchy of IFDs (similar to HDF5 groups). There is no
+    support for that in most TIFF-using software, notably GDAL, and no compelling
+    need to have a nested hiearchy, so "flat" organization with the standard IFD chaining
+    mechanism is adopted.
+
+Examples of multi-grid dataset
+++++++++++++++++++++++++++++++
+
+https://github.com/rouault/sample_proj_gtiff_grids/blob/master/GDA94_GDA2020_conformal.tif has
+been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/oceania/GDA94_GDA2020_conformal.gsb
+with https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_to_gtiff.py
+
+It contains 5 subgrids. All essential metadata to list the subgrids and their
+georeferencing is contained within the first 3 KB of the file.
+
+The file size is 4.8 MB using DEFLATE compression and floating-point predictor.
+To be compared with the 83 MB of the original .gsb file.
+
+https://github.com/rouault/sample_proj_gtiff_grids/blob/master/ntv2_0.tif  has
+been converted from https://github.com/OSGeo/proj-datumgrid/blob/master/north-america/ntv2_0.gsb
+
+It contains 114 subgrids. All essential metadata to list the subgrids and their
+georeferencing is contained within the first 40 KB of the file.
diff --git a/docs/source/specifications/index.rst b/docs/source/specifications/index.rst
new file mode 100644
index 00000000..1853ca53
--- /dev/null
+++ b/docs/source/specifications/index.rst
@@ -0,0 +1,14 @@
+.. _specifications:
+
+================================================================================
+Specifications
+================================================================================
+
+PROJ implements a number of extensions to standards, that are described below
+for the sake of broader interoperability.
+
+.. toctree::
+   :maxdepth: 1
+
+   projjson
+   geodetictiffgrids
diff --git a/docs/source/usage/projjson.rst b/docs/source/specifications/projjson.rst
index 6bcce585..6bcce585 100644
--- a/docs/source/usage/projjson.rst
+++ b/docs/source/specifications/projjson.rst
diff --git a/docs/source/usage/index.rst b/docs/source/usage/index.rst
index c31c6dce..a074b18b 100644
--- a/docs/source/usage/index.rst
+++ b/docs/source/usage/index.rst
@@ -17,5 +17,4 @@ command line applications or the C API that is a part of the software package.
    transformation
    environmentvars
    differences
-   projjson
-
+   network
diff --git a/docs/source/usage/network.rst b/docs/source/usage/network.rst
new file mode 100644
index 00000000..295232b0
--- /dev/null
+++ b/docs/source/usage/network.rst
@@ -0,0 +1,102 @@
+.. _network:
+
+================================================================================
+Network capabilities
+================================================================================
+
+.. versionadded:: 7.0
+
+PROJ 7.0 has introduced, per :ref:`rfc4`, the capability to work with grid files
+that are not installed on the local machine where PROJ is executed.
+
+This enables to transparently download the parts of grids that are needed to
+perform a coordinate transformation.
+
+.. _cdn_grids:
+
+CDN of GeoTIFF grids
+--------------------
+
+Files are accessed by default through a CDN (Content Delivery Network),
+accessible through https://cdn.proj.org, that contains :ref:`geodetictiffgrids`
+datasets which are mirrored and managed by the
+https://github.com/OSGeo/proj-datumgrid-geotiff/ GitHub project.
+Files in the CDN are designed to be used by PROJ 7 or later, but any software
+project wishing to use the CDN for shifting support are encouraged to
+participate in the project and leverage the CDN. 
+
+How to enable network capabilities ?
+------------------------------------
+
+This capability assumes that PROJ has been build against `libcurl`, and that
+the user authorizes network access.
+
+Authorizing network access can be done in multiple ways:
+
+  - enabling / uncommenting the ``network = on`` line of :ref:`proj-ini`
+  - definiting the :envvar:`PROJ_NETWORK` environment variable to ON
+  - or using the :cpp:func:`proj_context_set_enable_network` with a
+    ``enabled`` = TRUE value.
+
+.. note::
+
+    Instead of using the `libcurl` implementation, an application using the PROJ
+    API can supply its own network implementation through C function callbacks
+    with :cpp:func:`proj_context_set_network_callbacks`. Enabling network use
+    must still be done with one of the above mentionned method.
+
+Setting endpoint
+----------------
+
+When this is enabled, and a grid is not found in the various locations where
+:ref:`resource files are looked for <resource_file_paths>`, PROJ will then
+attempt at loading the file from a remote server, which defaults to
+https://cdn.proj.org in :ref:`proj-ini`. This location can be changed with
+the :envvar:`PROJ_NETWORK_ENDPOINT` environment variable or with
+:cpp:func:`proj_context_set_url_endpoint`.
+
+Caching
+-------
+
+To avoid repeated access to network, a local cache of downloaded chunks of grids
+is implemented as SQLite3 database, ``cache.db``, stored in the
+:ref:`PROJ user writable directory <user_writable_directory>`.
+
+This local caching is enabled by default (can be changed in :ref:`proj-ini` or
+with :cpp:func:`proj_grid_cache_set_enable`). The default maximum size of the
+cache is 300 MB, which is more than half of the total size of grids available,
+at time of writing. This size can also be customized in :ref:`proj-ini` or
+with :cpp:func:`proj_grid_cache_set_max_size`
+
+Download API
+------------
+
+When on-demand loading of grid is not desirable, the PROJ API also offers the
+capability to download whole grids in the
+:ref:`PROJ user writable directory <user_writable_directory>` by using the
+:cpp:func:`proj_is_download_needed` and :cpp:func:`proj_download_file` functions.
+
+
+Mirroring
+---------
+
+If you are able, you are encouraged to mirror the grids via AWS S3 command line:
+
+::
+
+    aws s3 sync s3://cdn.proj.org .
+
+If direct S3 access is not possible, you can also use wget to locally mirror the
+data:
+
+::
+
+    wget --mirror https://cdn.proj.org/
+
+Acknowledgments
+---------------
+
+The s3://cdn.proj.org bucket is hosted by the
+`Amazon Public Datasets program <https://aws.amazon.com/opendata/public-datasets/>`_.
+CDN services are provided by the AWS Public Dataset team via
+`CloudFront <https://aws.amazon.com/cloudfront/>`_
diff --git a/scripts/reformat_cpp.sh b/scripts/reformat_cpp.sh
index 1b54fb3b..20c32b29 100755
--- a/scripts/reformat_cpp.sh
+++ b/scripts/reformat_cpp.sh
@@ -19,7 +19,7 @@ for i in "$TOPDIR"/include/proj/*.hpp "$TOPDIR"/include/proj/internal/*.hpp \
          "$TOPDIR"/src/iso19111/*.cpp "$TOPDIR"/test/unit/*.cpp "$TOPDIR"/src/apps/projinfo.cpp \
          "$TOPDIR"/src/tracing.cpp "$TOPDIR"/src/grids.hpp "$TOPDIR"/src/grids.cpp \
          "$TOPDIR"/src/filemanager.hpp "$TOPDIR"/src/filemanager.cpp \
-         "$TOPDIR"/src/sqlite3.hpp "$TOPDIR"/src/sqlite3.cpp ; do
+         "$TOPDIR"/src/sqlite3_utils.hpp "$TOPDIR"/src/sqlite3_utils.cpp ; do
     if ! echo "$i" | grep -q "lru_cache.hpp"; then
         "$SCRIPT_DIR"/reformat.sh "$i";
     fi
diff --git a/src/4D_api.cpp b/src/4D_api.cpp
index 71393dab..dabd44a0 100644
--- a/src/4D_api.cpp
+++ b/src/4D_api.cpp
@@ -1487,7 +1487,7 @@ PJ_INFO proj_info (void) {
         // Env var mostly for testing purposes and being independent from
         // an existing installation
         const char* ignoreUserWritableDirectory =
-            getenv("PROJ_IGNORE_USER_WRITABLE_DIRECTORY");
+            getenv("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY");
         if( ignoreUserWritableDirectory == nullptr ||
             ignoreUserWritableDirectory[0] == '\0'  ) {
             buf = path_append(buf,
diff --git a/src/filemanager.cpp b/src/filemanager.cpp
index 592bada4..005e734b 100644
--- a/src/filemanager.cpp
+++ b/src/filemanager.cpp
@@ -1156,7 +1156,7 @@ std::string pj_context_get_user_writable_directory(PJ_CONTEXT *ctx,
             const char *home = getenv("HOME");
             if (home) {
 #if defined(__MACH__) && defined(__APPLE__)
-                path = std::string(home) + "/Library/Logs";
+                path = std::string(home) + "/Library/Application Support";
 #else
                 path = std::string(home) + "/.local/share";
 #endif
@@ -1258,13 +1258,11 @@ static const char *proj_lib_name =
     nullptr;
 #endif
 
-static bool ignoreUserWritableDirectory() {
+static bool dontReadUserWritableDirectory() {
     // Env var mostly for testing purposes and being independent from
     // an existing installation
-    const char *envVarIgnoreUserWritableDirectory =
-        getenv("PROJ_IGNORE_USER_WRITABLE_DIRECTORY");
-    return envVarIgnoreUserWritableDirectory != nullptr &&
-           envVarIgnoreUserWritableDirectory[0] != '\0';
+    const char *envVar = getenv("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY");
+    return envVar != nullptr && envVar[0] != '\0';
 }
 
 static void *
@@ -1336,7 +1334,7 @@ pj_open_lib_internal(projCtx ctx, const char *name, const char *mode,
             }
         }
 
-        else if (!ignoreUserWritableDirectory() &&
+        else if (!dontReadUserWritableDirectory() &&
                  (fid = open_file(
                       ctx, (pj_context_get_user_writable_directory(ctx, false) +
                             DIR_CHAR + name)
diff --git a/src/grids.cpp b/src/grids.cpp
index 24fcfe83..68bf8600 100644
--- a/src/grids.cpp
+++ b/src/grids.cpp
@@ -1837,17 +1837,20 @@ std::unique_ptr<NTv2GridSet> NTv2GridSet::open(PJ_CONTEXT *ctx,
         pj_ctx_set_errno(ctx, PJD_ERR_FAILED_TO_LOAD_GRID);
         return nullptr;
     }
-    if (memcmp(header + 56, "SECONDS", 7) != 0) {
+
+    constexpr int OFFSET_GS_TYPE = 56;
+    if (memcmp(header + OFFSET_GS_TYPE, "SECONDS", 7) != 0) {
         pj_log(ctx, PJ_LOG_ERROR, "Only GS_TYPE=SECONDS is supported");
         pj_ctx_set_errno(ctx, PJD_ERR_FAILED_TO_LOAD_GRID);
         return nullptr;
     }
 
     const bool must_swap = (header[8] == 11) ? !IS_LSB : IS_LSB;
+    constexpr int OFFSET_NUM_SUBFILES = 8 + 32;
     if (must_swap) {
         // swap_words( header+8, 4, 1 );
         // swap_words( header+8+16, 4, 1 );
-        swap_words(header + 8 + 32, 4, 1);
+        swap_words(header + OFFSET_NUM_SUBFILES, 4, 1);
         // swap_words( header+8+7*16, 8, 1 );
         // swap_words( header+8+8*16, 8, 1 );
         // swap_words( header+8+9*16, 8, 1 );
@@ -1858,7 +1861,7 @@ std::unique_ptr<NTv2GridSet> NTv2GridSet::open(PJ_CONTEXT *ctx,
     /*      Get the subfile count out ... all we really use for now.        */
     /* -------------------------------------------------------------------- */
     unsigned int num_subfiles;
-    memcpy(&num_subfiles, header + 8 + 32, 4);
+    memcpy(&num_subfiles, header + OFFSET_NUM_SUBFILES, 4);
 
     std::map<std::string, NTv2Grid *> mapGrids;
 
@@ -1878,25 +1881,31 @@ std::unique_ptr<NTv2GridSet> NTv2GridSet::open(PJ_CONTEXT *ctx,
         }
 
         // Byte swap interesting fields if needed.
+        constexpr int OFFSET_GS_COUNT = 8 + 16 * 10;
+        constexpr int OFFSET_SOUTH_LAT = 8 + 16 * 4;
         if (must_swap) {
-            swap_words(header + 8 + 16 * 4, sizeof(double), 6);
-            swap_words(header + 8 + 16 * 10, sizeof(int), 1);
+            // 6 double values: southLat, northLat, eastLon, westLon, resLat,
+            // resLon
+            swap_words(header + OFFSET_SOUTH_LAT, sizeof(double), 6);
+            swap_words(header + OFFSET_GS_COUNT, sizeof(int), 1);
         }
 
         std::string gridName;
         gridName.append(header + 8, 8);
 
         ExtentAndRes extent;
-        extent.westLon =
-            -to_double(header + 7 * 16 + 8) * DEG_TO_RAD / 3600.0; /* W_LONG */
-        extent.southLat =
-            to_double(header + 4 * 16 + 8) * DEG_TO_RAD / 3600.0; /* S_LAT */
-        extent.eastLon =
-            -to_double(header + 6 * 16 + 8) * DEG_TO_RAD / 3600.0; /* E_LONG */
-        extent.northLat =
-            to_double(header + 5 * 16 + 8) * DEG_TO_RAD / 3600.0; /* N_LAT */
-        extent.resLon = to_double(header + 9 * 16 + 8) * DEG_TO_RAD / 3600.0;
-        extent.resLat = to_double(header + 8 * 16 + 8) * DEG_TO_RAD / 3600.0;
+        extent.southLat = to_double(header + OFFSET_SOUTH_LAT) * DEG_TO_RAD /
+                          3600.0; /* S_LAT */
+        extent.northLat = to_double(header + OFFSET_SOUTH_LAT + 16) *
+                          DEG_TO_RAD / 3600.0; /* N_LAT */
+        extent.eastLon = -to_double(header + OFFSET_SOUTH_LAT + 16 * 2) *
+                         DEG_TO_RAD / 3600.0; /* E_LONG */
+        extent.westLon = -to_double(header + OFFSET_SOUTH_LAT + 16 * 3) *
+                         DEG_TO_RAD / 3600.0; /* W_LONG */
+        extent.resLat =
+            to_double(header + OFFSET_SOUTH_LAT + 16 * 4) * DEG_TO_RAD / 3600.0;
+        extent.resLon =
+            to_double(header + OFFSET_SOUTH_LAT + 16 * 5) * DEG_TO_RAD / 3600.0;
 
         if (!(fabs(extent.westLon) <= 4 * M_PI &&
               fabs(extent.eastLon) <= 4 * M_PI &&
@@ -1923,7 +1932,7 @@ std::unique_ptr<NTv2GridSet> NTv2GridSet::open(PJ_CONTEXT *ctx,
                extent.northLat * RAD_TO_DEG);
 
         unsigned int gs_count;
-        memcpy(&gs_count, header + 8 + 16 * 10, 4);
+        memcpy(&gs_count, header + OFFSET_GS_COUNT, 4);
         if (gs_count / columns != static_cast<unsigned>(rows)) {
             pj_log(ctx, PJ_LOG_ERROR,
                    "GS_COUNT(%u) does not match expected cells (%dx%d)",
@@ -2845,6 +2854,7 @@ ListOfHGrids pj_hgrid_init(PJ *P, const char *gridkey) {
 
 typedef struct { pj_int32 lam, phi; } ILP;
 
+// Apply bilinear interpolation for horizontal shift grids
 static PJ_LP pj_hgrid_interpolate(PJ_LP t, const HorizontalShiftGrid *grid,
                                   bool compensateNTConvention) {
     PJ_LP val, frct;
@@ -3283,6 +3293,8 @@ const GenericShiftGrid *pj_find_generic_grid(const ListOfGenericGrids &grids,
 
 // ---------------------------------------------------------------------------
 
+// Used by +proj=deformation and +proj=xyzgridshift to do bilinear interpolation
+// on 3 sample values per node.
 bool pj_bilinear_interpolation_three_samples(const GenericShiftGrid *grid,
                                              const PJ_LP &lp, int idx1,
                                              int idx2, int idx3, double &v1,
diff --git a/src/proj_internal.h b/src/proj_internal.h
index bb0e3453..a587c037 100644
--- a/src/proj_internal.h
+++ b/src/proj_internal.h
@@ -682,7 +682,7 @@ struct projGridChunkCache
 {
     bool enabled = true;
     std::string filename{};
-    long long max_size = 100 * 1024 * 1024;
+    long long max_size = 300 * 1024 * 1024;
     int ttl = 86400; // 1 day
 };
 
diff --git a/test/cli/Makefile.am b/test/cli/Makefile.am
index 42ee466d..417ec137 100644
--- a/test/cli/Makefile.am
+++ b/test/cli/Makefile.am
@@ -29,7 +29,7 @@ EXTRA_DIST = pj_out27.dist pj_out83.dist td_out.dist \
 		CMakeLists.txt
 
 testprojinfo-check:
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTPROJINFO) $(PROJINFOEXE)
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTPROJINFO) $(PROJINFOEXE)
 
 test27-check:
 	$(TEST27) $(PROJEXE)
@@ -41,24 +41,24 @@ testproj-check:
 	$(TESTPROJ) $(PROJEXE)
 
 testvarious-check:
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTVARIOUS) $(CS2CSEXE)
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTVARIOUS) $(CS2CSEXE)
 
 testdatumfile-check:
 	@if [ -f $(PROJ_LIB)/conus -a -f $(PROJ_LIB)/ntv1_can.dat -a -f $(PROJ_LIB)/MD -a -f $(PROJ_LIB)/ntf_r93.gsb -a -f $(PROJ_LIB)/egm96_15.gtx ]; then \
-	  PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTDATUMFILE) $(CS2CSEXE) ; \
+	  PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTDATUMFILE) $(CS2CSEXE) ; \
 	fi
 
 testign-check:
 	@if [ -f $(PROJ_LIB)/ntf_r93.gsb ] ; then \
-	  PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTIGN) $(CS2CSEXE) ; \
+	  PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTIGN) $(CS2CSEXE) ; \
 	fi
 
 testntv2-check:
 	@if [ -f $(PROJ_LIB)/ntv2_0.gsb -a -f $(PROJ_LIB)/conus -a -f $(PROJ_LIB)/ntv1_can.dat ] ; then \
-	  PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTNTV2) $(CS2CSEXE) ; \
+	  PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTNTV2) $(CS2CSEXE) ; \
 	fi
 
 testcct-check:
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTCCT) $(CCTEXE)
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(TESTCCT) $(CCTEXE)
 
 check-local: testprojinfo-check test27-check test83-check testproj-check testvarious-check testdatumfile-check testign-check testntv2-check testcct-check
diff --git a/test/gie/Makefile.am b/test/gie/Makefile.am
index 783dfb7c..560fea0b 100644
--- a/test/gie/Makefile.am
+++ b/test/gie/Makefile.am
@@ -15,33 +15,33 @@ EXTRA_DIST = 4D-API_cs2cs-style.gie \
 PROJ_LIB ?= ../../data/for_tests
 
 4D-API-cs2cs-style: 4D-API_cs2cs-style.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 GDA: GDA.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 axisswap: axisswap.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 builtins: builtins.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 deformation: deformation.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 ellipsoid: ellipsoid.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 more_builtins: more_builtins.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 unitconvert: unitconvert.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 DHDN_ETRS89: DHDN_ETRS89.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 geotiff_grids: geotiff_grids.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 check-local: 4D-API-cs2cs-style GDA axisswap builtins deformation ellipsoid more_builtins unitconvert DHDN_ETRS89 geotiff_grids
diff --git a/test/gigs/Makefile.am b/test/gigs/Makefile.am
index 22ab099b..436a6e89 100644
--- a/test/gigs/Makefile.am
+++ b/test/gigs/Makefile.am
@@ -23,54 +23,54 @@ EXTRA_DIST = \
 PROJ_LIB ?= ../../data/for_tests
 
 5101.1: 5101.1-jhs.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5101.2: 5101.2-jhs.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5101.3: 5101.3-jhs.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5101.4: 5101.4-jhs-etmerc.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5102.1: 5102.1.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5103.1: 5103.1.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5103.2: 5103.2.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5103.3: 5103.3.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5105.2: 5105.2.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5106: 5106.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5107: 5107.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5109: 5109.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5111.1: 5111.1.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5112: 5112.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5113: 5113.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5201: 5201.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 5208: 5208.gie
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) $(GIEEXE) $<
 
 check-local: 5101.1 5101.2 5101.3 5101.4 5102.1 5103.1 5103.2 5103.3 5105.2 5106 5107 5109 5111.1 5112 5113 5201 5208
diff --git a/test/unit/CMakeLists.txt b/test/unit/CMakeLists.txt
index f5890f1a..845d07e5 100644
--- a/test/unit/CMakeLists.txt
+++ b/test/unit/CMakeLists.txt
@@ -71,7 +71,7 @@ target_link_libraries(proj_pj_transform_test
   ${PROJ_LIBRARIES})
 add_test(NAME proj_pj_transform_test COMMAND proj_pj_transform_test)
 set_property(TEST proj_pj_transform_test
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 
 add_executable(proj_errno_string_test
@@ -82,7 +82,7 @@ target_link_libraries(proj_errno_string_test
   ${PROJ_LIBRARIES})
 add_test(NAME proj_errno_string_test COMMAND proj_errno_string_test)
 set_property(TEST proj_errno_string_test
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 add_executable(proj_angular_io_test
   main.cpp
@@ -92,7 +92,7 @@ target_link_libraries(proj_angular_io_test
   ${PROJ_LIBRARIES})
 add_test(NAME proj_angular_io_test COMMAND proj_angular_io_test)
 set_property(TEST proj_angular_io_test
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 add_executable(proj_context_test
   main.cpp
@@ -102,7 +102,7 @@ target_link_libraries(proj_context_test
   ${PROJ_LIBRARIES})
 add_test(NAME proj_context_test COMMAND proj_context_test)
 set_property(TEST proj_context_test
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 if(MSVC AND BUILD_LIBPROJ_SHARED)
   # ph_phi2_test not compatible of a .dll build
@@ -115,7 +115,7 @@ else()
     ${PROJ_LIBRARIES})
   add_test(NAME pj_phi2_test COMMAND pj_phi2_test)
   set_property(TEST pj_phi2_test
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 endif()
 
 add_executable(proj_test_cpp_api
@@ -136,7 +136,7 @@ target_link_libraries(proj_test_cpp_api
   ${SQLITE3_LIBRARY})
 add_test(NAME proj_test_cpp_api COMMAND proj_test_cpp_api)
 set_property(TEST proj_test_cpp_api
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 
 add_executable(gie_self_tests
@@ -147,7 +147,7 @@ target_link_libraries(gie_self_tests
   ${PROJ_LIBRARIES})
 add_test(NAME gie_self_tests COMMAND gie_self_tests)
 set_property(TEST gie_self_tests
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests")
 
 
 add_executable(test_network
@@ -163,4 +163,4 @@ target_link_libraries(test_network
   ${SQLITE3_LIBRARY})
 add_test(NAME test_network COMMAND test_network)
 set_property(TEST test_network
-        PROPERTY ENVIRONMENT "PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests;PROJ_SOURCE_DATA=${PROJECT_SOURCE_DIR}/data")
+        PROPERTY ENVIRONMENT "PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES;PROJ_LIB=${PROJECT_BINARY_DIR}/data/for_tests;PROJ_SOURCE_DATA=${PROJECT_SOURCE_DIR}/data")
diff --git a/test/unit/Makefile.am b/test/unit/Makefile.am
index 86ed383a..11a6473c 100644
--- a/test/unit/Makefile.am
+++ b/test/unit/Makefile.am
@@ -23,7 +23,7 @@ pj_transform_test_SOURCES = pj_transform_test.cpp main.cpp
 pj_transform_test_LDADD = ../../src/libproj.la @GTEST_LIBS@
 
 pj_transform_test-check: pj_transform_test
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./pj_transform_test
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./pj_transform_test
 
 pj_phi2_test_SOURCES = pj_phi2_test.cpp main.cpp
 pj_phi2_test_LDADD = ../../src/libproj.la @GTEST_LIBS@
@@ -47,19 +47,19 @@ proj_context_test_SOURCES = proj_context_test.cpp main.cpp
 proj_context_test_LDADD = ../../src/libproj.la @GTEST_LIBS@
 
 proj_context_test-check: proj_context_test
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES ./proj_context_test
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES ./proj_context_test
 
 test_cpp_api_SOURCES = test_util.cpp test_common.cpp test_crs.cpp test_metadata.cpp test_io.cpp test_operation.cpp test_datum.cpp test_factory.cpp test_c_api.cpp test_grids.cpp main.cpp
 test_cpp_api_LDADD = ../../src/libproj.la @GTEST_LIBS@ @SQLITE3_LIBS@
 
 test_cpp_api-check: test_cpp_api
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./test_cpp_api
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./test_cpp_api
 
 gie_self_tests_SOURCES = gie_self_tests.cpp main.cpp
 gie_self_tests_LDADD = ../../src/libproj.la @GTEST_LIBS@ @SQLITE3_LIBS@
 
 gie_self_tests-check: gie_self_tests
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./gie_self_tests
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) ./gie_self_tests
 
 include_proj_h_from_c_SOURCES = include_proj_h_from_c.c
 
@@ -68,6 +68,6 @@ test_network_CXXFLAGS = @CURL_CFLAGS@ @CURL_ENABLED_FLAGS@
 test_network_LDADD = ../../src/libproj.la @GTEST_LIBS@ @SQLITE3_LIBS@ @CURL_LIBS@
 
 test_network-check: test_network
-	PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) PROJ_SOURCE_DATA=$(PROJ_LIB) ./test_network
+	PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES PROJ_LIB=$(PROJ_LIB) PROJ_SOURCE_DATA=$(PROJ_LIB) ./test_network
 
 check-local: pj_transform_test-check pj_phi2_test-check proj_errno_string_test-check proj_angular_io_test-check proj_context_test-check test_cpp_api-check gie_self_tests-check test_network-check
diff --git a/test/unit/gie_self_tests.cpp b/test/unit/gie_self_tests.cpp
index fc0f0748..bcf31139 100644
--- a/test/unit/gie_self_tests.cpp
+++ b/test/unit/gie_self_tests.cpp
@@ -348,9 +348,9 @@ TEST(gie, info_functions) {
 
     /* proj_info()                                                            */
     /* this one is difficult to test, since the output changes with the setup */
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY="));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY="));
     info = proj_info();
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES"));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES"));
 
     if (info.version[0] != '\0') {
         char tmpstr[64];
diff --git a/test/unit/proj_context_test.cpp b/test/unit/proj_context_test.cpp
index ec59590d..92f251cd 100644
--- a/test/unit/proj_context_test.cpp
+++ b/test/unit/proj_context_test.cpp
@@ -159,18 +159,19 @@ TEST(proj_context, read_grid_from_user_writable_directory) {
     auto filename = path + DIR_CHAR + "temp_proj_dic3";
     EXPECT_TRUE(createTmpFile(filename));
     {
-        // Check that with PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES (set by
+        // Check that with PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES (set by
         // calling script), we cannot find the file
         auto P = proj_create(ctx, "+init=temp_proj_dic3:MY_PIPELINE");
         EXPECT_EQ(P, nullptr);
         proj_destroy(P);
     }
     {
-        // Cancel the effect of PROJ_IGNORE_USER_WRITABLE_DIRECTORY
-        putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY="));
+        // Cancel the effect of PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY
+        putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY="));
         auto P = proj_create(ctx, "+init=temp_proj_dic3:MY_PIPELINE");
         EXPECT_NE(P, nullptr);
-        putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES"));
+        putenv(
+            const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES"));
         proj_destroy(P);
     }
     proj_context_destroy(ctx);
diff --git a/test/unit/test_network.cpp b/test/unit/test_network.cpp
index 688e61e5..241e6bbc 100644
--- a/test/unit/test_network.cpp
+++ b/test/unit/test_network.cpp
@@ -1577,7 +1577,7 @@ TEST(networking, download_whole_files) {
     unlink("proj_test_tmp/dvr90.tif");
     rmdir("proj_test_tmp");
 
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY="));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY="));
     putenv(const_cast<char *>("PROJ_USER_WRITABLE_DIRECTORY=./proj_test_tmp"));
     putenv(const_cast<char *>("PROJ_FULL_FILE_CHUNK_SIZE=100000"));
     auto ctx = proj_context_create();
@@ -1699,7 +1699,7 @@ TEST(networking, download_whole_files) {
     ASSERT_EQ(vectPct.back().second, 1.0);
 
     proj_context_destroy(ctx);
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES"));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES"));
     putenv(const_cast<char *>("PROJ_USER_WRITABLE_DIRECTORY="));
     putenv(const_cast<char *>("PROJ_FULL_FILE_CHUNK_SIZE="));
     unlink("proj_test_tmp/cache.db");
@@ -1719,7 +1719,7 @@ TEST(networking, file_api) {
     unlink("proj_test_tmp/dvr90.tif");
     rmdir("proj_test_tmp");
 
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY="));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY="));
     putenv(const_cast<char *>("PROJ_USER_WRITABLE_DIRECTORY=./proj_test_tmp"));
     putenv(const_cast<char *>("PROJ_FULL_FILE_CHUNK_SIZE=30000"));
     auto ctx = proj_context_create();
@@ -1843,7 +1843,7 @@ TEST(networking, file_api) {
     ASSERT_TRUE(userData.in_seek);
 
     proj_context_destroy(ctx);
-    putenv(const_cast<char *>("PROJ_IGNORE_USER_WRITABLE_DIRECTORY=YES"));
+    putenv(const_cast<char *>("PROJ_SKIP_READ_USER_WRITABLE_DIRECTORY=YES"));
     putenv(const_cast<char *>("PROJ_USER_WRITABLE_DIRECTORY="));
     putenv(const_cast<char *>("PROJ_FULL_FILE_CHUNK_SIZE="));
     unlink("proj_test_tmp/cache.db");