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itemscope="itemscope" itemtype="http://schema.org/Article"> + <div itemprop="articleBody"> + + <section id="geodetic-tiff-grids-gtg"> +<span id="geodetictiffgrids"></span><h1>Geodetic TIFF grids (GTG)<a class="headerlink" href="#geodetic-tiff-grids-gtg" title="Permalink to this headline">¶</a></h1> +<div class="versionadded"> +<p><span class="versionmodified added">New in version 7.0.</span></p> +</div> +<section id="introduction"> +<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2> +<p>The Geodetic TIFF grid format has been introduced per <a class="reference internal" href="../community/rfc/rfc-4.html#rfc4"><span class="std std-ref">PROJ RFC 4: Remote access to grids and GeoTIFF grids</span></a>. 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 <a class="reference external" href="http://cogeo.org">Cloud Optimized GeoTIFF</a> principles.</p> +<p>Such grids are available on a <a class="reference internal" href="../usage/network.html#cdn-grids"><span class="std std-ref">CDN of GeoTIFF grids</span></a>.</p> +</section> +<section id="general-description"> +<span id="gtg-general-description"></span><h2>General description<a class="headerlink" href="#general-description" title="Permalink to this headline">¶</a></h2> +<p>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:</p> +<ul> +<li><p><a class="reference external" href="https://www.awaresystems.be/imaging/tiff/specification/TIFF6.pdf">TIFF 6.0</a> +based (could possibly be BigTIFF without code changes, if we ever +need some day to handle grids larger than 4GB)</p></li> +<li><p><a class="reference external" href="http://docs.opengeospatial.org/is/19-008r4/19-008r4.html">GeoTIFF 1.1</a> 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.</p></li> +<li><p>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 +<a class="reference external" href="http://docs.opengeospatial.org/as/18-005r4/18-005r4.html">OGC Abstract Specification Topic 2</a>, +that is the CRS to which grid values are referred to.</p> +<p>Given that they will nominally be related to the EPSG dataset, the <a class="reference external" href="http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_geodeticcrsgeokey">GeodeticCRSGeoKey</a> +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 <code class="docutils literal notranslate"><span class="pre">interpolation_crs_wkt</span></code> metadata +item detailed afterwards should be used.</p> +<p>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…</p> +<p>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.</p> +<p>For geographic to vertical shift files (geoid models), the GeoTIFF 1.1 +convention will be used to store the value of the <a class="reference external" href="http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_verticalgeokey">VerticalGeoKey</a> +So a geoid model that apply to WGS 84 EPSG:4979 will have GeodeticCRSGeoKey = 4326 +and VerticalGeoKey = 4979.</p> +</li> +<li><p>Files hosted on the CDN will use the GeoTIFF defined <a class="reference external" href="http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_raster_to_model_coordinate_transformation_requirements">ModelTiepointTag and ModelPixelScaleTag</a> 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.</p> +<div class="admonition note"> +<p class="admonition-title">Note</p> +<p>Regarding anti-meridian handling, a variety of possibilities exist. +We do not attempt to standardize this and files 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.</p> +</div> +</li> +<li><p>Files hosted on the CDN will use the <a class="reference external" href="http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#_requirements_class_gtrastertypegeokey">GTRasterTypeGeoKey</a> += 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 behavior if this GeoKey is absent.</p></li> +<li><p>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.</p></li> +<li><p>Files hosted on the CDN will use <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/compression.html">Compression</a> = DEFLATE +or LZW (to be determined, possibly with +<a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/predictor.html">Predictor</a> = 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.</p></li> +<li><p>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.</p></li> +<li><p>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.</p></li> +<li><p>Files hosted on the CDN will generally use Float32 (BitsPerSample=32 and SampleFormat=IEEEFP) +Files may be created using Signed Int 16 ( +<a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/bitspersample.html">BitsPerSample</a> =16 and +<a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/sampleformat.html">SampleFormat</a> = 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.</p></li> +<li><p>Files hosted on the CDN will have a <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/photometricinterpretation.html">PhotometricInterpretation</a> = MinIsBlack. +It will be assumed, and ignored on the reading side.</p></li> +<li><p>Files hosted on the CDN will nominally have:</p> +<blockquote> +<div><blockquote> +<div><ul class="simple"> +<li><p><a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/samplesperpixel.html">SamplesPerPixel</a> = 2 +for horizontal shift grid, with the first sample being the longitude offset +and the second sample being the latitude offset.</p></li> +<li><p>SamplesPerPixel = 1 for vertical shift grids.</p></li> +<li><p>SamplesPerPixel = 3 for deformation models combining +horizontal and vertical adjustments.</p></li> +</ul> +</div></blockquote> +<p>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.</p> +<p>The <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/extrasamples.html">ExtraSamples</a> +tag should be set to a value of SamplesPerPixel - 1 (given the rules that +apply for PhotometricInterpretation = MinIsBlack)</p> +</div></blockquote> +</li> +<li><p>The <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/imagedescription.html">ImageDescription</a> +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 for files hosted on the CDN. +Ignored by PROJ.</p></li> +<li><p>The <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/copyright.html">Copyright</a> +tag may be used to convey extra information about the copyright and license of the grid. +Will be set when possible for files hosted on the CDN. +Ignored by PROJ.</p></li> +<li><p>The <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/datetime.html">DateTime</a> +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 <code class="docutils literal notranslate"><span class="pre">ImageDescription</span></code> +tag however will contain the latest of the CREATED or UPDATED fields from the NTv2 file. +Will be set when possible for files hosted on the CDN. +Ignored by PROJ.</p></li> +<li><p>Files hosted on the CDN will use the <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/gdal_nodata.html">GDAL_NODATA</a> tag to encode +the value of the nodata / missing value, when it applies to the grid.</p> +<p>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.</p> +</li> +<li><p>Files hosted on the CDN will use the <a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/gdal_metadata.html">GDAL_METADATA</a> tag to encode extra +metadata not supported by baseline or extended TIFF.</p> +<ul> +<li><p>The root XML node should be <code class="docutils literal notranslate"><span class="pre">GDALMetadata</span></code></p></li> +<li><p>Zero, one or several child XML nodes <code class="docutils literal notranslate"><span class="pre">Item</span></code> may be present.</p></li> +<li><p>A Item should have a <code class="docutils literal notranslate"><span class="pre">name</span></code> attribute, and a child text node with its value. +<code class="docutils literal notranslate"><span class="pre">role</span></code> and <code class="docutils literal notranslate"><span class="pre">sample</span></code> attributes may be present for attributes that have +a special semantics (recognized by GDAL). The value of <cite>sample</cite> should be +a integer value between 0 and number_of_samples - 1.</p></li> +<li><p>Scale and offset to convert integer raw values to floating point values +may be expressed with XML <cite>Item</cite> elements whose name attribute is respectively +<code class="docutils literal notranslate"><span class="pre">SCALE</span></code> and <code class="docutils literal notranslate"><span class="pre">OFFSET</span></code>, and their <code class="docutils literal notranslate"><span class="pre">role</span></code> attribute is respectively <code class="docutils literal notranslate"><span class="pre">scale</span></code> +and <code class="docutils literal notranslate"><span class="pre">offset</span></code>. The decoded value will be: {offset} + {scale} * raw_value_from_geotiff_file</p> +<p>For a offset value of 1 and scaling of 2, the following payload should be +stored:</p> +<div class="highlight-xml notranslate"><div class="highlight"><pre><span></span><span class="nt"><GDALMetadata></span> + <span class="nt"><Item</span> <span class="na">name=</span><span class="s">"OFFSET"</span> <span class="na">sample=</span><span class="s">"0"</span> <span class="na">role=</span><span class="s">"offset"</span><span class="nt">></span>1<span class="nt"></Item></span> + <span class="nt"><Item</span> <span class="na">name=</span><span class="s">"SCALE"</span> <span class="na">sample=</span><span class="s">"0"</span> <span class="na">role=</span><span class="s">"scale"</span><span class="nt">></span>2<span class="nt"></Item></span> +<span class="nt"></GDALMetadata></span> +</pre></div> +</div> +</li> +<li><p>The type of the grid must be specified with a <cite>Item</cite> whose <code class="docutils literal notranslate"><span class="pre">name</span></code> is set +to <code class="docutils literal notranslate"><span class="pre">TYPE</span></code>.</p> +<p>Values recognized by PROJ currently are:</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">HORIZONTAL_OFFSET</span></code>: 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 <a class="reference internal" href="../operations/transformations/hgridshift.html#hgridshift"><span class="std std-ref">Horizontal grid shift</span></a> method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL</span></code>: 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 <a class="reference internal" href="../operations/transformations/vgridshift.html#vgridshift"><span class="std std-ref">Vertical grid shift</span></a> method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">VERTICAL_OFFSET_VERTICAL_TO_VERTICAL</span></code>: 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 <a class="reference internal" href="../operations/transformations/vgridshift.html#vgridshift"><span class="std std-ref">Vertical grid shift</span></a> method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">GEOCENTRIC_TRANSLATION</span></code>: 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 <a class="reference internal" href="../operations/transformations/xyzgridshift.html#xyzgridshift"><span class="std std-ref">Geocentric grid shift</span></a> method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">VELOCITY</span></code>: 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 <a class="reference internal" href="../operations/transformations/deformation.html#deformation"><span class="std std-ref">Kinematic datum shifting utilizing a deformation model</span></a> method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">DEFORMATION_MODEL</span></code>: implies the presence of the <code class="docutils literal notranslate"><span class="pre">DISPLACEMENT_TYPE</span></code> +and <code class="docutils literal notranslate"><span class="pre">UNCERTAINTY_TYPE</span></code> metadata items. +Corresponds to PROJ <a class="reference internal" href="../operations/transformations/defmodel.html#defmodel"><span class="std std-ref">Multi-component time-based deformation model</span></a> method.</p></li> +</ul> +<p>For example:</p> +<div class="highlight-xml notranslate"><div class="highlight"><pre><span></span><span class="nt"><Item</span> <span class="na">name=</span><span class="s">"TYPE"</span><span class="nt">></span>HORIZONTAL_OFFSET<span class="nt"></Item></span> +</pre></div> +</div> +</li> +<li><p>The description of each sample must be specified with a Item whose <code class="docutils literal notranslate"><span class="pre">name</span></code> +attribute is set to <code class="docutils literal notranslate"><span class="pre">DESCRIPTION</span></code> and <code class="docutils literal notranslate"><span class="pre">role</span></code> attribute to <code class="docutils literal notranslate"><span class="pre">description</span></code>.</p> +<p>Values recognized by PROJ for this Item are currently:</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">latitude_offset</span></code>: 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.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">longitude_offset</span></code>: 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.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">geoid_undulation</span></code>: 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 <a class="reference external" href="https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:method:EPSG::9665">EPSG:9665</a> +operation method.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">vertical_offset</span></code>: 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.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">x_translation</span></code> / <code class="docutils literal notranslate"><span class="pre">y_translation</span></code> / <code class="docutils literal notranslate"><span class="pre">z_translation</span></code>: 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.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">east_velocity</span></code> / <code class="docutils literal notranslate"><span class="pre">north_velocity</span></code> / <code class="docutils literal notranslate"><span class="pre">up_velocity</span></code>: valid for +TYPE=VELOCITY. +Sample values should be the velocity in a linear/time unit in a ENU local +topocentric coordinate system.</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">east_offset</span></code> / <code class="docutils literal notranslate"><span class="pre">north_offset</span></code> / <code class="docutils literal notranslate"><span class="pre">vertical_offset</span></code>: valid for +TYPE=DEFORMATION_MODEL. +For east_offset and north_offset, the unit might be degree or metre. +For vertical_offset, the unit must be metre.</p></li> +</ul> +<p>For example:</p> +<div class="highlight-xml notranslate"><div class="highlight"><pre><span></span><span class="nt"><Item</span> <span class="na">name=</span><span class="s">"DESCRIPTION"</span> <span class="na">sample=</span><span class="s">"0"</span> <span class="na">role=</span><span class="s">"description"</span><span class="nt">></span>latitude_offset<span class="nt"></Item></span> +<span class="nt"><Item</span> <span class="na">name=</span><span class="s">"DESCRIPTION"</span> <span class="na">sample=</span><span class="s">"1"</span> <span class="na">role=</span><span class="s">"description"</span><span class="nt">></span>longitude_offset<span class="nt"></Item></span> +</pre></div> +</div> +<p>Other values may be used (not used by PROJ):</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">latitude_offset_accuracy</span></code>: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be +the accuracy of corresponding latitude_offset samples. Generally in metre (if converted from NTv2)</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">longitude_offset_accuracy</span></code>: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be +the accuracy of corresponding longitude_offset samples. Generally in metre (if converted from NTv2)</p></li> +</ul> +</li> +<li><p>The sign convention for the values of the <code class="docutils literal notranslate"><span class="pre">longitude_offset</span></code> channel +should be indicated with an Item named <code class="docutils literal notranslate"><span class="pre">positive_value</span></code> whose value +can be <code class="docutils literal notranslate"><span class="pre">west</span></code> or <code class="docutils literal notranslate"><span class="pre">east</span></code>. NTv2 products originally use a <code class="docutils literal notranslate"><span class="pre">west</span></code> +convention, but when converting from them to GeoTIFF, the sign of those +samples will be inverted so they use a more natural <code class="docutils literal notranslate"><span class="pre">east</span></code> convention. +If this item is absent, the default value is <code class="docutils literal notranslate"><span class="pre">east</span></code>.</p></li> +<li><p>The unit of the values stored in the grid must be specified for each +sample through an Item of name <code class="docutils literal notranslate"><span class="pre">UNITTYPE</span></code> and role <code class="docutils literal notranslate"><span class="pre">unittype</span></code> +Valid values should be the name of entries from the EPSG <code class="docutils literal notranslate"><span class="pre">unitofmeasure</span></code> +table. To maximize interoperability, writers are strongly encouraged to +limit themselves to the following values:</p> +<p>For linear units:</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">metre</span></code> (default value assumed if absent for vertical shift grid files, and value used for files stored on PROJ CDN)</p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">US</span> <span class="pre">survey</span> <span class="pre">foot</span></code></p></li> +</ul> +<p>For angular units:</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">degree</span></code></p></li> +<li><p><code class="docutils literal notranslate"><span class="pre">arc-second</span></code> (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)</p></li> +</ul> +<p>For velocity units:</p> +<ul class="simple"> +<li><p><code class="docutils literal notranslate"><span class="pre">millimetres</span> <span class="pre">per</span> <span class="pre">year</span></code></p></li> +</ul> +<p>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.</p> +<p>Example:</p> +<div class="highlight-xml notranslate"><div class="highlight"><pre><span></span><span class="nt"><Item</span> <span class="na">name=</span><span class="s">"UNITTYPE"</span> <span class="na">sample=</span><span class="s">"0"</span> <span class="na">role=</span><span class="s">"unittype"</span><span class="nt">></span>arc-second<span class="nt"></Item></span> +<span class="nt"><Item</span> <span class="na">name=</span><span class="s">"UNITTYPE"</span> <span class="na">sample=</span><span class="s">"1"</span> <span class="na">role=</span><span class="s">"unittype"</span><span class="nt">></span>arc-second<span class="nt"></Item></span> +</pre></div> +</div> +</li> +<li><p>For TYPE=DEFORMATION_MODEL, the type of the displacement must be specified +with a <cite>Item</cite> whose <code class="docutils literal notranslate"><span class="pre">name</span></code> is set to <code class="docutils literal notranslate"><span class="pre">DISPLACEMENT_TYPE</span></code>.</p> +<p>The accepted values are: <code class="docutils literal notranslate"><span class="pre">HORIZONTAL</span></code>, <code class="docutils literal notranslate"><span class="pre">VERTICAL</span></code>, <code class="docutils literal notranslate"><span class="pre">3D</span></code> or <code class="docutils literal notranslate"><span class="pre">NONE</span></code></p> +</li> +<li><p>For TYPE=DEFORMATION_MODEL, the type of the uncertainty must be specified +with a <cite>Item</cite> whose <code class="docutils literal notranslate"><span class="pre">name</span></code> is set to <code class="docutils literal notranslate"><span class="pre">UNCERTAINTY_TYPE</span></code>.</p> +<p>The accepted values are: <code class="docutils literal notranslate"><span class="pre">HORIZONTAL</span></code>, <code class="docutils literal notranslate"><span class="pre">VERTICAL</span></code>, <code class="docutils literal notranslate"><span class="pre">3D</span></code> or <code class="docutils literal notranslate"><span class="pre">NONE</span></code></p> +</li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">target_crs_epsg_code</span></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, <code class="docutils literal notranslate"><span class="pre">target_crs_wkt</span></code> must be +used. +Ignored by PROJ currently.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">target_crs_wkt</span></code> metadata item must be present if the +<code class="docutils literal notranslate"><span class="pre">target_crs_epsg_code</span></code> cannot be used. +Its value should be a valid WKT string according to +<a class="reference external" href="http://docs.opengeospatial.org/is/12-063r5/12-063r5.html">WKT:2015</a> +or <a class="reference external" href="hhttp://docs.opengeospatial.org/is/18-010r7/18-010r7.html">WKT:2019</a> +Ignored by PROJ currently.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">source_crs_epsg_code</span></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, <code class="docutils literal notranslate"><span class="pre">source_crs_wkt</span></code> must be +used. +Ignored by PROJ currently.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">source_crs_wkt</span></code> metadata item must be present if the +<code class="docutils literal notranslate"><span class="pre">source_crs_epsg_code</span></code> cannot be used. +Its value should be a valid WKT string according to WKT:2015 or WKT:2019. +Ignored by PROJ currently.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">interpolation_crs_wkt</span></code> 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.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">recommended_interpolation_method</span></code> 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: <code class="docutils literal notranslate"><span class="pre">bilinear</span></code>, +<code class="docutils literal notranslate"><span class="pre">bicubic</span></code>. +Ignored by PROJ currently.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">area_of_use</span></code> 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.</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">grid_name</span></code> 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)</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">parent_grid_name</span></code> metadata item should be present if this is a +subgrid and its value should be equal to the parent’s <code class="docutils literal notranslate"><span class="pre">grid_name</span></code> +Will be ignored by PROJ (this information can be inferred by the grids extent)</p></li> +<li><p>The <code class="docutils literal notranslate"><span class="pre">number_of_nested_grids</span></code> 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)</p></li> +</ul> +</li> +</ul> +</section> +<section id="example"> +<h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2> +<p><a class="reference external" href="https://github.com/OSGeo/PROJ-data/blob/master/fr_ign/fr_ign_ntf_r93.tif">https://github.com/OSGeo/PROJ-data/blob/master/fr_ign/fr_ign_ntf_r93.tif</a> has +been converted from <a class="reference external" href="https://github.com/OSGeo/proj-datumgrid/blob/master/ntf_r93.gsb">https://github.com/OSGeo/proj-datumgrid/blob/master/ntf_r93.gsb</a> +with <a class="reference external" href="https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/ntv2_to_gtiff.py">https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/ntv2_to_gtiff.py</a></p> +<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ 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) +</pre></div> +</div> +<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ 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) +</pre></div> +</div> +<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ 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 +</pre></div> +</div> +</section> +<section id="multi-grid-storage"> +<h2>Multi-grid storage<a class="headerlink" href="#multi-grid-storage" title="Permalink to this headline">¶</a></h2> +<p>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.</p> +<p>The first IFD should have a full description according to the +<a class="reference internal" href="#gtg-general-description"><span class="std std-ref">General description</span></a>. +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.</p> +<p>Each IFD will have its +<a class="reference external" href="https://www.awaresystems.be/imaging/tiff/tifftags/newsubfiletype.html">NewSubfileType</a> +tag set to 0.</p> +<p>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-resolution grid that contains the point of interest.</p> +<p>For efficient reading from the network, files hosted on the CDN will use +a layout similar to the one described in the <a class="reference external" href="https://gdal.org/drivers/raster/cog.html#low-level">low level paragraph of the Cloud Optimized GeoTIFF +GDAL driver page</a></p> +<p>The layout for a file converted from NTv2 will for example be:</p> +<ul class="simple"> +<li><p>TIFF/BigTIFF header/signature and pointer to first IFD (Image File Directory)</p></li> +<li><p>“ghost area” indicating the generated process</p></li> +<li><p>IFD of the first grid, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays</p></li> +<li><p>…</p></li> +<li><p>IFD of the last grid, followed by TIFF tags values, excluding the GDAL_METADATA tag, TileOffsets and TileByteCounts arrays</p></li> +<li><p>TileOffsets and TileByteCounts arrays for first IFD</p></li> +<li><p>…</p></li> +<li><p>TileOffsets and TileByteCounts arrays for last IFD</p></li> +<li><p>Value of GDAL_METADATA tag for IFDs following the first IFD</p></li> +<li><p>First IFD: Data corresponding to latitude offset of Block_0_0</p></li> +<li><p>First IFD: Data corresponding to longitude offset of Block_0_0</p></li> +<li><p>First IFD: Data corresponding to latitude offset of Block_0_1</p></li> +<li><p>First IFD: Data corresponding to longitude offset of Block_0_1</p></li> +<li><p>…</p></li> +<li><p>First IFD: Data corresponding to latitude offset of Block_n_m</p></li> +<li><p>First IFD: Data corresponding to longitude offset of Block_n_m</p></li> +<li><p>…</p></li> +<li><p>Last IFD: Data corresponding to latitude offset of Block_0_0</p></li> +<li><p>Last IFD: Data corresponding to longitude offset of Block_0_0</p></li> +<li><p>Last IFD: Data corresponding to latitude offset of Block_0_1</p></li> +<li><p>Last IFD: Data corresponding to longitude offset of Block_0_1</p></li> +<li><p>…</p></li> +<li><p>Last IFD: Data corresponding to latitude offset of Block_n_m</p></li> +<li><p>Last IFD: Data corresponding to longitude offset of Block_n_m</p></li> +</ul> +<p>If longitude_offset_accuracy and latitude_offset_accuracy are present, this +will be followed by:</p> +<ul class="simple"> +<li><p>First IFD: Data corresponding to latitude offset accuracy of Block_0_0</p></li> +<li><p>First IFD: Data corresponding to longitude offset accuracy of Block_0_0</p></li> +<li><p>…</p></li> +<li><p>First IFD: Data corresponding to latitude offset accuracy of Block_n_m</p></li> +<li><p>First IFD: Data corresponding to longitude offset accuracy of Block_n_m</p></li> +<li><p>…</p></li> +<li><p>Last IFD: Data corresponding to latitude offset accuracy of Block_0_0</p></li> +<li><p>Last IFD: Data corresponding to longitude offset accuracy of Block_0_0</p></li> +<li><p>…</p></li> +<li><p>Last IFD: Data corresponding to latitude offset accuracy of Block_n_m</p></li> +<li><p>Last IFD: Data corresponding to longitude offset accuracy of Block_n_m</p></li> +</ul> +<div class="admonition note"> +<p class="admonition-title">Note</p> +<p>TIFF has another mechanism to link IFDs, the SubIFD tag. This potentially +enables to define a hierarchy 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 hierarchy, so “flat” organization with the standard IFD chaining +mechanism is adopted.</p> +</div> +</section> +<section id="examples-of-multi-grid-dataset"> +<h2>Examples of multi-grid dataset<a class="headerlink" href="#examples-of-multi-grid-dataset" title="Permalink to this headline">¶</a></h2> +<p><a class="reference external" href="https://github.com/OSGeo/PROJ-data/blob/master/au_icsm/au_icsm_GDA94_GDA2020_conformal.tif">https://github.com/OSGeo/PROJ-data/blob/master/au_icsm/au_icsm_GDA94_GDA2020_conformal.tif</a> has +been converted from <a class="reference external" href="https://github.com/OSGeo/proj-datumgrid/blob/master/oceania/GDA94_GDA2020_conformal.gsb">https://github.com/OSGeo/proj-datumgrid/blob/master/oceania/GDA94_GDA2020_conformal.gsb</a> +with <a class="reference external" href="https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/ntv2_to_gtiff.py">https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/ntv2_to_gtiff.py</a></p> +<p>It contains 5 subgrids. All essential metadata to list the subgrids and their +georeferencing is contained within the first 3 KB of the file.</p> +<p>The file size is 4.8 MB using DEFLATE compression and floating-point predictor. +It results from a lossless conversion from the 83 MB of the original .gsb file.</p> +<p><a class="reference external" href="https://github.com/OSGeo/PROJ-data/blob/master/ca_nrc/ca_nrc_ntv2_0.tif">https://github.com/OSGeo/PROJ-data/blob/master/ca_nrc/ca_nrc_ntv2_0.tif</a> has +been converted from <a class="reference external" href="https://github.com/OSGeo/proj-datumgrid/blob/master/north-america/ntv2_0.gsb">https://github.com/OSGeo/proj-datumgrid/blob/master/north-america/ntv2_0.gsb</a></p> +<p>It contains 114 subgrids. All essential metadata to list the subgrids and their +georeferencing is contained within the first 40 KB of the file.</p> +</section> +</section> + + + </div> + </div> + <footer><div class="rst-footer-buttons" role="navigation" aria-label="Footer"> + <a href="projjson.html" class="btn btn-neutral float-left" title="PROJJSON" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left" aria-hidden="true"></span> Previous</a> + <a href="../community/index.html" class="btn btn-neutral float-right" title="Community" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right" aria-hidden="true"></span></a> + </div> + + <hr/> + + <div role="contentinfo"> + <p>© Copyright 1983-2022. + <span class="lastupdated">Last updated on 22 Mar 2022. + </span></p> + </div> + + Built with <a href="https://www.sphinx-doc.org/">Sphinx</a> using a + <a href="https://github.com/readthedocs/sphinx_rtd_theme">theme</a> + provided by <a href="https://readthedocs.org">Read the Docs</a>. + + +</footer> + </div> + </div> + </section> + </div> + <script> + jQuery(function () { + SphinxRtdTheme.Navigation.enable(true); + }); + </script> + +</body> +</html>
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