Merge RFC4 (#1865)

This commit is the result of the squashing of rfc4_dev branch in a single
commit. It implements mostly RFC 4 related work.

* Grid handling:
  - remove obsolete and presumably unfinished implementation of grid catalog functionality
  - all grid functionality is in grids.cpp/.hpp
  - vertical and horizontal grid shift: rework to no longer load whole grid into memory
  - remove hgrids and vgrids member from PJ structure, and store them in hgridshift/vgridshift/deformation structures
  - build systems: add optional libtiff dependency. Must be explicitly disabled if not desired
  - add support for horizontal and vertical grids in GeoTIFF, if libtiff is available
  - add GenericShiftGridSet and GenericShiftGrid classes, relying on TIFF grids, that can be used for generic purpose grid-based adjustment
  - add a +proj=xyzgridshift method to perform geocentric translation by grid. Used for French NTF to RGF93 transformation using gr3df97a.tif grid
  - deformation: add support for +grids= for GeoTIFF grids
  - horizontal grid shift: fix failures on points slightly outside a subgrid (fixes #209)

* File management:
  - add a filemanager.cpp/.hpp to deal with file related work
  - test for legacy proj_api.h fileapi
  - proj.h: add proj_context_set_fileapi() and proj_context_set_sqlite3_vfs_name() (fixes #866)
  - add capability to read resource files from the user writable directory

* Network access:
  - build systems: add optional curl dependency
  - add a curl-based default implementation for network related functionality
  - proj.h: add C API to control network functionality, and optionaly provide network callbacks
  - add data/proj.ini with default settings
  - add a SQLite3 local cache of downloaded chunks
  - add proj_is_download_needed() and proj_download_file()

* Use Win32 Unicode APIs and expect all strings to be UTF-8 (fixes #1765)
  For backward compatibility, if PROJ_LIB content is found to be not UTF-8 or
  pointing to a non existing directory, then an attempt at interpretating it
  in the ANSI page encoding is done.
  proj_context_set_search_paths() now assumes strings to be in UTF-8, and
  functions returning paths will also return values in UTF-8.

1 files changed, 37 insertions, 2 deletions

diff --git a/docs/source/operations/transformations/deformation.rst b/docs/source/operations/transformations/deformation.rst
index 3a9d025c..02924a25 100644
--- a/docs/source/operations/transformations/deformation.rst
+++ b/docs/source/operations/transformations/deformation.rst
@@ -35,6 +35,9 @@ GTX format. Both grids are expected to contain grid-values in units of
 mm/year. GDAL both reads and writes both file formats. Using GDAL for
 construction of new grids is recommended.
 
+Starting with PROJ 7.0, use of a GeoTIFF format is recommended to store both
+the horizontal and vertical velocities.
+
 Example
 -------------------------------------------------------------------------------
 
@@ -89,13 +92,15 @@ Parameters
 
 .. option:: +xy_grids=<list>
 
-    Comma-separated list of grids to load. If a grid is prefixed by an `@` the
+    Comma-separated list of grids to load. If a grid is prefixed by an ``@`` the
     grid is considered optional and PROJ will the not complain if the grid is
     not available.
 
-    Grids for the horizontla component of a deformation model is expected to be
+    Grids for the horizontal component of a deformation model is expected to be
     in CTable2 format.
 
+    .. note:: :option:`+xy_grids` is mutually exclusive with :option:`+grids`
+
 .. option:: +z_grids=<list>
 
     Comma-separated list of grids to load. If a grid is prefixed by an `@` the
@@ -105,6 +110,36 @@ Parameters
     Grids for the vertical component of a deformation model is expected to be
     in either GTX format.
 
+    .. note:: :option:`+z_grids` is mutually exclusive with :option:`+grids`
+
+.. option:: +grids=<list>
+
+    .. versionadded:: 7.0.0
+
+    Comma-separated list of grids to load. If a grid is prefixed by an `@` the
+    grid is considered optional and PROJ will the not complain if the grid is
+    not available.
+
+    Grids should be in GeoTIFF format with the first 3 components being
+    respectively the easting, northing and up velocities in mm/year.
+    Setting the Description and Unit Type GDAL band metadata items is strongly
+    recommended, so that gdalinfo reports:
+
+    ::
+
+        Band 1 Block=... Type=Float32, ColorInterp=Gray
+            Description = east_velocity
+            Unit Type: mm/year
+        Band 2 Block=... Type=Float32, ColorInterp=Undefined
+            Description = north_velocity
+            Unit Type: mm/year
+        Band 3 Block=... Type=Float32, ColorInterp=Undefined
+            Description = up_velocity
+            Unit Type: mm/year
+
+    .. note:: :option:`+grids` is mutually exclusive with :option:`+xy_grids`
+              and :option:`+z_grids`
+
 .. option:: +t_epoch=<value>
 
     Central epoch of transformation given in decimalyears. Will be used in