[PROJ] Gridshift files

[Erixen Cruz]

Mladen,
See this thread where Even suggests how to use arbitrary grid shift in WKT CRS definition.
https://lists.osgeo.org/pipermail/proj/2019-December/009143.html

You would need to use ABRIDGEDTRANSFORMATION. https://docs.ogc.org/is/18-010r7/18-010r7.html#151

Sincerely, Erixen
________________________________
From: PROJ <proj-bounces at lists.osgeo.org> on behalf of Mladen Mijatovic via PROJ <proj at lists.osgeo.org>
Sent: Monday, September 25, 2023 5:56 AM
To: proj at lists.osgeo.org <proj at lists.osgeo.org>
Subject: [PROJ] Gridshift files

Dear,

We are trying to substitute old Proj 5.1 in our project and use new Proj
9.2.1.

When crs is defind with proj4string or wkt, we do not have issue with
transforming coordinates from/to EPSG:4326, except when we are using
grid files e.g.

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+datum=OSGB36 +units=m +ellps=airy +no_defs
+nadgrids=OSTN15_NTv2_OSGBtoETRS.gsb  +type=crs

PROJCS["ED50 / 3-degree Gauss-Kruger zone 10",
     GEOGCS["ED50",
         DATUM["European_Datum_1950",
             SPHEROID["International 1924",6378388,297],
             EXTENSION["PROJ4_GRIDS","SPED2ETV2.gsb"]],
         PRIMEM["Greenwich",0,
             AUTHORITY["EPSG","8901"]],
         UNIT["degree",0.0174532925199433,
             AUTHORITY["EPSG","9122"]],
         AUTHORITY["EPSG","4230"]],
     PROJECTION["Transverse_Mercator"],
     PARAMETER["latitude_of_origin",0],
     PARAMETER["central_meridian",30],
     PARAMETER["scale_factor",1],
     PARAMETER["false_easting",10500000],
     PARAMETER["false_northing",0],
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]],
     AUTHORITY["EPSG","2207"]]

Conversion fails with (HUGE_VAL, HUGE_VAL) coordinates.

If we allow user to define crs by proj4string or wkt and use gridshift
files:

- How proj4string/wkt need to looks like?

- What c++ functions we need to call to get these coordinates converted
without any errors?

Best regards,
Mladen.
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