[gdal-dev] GDAL (.NET) coordinate transformation via BETA2007

[Carsten Lockenkötter]

Hello,

because SQLServer can't transform geometries, i have developed a .NET library (C#) that transforms the geometries based on GDAL (NuGet Package GDAL.Native v2.4.2).
In principle, this also works.

If i transform from 31466 to 3857, i have an offset of about 1.5m because gdal transformed via Helmert, not via Beta2007.
If i transform the same geometrie via proj4 string to use my Beta2007.gsb file, i get a completly wrong geometry.

sample code to transform via epsg
Example:

                var inputGeom = "POINT (2529815.8 5642182.4)";

                GdalConfiguration.ConfigureOgr();

                var source = new SpatialReference("");
                var destination = new SpatialReference("");

                source.ImportFromEPSG(31466);
                destination.ImportFromEPSG(3857);

                CoordinateTransformation transform = new CoordinateTransformation(source, destination);
                Geometry ogrGeom = Geometry.CreateFromWkt(inputGeom);
                ogrGeom.Transform(transform);

                ogrGeom.ExportToWkt(out string outGeom);

                return outGeom;

value of outGeom:
                POINT (776704.423009968 6626706.00572367)

if i transform the same geometrie via FME with the CsmapReprojector, i get this (correct) coordinates:
(776702.2109972419,6626706.521023605)

FME used the gridfile Beta2007, GDAL not.
So i tried to transform my geometrie via proj4 string, where I have the possibility to specify the gridfile.
My gridfile Beta2007.gsb is located here: {ProjectRoot}\bin\Debug\gdal\share\Beta2007.gsb

Example:

                var inputGeom = "POINT (2529815.8 5642182.4)";

                GdalConfiguration.ConfigureOgr();

                var source = new SpatialReference("");
                var destination = new SpatialReference("");

                source.ImportFromProj4("+proj=tmerc +lat_0=0 +lon_0=6 +k=1 +x_0=2500000 +y_0=0 +ellps=bessel +nadgrids=@BETA2007.gsb<mailto:+nadgrids=@BETA2007.gsb>");
                destination.ImportFromEPSG(3857);

                CoordinateTransformation transform = new CoordinateTransformation(source, destination);
                Geometry ogrGeom = Geometry.CreateFromWkt(inputGeom);
                ogrGeom.Transform(transform);

                ogrGeom.ExportToWkt(out string outGeom);

                return outGeom;

value of outGeom:
                POINT (776787.364952423 6626930.71048604)


What i need:
Beta2007 Transformation via EPSG code, but i don't know, where i have to change it.
(optional) if i can't change the transformation via EPSG code via Gridfile, i need to know the correct proj4 string for my transformation.

Best regards,
Carsten

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