SharpMap Coordinate transformation Problem

Question

I want to transform my layers from SRID 31463 (Gauß Krüger) to Webmercator. Since SharpMap does only know webmercator and WGS84 i tried to create the Coordinate System 31463 by WKT and applied it on the layer. But i have a small offset of a few meters.

        ProjNet.CoordinateSystems.CoordinateSystemFactory csFact = new ProjNet.CoordinateSystems.CoordinateSystemFactory();

        string wkt_gk = "PROJCS[\"DHDN / 3-degree Gauss zone 3 (deprecated)\",GEOGCS[\"DHDN\",DATUM[\"Deutsches_Hauptdreiecksnetz\",SPHEROID[\"Bessel 1841\",6377397.155,299.1528128,AUTHORITY[\"EPSG\",\"7004\"]],AUTHORITY[\"EPSG\",\"6314\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.01745329251994328,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4314\"]],UNIT[\"metre\",1,AUTHORITY[\"EPSG\",\"9001\"]],PROJECTION[\"Transverse_Mercator\"],PARAMETER[\"latitude_of_origin\",0],PARAMETER[\"central_meridian\",9],PARAMETER[\"scale_factor\",1],PARAMETER[\"false_easting\",3500000],PARAMETER[\"false_northing\",0],AUTHORITY[\"EPSG\",\"31463\"],AXIS[\"X\",EAST],AXIS[\"Y\",NORTH]]";

        GeoAPI.CoordinateSystems.ICoordinateSystem gauss_krueger_3 = csFact.CreateFromWkt(wkt_gk);

        GeoAPI.CoordinateSystems.ICoordinateSystem webmercator = ProjNet.CoordinateSystems.ProjectedCoordinateSystem.WebMercator;

        ProjNet.CoordinateSystems.Transformations.CoordinateTransformationFactory ctFact = new ProjNet.CoordinateSystems.Transformations.CoordinateTransformationFactory();

        layer.CoordinateTransformation = ctFact.CreateFromCoordinateSystems(gauss_krueger_3, webmercator);
        layer.ReverseCoordinateTransformation = ctFact.CreateFromCoordinateSystems(webmercator, gauss_krueger_3);

Answer 1

Your transformation omits the offset between the Bessel and the WGS84 Ellipsoid.

The full WKT would be:

PROJCS["DHDN / 3-degree Gauss-Kruger zone 3",
    GEOGCS["DHDN",
        DATUM["Deutsches_Hauptdreiecksnetz",
            SPHEROID["Bessel 1841",6377397.155,299.1528128,
                AUTHORITY["EPSG","7004"]],
            TOWGS84[598.1,73.7,418.2,0.202,0.045,-2.455,6.7],
            AUTHORITY["EPSG","6314"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4314"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",9],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",3500000],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","31467"]]

I have no experience with sharpmap. If it does not know about TOWGS84, you have to use QGIS (or pure GDAL) to reproject the data before feeding to sharpmap.

Answer 2

you can use this code to transform. it works for me.

 layCountries.CoordinateTransformation = fact.CreateFromCoordinateSystems(
           shpLaye.CoordinateSystem ?? ProjNet.CoordinateSystems.GeographicCoordinateSystem.WGS84,
           ProjNet.CoordinateSystems.ProjectedCoordinateSystem.WebMercator);
        layCountries.ReverseCoordinateTransformation = fact.CreateFromCoordinateSystems(
            ProjNet.CoordinateSystems.ProjectedCoordinateSystem.WebMercator,
            shpLaye.CoordinateSystem ?? ProjNet.CoordinateSystems.GeographicCoordinateSystem.WGS84);