Issues with 3857 to 25832

[mpallesen]

Hi

First off, good job with this port!

I'm having issues transforming my geometry from 25832 to 3857.

I have a point:
x = 702575 y = 6153153 in epsg:25832

When i try to convert to epsg:3857 with this wkt string:

PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]],
PROJECTION["Mercator_1SP"],
PARAMETER["central_meridian",0],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["metre",1,AUTHORITY["EPSG","9001"]],
AXIS["X",EAST],
AXIS["Y",NORTH],
EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"],
AUTHORITY["EPSG","3857"]]

I get x = 1358761 y = 7420836
which is about 36 km too much to the south..
I have tried different wkt strings found on the net, but i can't find one that works.

If i try epsg.io's transformer i get the expected results:
https://epsg.io/transform#s_srs=25832&t_srs=3857&x=702575.0000000&y=6153153.0000000

Also, my old native dll implementation of proj4.dll works as intenden.

Can you fix this issue or point me in the right direction to find a solution?

Thanks!
Michael Pallesen

[FObermaier]

The issue is that the projection "Mercator_1SP" needs additional help to perform the correct conversion.
Out of the box, proj4 wouldn't compute the correct result either, that is why the Extension token is there.

Add [PARAMETER["semi_minor",6378137]] anywhere in the parameter block and you should be all set.
Or use ProjectedCoordinateSystem.WebMercator in the first place.