I am working on a project to convert between one coordinate system, provided by an EPSG code, and EPSG:4326 or EPSG:4979 (WGS84 CS to include heights above ellipsoid).

Testing the following coordinates in British National Grid, EPSG:27700 using gdal transform and epsg.io generates the same results:

x = 330231.2515625854;
y = 376174.72374993574;
z = -7.352000006675735;

gdaltransform returns the correct results, apart from the elevation which is something I'll be looking into at a later date - the lat/long are the important values at the moment.

gdaltransform -s_srs epsg:27700 -t_srs epsg:4979
Enter X Y [Z [T]] values separated by space, and press Return.
330231.2515625854 376174.72374993574 -7.352000006675735
-3.04778889624274 53.2779570544652 -7.35200000667573

epsg.io also returns the same results as above https://epsg.io/transform#s_srs=27700&t_srs=4979&x=330231.2515626&y=376174.7237499

gdaltransform and epsg.io also return the same results for epsg:4236.

I have implemented GDAL using C#, .NET 7, using the MaxRev.Gdal.Core package.

When using this the results are incorrect:


    var x = 330231.25156258536;
    var y = 376174.72374993586;
    var z = -7.352000006675735;

    var epsg27700 = 27700;
    var epsg4979 = 4979; // same result for 4326

    var inputCS= new SpatialReference(null);

    var wgs84 = new SpatialReference(null);

    var ct = new CoordinateTransformation(inputCS, wgs84);
    var transformed = new double[3];
    ct.TransformPoint(transformed, x, y, z);
    Console.WriteLine($"long: {transformed[0]}, lat: {transformed[1]}, z: {transformed[2]}");

Which returns:

long: -3.047789419352417, lat: 53.277941280668884, z: -7.352000006675735

The correct values are:

long: -3.04778889624274, lat: 53.2779570544652, z: -7.35200000667573

This is just over a meter difference.

Does anyone know what I might be doing wrong here?

  • With GDAL 3.7.0dev-5ce7ba068f, released 2023/01/17 and Proj Rel. 8.2.0, November 1st, 2021 the result is -3.04778941935242 53.2779412806689 -7.35200000667573. So pretty close to the "wrong" result. I believe that your programs are having different Proj versions and transformation parameters. I would trust the newest version but if you have independent reference data that the result is wrong. In that case I would file a Proj issue into bug tracker.
    – user30184
    Jan 25, 2023 at 15:23
  • Thanks that's really helpful, are those results from using gdaltransform? I'm unsure if I need to add some sort of shift as EPSG:4979 needs handling in the vertical datum
    – TJames
    Jan 25, 2023 at 15:51
  • 1
    I noticed that my GDAL 3.7.0dev was missing a grid shift file "uk_os_OSTN15_NTv2_OSGBtoETRS.tif". Maybe you have the same issue.
    – user30184
    Jan 25, 2023 at 16:18
  • Gotcha, that could be it, I can't seem to find where Gdal Net installs the grid shift files but this points me in the right direction!
    – TJames
    Jan 25, 2023 at 16:37
  • 1
    @user30184, you were right - Gdal Net didn't reference this file, I added this and it works as expected. Thanks for your time!
    – TJames
    Jan 25, 2023 at 16:59

1 Answer 1


As mentioned in the comments, the issue was due to Gdal Net not including the grid shift files. Adding a reference to these using Proj.Configure("path/to/grid_shift/dir"); fixed the issue.

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