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I am converting coordinate values from EPSG 4326 (lat long) to EPSG 27700 (British National Grid). I have used QGIS and am then verifying the results using GDAL, but I am find that for higher latitudes the results do not match:

Using QGIS

Long Lat BNG_X BNG_Y
-3 61 346015 1235698
-4 61 291937 1236937

Using GDAL (gdaltransform -s_srs EPSG:4326 -t_srs EPSG:27700)

Long Lat BNG_X BNG_Y
-3 61 345920 1235625
-4 61 291850 1236864

What is causing this divergence of results at more northern latitudes?

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    QGIS is also using the Proj library for coordinate operations. Compare the versions you have. From QGIS you find the information from Help-About.
    – user30184
    Dec 14, 2020 at 9:22

1 Answer 1

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Try this command from your GDAL shell:

projinfo  -s epsg:4326 -t epsg:27700

You will probably see at the end of the output

Grid OSTN15_NTv2_OSGBtoETRS.gsb needed but not found on the system. Can be obtained from the proj-datumgrid-europe package at https://download.osgeo.org/proj/proj-datumgrid-europe-latest.zip

Download the grid and place it into the proj data directory, it may be share/proj. After that you should get the same results than from QGIS, that obviously has the datum grid file.

gdaltransform -s_srs epsg:4326 -t_srs epsg:27700
Enter X Y [Z [T]] values separated by space, and press Return.
-3 61
346015.472779439 1235697.68734433 0

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