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We have PostGIS data in BNG and are displaying it over Google satellite imagery.

This worked fine previously - using a Ubuntu server running 18.04, with MapServer 7.0.7, GDAL 2.2.3, proj 4.9.3.

This server is long out of date and we have a new one running 20.02, with MapServer 7.4.3, GDAL 3.0.4 and proj 6.3.1. However this is projecting the BNG layers incorrectly (~200m ESE).

This appears to be similar to the issue reported at https://github.com/OSGeo/gdal/issues/3695 although that claims the problem only arose after proj 7.2.1

I have installed the updated transform OSTN15_NTv2_OSGBtoETRS.gsb but this had no effect.

Is there a way to update the transforms used by MapServer or get MapServer to pass the transformation to PostGIS ?

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  • Project-on-the-fly is suboptimal for mapping services, even more so when it's a deproject-transform-reproject on the fly. Your accuracy issue is likely a failure to transform between datums correctly, so once you get the transform right, it would probably be best to make a "cooked" version of the geometries and use that in your app.
    – Vince
    Commented Sep 3, 2021 at 13:21
  • It should be possible to create a view in PostGIS and make MapServer to read that. You may need to help a bit with using SRID= in the DATA mapserver.org/input/vector/postgis.html.
    – user30184
    Commented Sep 13, 2021 at 23:42

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