[mapserver-users] MapServer 7.4.3 - Poor projection from British National Grid (EPSG 27700) to WebMercator (EPSG 3857)

[mapserver at brainspeedjelly.co.uk]

Reposted from 
https://gis.stackexchange.com/questions/410874/mapserver-7-4-3-poor-projection-from-british-national-grid-epsg-27700-to-web

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This server is long out of date and we have a new one running 20.02, 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 
<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 ?

Thanks

Ben

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