[postgis-users] Differences in st_transform between Postgis 2.3 and Postgis 3.0

[rmrodriguez at carto.com]

What proj releases are you using? `Select postgis_full_version()` will show it.

It is likely that your 3.0 is using PROJ6+ which has more precise
transformations.

On Thu, Apr 23, 2020 at 2:55 PM David McKelvie <dmck at interactive.co.uk> wrote:
>
>
> Hello;
>
> Ive noticed a difference between the results returned by st_transform
> between Postgis 2.3 and Postgis 3.0 and wonder if anyone on this list
> might be able to explain why that is so.
>
> I have a home rolled entry in the spatial_ref_sys table for a SRID called
> 927700.  This is the UK Ordnance Survey coordinate system using OSTN02.
>
> The spatial_ref_sys.proj4text for this is
> (newline added for readability)
>
> '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000
> +y_0=-100000 +ellps=airy +units=m +no_defs +nadgrids=OSTN02_NTv2.gsb'
>
> So it is using a gsb data file.
>
> The following query shows a difference
> converting a point in 4326 'WGS 84' to 927700 "OSGB 1936 / British National Grid"
>
> SELECT st_asewkt(st_transform(geomfromEWKT(
>    'SRID=4326;POINT(-0.077731 54.116851 86.778)'
> ), 927700)) AS a;
>
> postgres / postgis     result
> 9.6  / 2.3  SRID=927700;POINT(525745.680454343 470703.162768111 86.778)
> 9.6  / 3.0  SRID=927700;POINT(525744.779056573 470705.423178893 86.778)
>
> A difference of -0.901 2.26 which is quite significant given that OSGB
> is measured in meters.
>
> Any ideas what is going on?
>
> David McKelvie
>
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-- 
Raúl Marín Rodríguez
carto.com