[postgis-users] PostGIS & PROJ 6 -- Y axis shift with 900913

[Shira Bezalel]

Thank you, Jeff. This is helpful.

Shira

On Thu, Jul 23, 2020 at 5:04 AM Jeff McKenna <jmckenna at gatewaygeomatics.com>
wrote:

> For the history of this: I first reported this shift back in 2018
> (http://lists.maptools.org/pipermail/proj/2018-February/008082.html).
> It was fixed for 3857 in the PROJ 5.2.0 release.
>
> My advice to you is to change to using EPSG:3857 as most software now no
> longer supports the non-standard "900913".
>
> -jeff
>
>
>
> --
> Jeff McKenna
> MapServer Consulting and Training Services
> co-founder of FOSS4G
> http://gatewaygeo.com/
>
>
>
>
>
> On 2020-07-20 3:25 p.m., Shira Bezalel wrote:
> > We recently recompiled PostGIS in one of our Postgres 12.3 environments
> > with PROJ 6 (6.3.2) from PROJ 4 (4.9.3).
> >
> > postgis_full_version() output:
> > POSTGIS="3.0.1 ec2a9aa" [EXTENSION] PGSQL="120" GEOS="3.7.1-CAPI-1.11.1
> > 27a5e771" PROJ="6.3.2" LIBXML="2.9.4" LIBJSON="0.12.1"
> >
> > We're now noticing that using ST_Transform(geometry, srid)**to transform
> > some points in California from SRID 4326 to 900913 is causing a shift
> > along the Y axis of about 20-30 meters. The difference is evidenced by a
> > comparison of the old and new ST_Y() values.
> >
> > If I force the PROJ 4 definition to be used, the transformation works
> > fine with no shift. (To force the PROJ 4 definition, I used the method
> > described in
> > http://blog.cleverelephant.ca/2019/02/proj4-postgis.html and set
> > authname, authsrid and srtext to null so that the proj4text column is
> used.)
> >
> > Is this shift expected with PROJ 6? I realize 900913 is deprecated and
> > we plan to migrate to 3857 as a longer term solution, but looking for
> > a shorter term fix if possible since much data and code is impacted by
> > this. Note we are seeing no shift issues with SRID 3857.
> >
> > Let me know if I can provide more information.
> >
> > Thanks much,
> > Shira
> >
>
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