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

[Shira Bezalel]

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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