[PROJ] Pseudo-mercator

[Andrew Patterson]

Hello!

We recently made the jump from GDAL 2.4 to 3.3.0 and while we had some
major headaches initially, in the end they boiled down to a single
(unnecessary) function call that resulted in an issue filed against GDAL.
The fix, by the way, was supplied within three or four hours, which is an
amazing turnaround time (and greatly appreciated by me)!

Since then, we've released our app (Avenza Maps) and immediately run
headlong into recurring issues with pseudo-mercator that I'm not sure how
to solve. In our previous version (using GDAL 2.4 & PROJ 6.1) we had
various pseudo-mercator maps that worked fine. When we upgrade to our
version running GDAL 3.3.0 & PROJ 8.1.1, many of these maps find their
referencing shifted. In one case, it was shifted toward the equator ~26km.
In another, the referencing is off by ~75km.

The former has 'deprecated' in its name -- though I know that's mostly a
cosmetic property -- but it made me wonder if there's some issue with these
deprecated versions of pseudo-mercator. We've seen other issues with these
'off brand' pseudo-mercator projections as well, but it's not clear if it's
that they're not as well supported any more or if there's something in
those WKTs that is problematic.

The latter one seems to be influenced by what appears to be a dropped
parameter: the WKT specifies a latitude_of_origin but when I asked the
resultant ORGSpatialReference to read the WKT back to me it's not included.
If I eliminate that parameter from the WKT in our previous app (using
2.4/6.1) I get a similar-ish result to the one in the current version
(using 3.3/8.1.1). That seems like it might be a bug about not considering
latitude_of_origin for pseudo-mercator when it should, but even if that's
true, the latitude is off by ~0.15 degrees so there's an additional
component.

To further confuse matters, we also have other pseudo-mercator maps that
work as well as before. I'm well versed enough in GIS from years of
exposure that I feel like I'd be able to spot obvious problems, but I'm not
seeing anything that jumps out at me. I'm hoping someone on this list has
experience and knowledge of how PROJ handles pseudo-mercator and can lend
me some guidance on what to do. I've figuratively got management breathing
down my neck on this and I'm running out of leads to pursue.

Any help would be greatly appreciated!

..............................
Andrew Patterson
Lead Software Architect
Avenza Systems Inc.

email: andrew at avenza.com
phone: 416.487.5116
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