[gdal-dev] OGRSpatialReference::IsSame returning TRUE unexpectedly

[Even Rouault]

> I assume my understanding of IsSame is flawed so any clarification would be
> a big help.
>

Matt,

Your understanding of IsSame() is correct. You just hit a bug. I've fixed it per https://
trac.osgeo.org/gdal/ticket/7029. You can see the changeset code as a workaround to add in 
your own code if you need.

IsSame() is a "smart" comparison, ie it tries to abstract away unsignificant differences like the 
PROJCS text value that don't participate really to the SRS and can vary over formats, data 
producers, etc... The issue here is that web-mercator (EPSG:3857) and WGS84-mercator 
(EPSG:3395) are represented the same way at the GEOGCS and projection levels. The only 
difference is that GDAL creates a EXTENSION["PROJ4", ...] node in the web-mercator case to 
really use the spherical definition. And IsSame() before my fix neglected to compare that 
extension node. So you were a bit unlucky since that must be pretty much the only pair of 
EPSG entries in that situation.

My initial tought was to compare the authority codes/nodes when they exist on both sides, 
but I was wondering if there are not cases where you'd have the same SRS registered in 2 
different catalogs. Presumably within the same catalog, a different code should mean a 
different CRS, but I'm not even completely sure you couldn't find oddities. I was thinking to 
EPSG:3857 vs the deprecated EPSG:3785, but this isn't a good example, since they differ on 
the DATUM. So currently IsSame() returns FALSE on that pair, although I guess one might 
consider them as equivalent (the deprecated version is just a buggy expression on the 
current EPSG:3857). Or not...
You also have the situation where really different CRS (IsSame() == FALSE) ends up being 
translated the same way as proj.4 strings...

So it is actually not obvious to define the semantics of IsSame(). It should at least return 
FALSE for CRS that are clearly different (coordinate transformation results in differences).

Even

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