[PROJ] Understanding dropped parameters for stere and latlong

[Even Rouault]

On lundi 13 avril 2020 00:10:14 CEST Martin Desruisseaux wrote:
> Why not using PRIMEM, for example like below?
> 
>     GEODCRS["WGS 84 (anti-meridian)",
>        DATUM["World Geodetic System 1984",
>          ELLIPSOID["WGS 84", 6378137, 298.257223563,
> LENGTHUNIT["metre",1.0]]], PRIMEM["Anti-meridian", 180,
> ANGLEUNIT["degree",0.01745329252]], CS[ellipsoidal,2],
>          AXIS["latitude",north,ORDER[1]],
>          AXIS["longitude",east,ORDER[2]],
>          ANGLEUNIT["degree",0.01745329252]]
> 
> It should create a CRS with longitude in -180 to 180° range, but where
> 0° is at 180° from Greenwich (I not sure if this is what you wanted).

That's a good point. If you use "+proj=longlat +pm=45", this will actually set the PRIMEM[]. 
"+proj=longlat +pm=" and "+proj=longlat +lon_0=" seem to behave quite simimarly from a 
quick test. The +over trick however is a PROJ specific thing. If you need to serialize this, 
there's no alternative than storing the PROJ string itself.

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