[PROJ] Understanding dropped parameters for stere and latlong

[Even Rouault]

David,

> I noticed that the `lat_ts` parameter gets dropped for any case where
> `lat_0` isn't -90 or 90. Is this expected?

Yes. Given what is supported in the mathematical part of PROJ, either you have Polar 
Stereographic (lat_0=+/-90, and lat_ts),  or Oblique Stereographic (lat_0 != +/-90, and k_0)
Previous PROJ versions might have returned the string you provided, but that didn't mean all 
the parameters you specified were taken into account.

> Is it no longer valid (or maybe it never was) to specify a "lon_0" for a
> lat/lon projection:
> 
> crs = CRS("+proj=latlong +lon_0=45")
> 
> crs.to_wkt()
> # Outputs:
> 'GEOGCRS["unknown",DATUM["World Geodetic System 1984",ELLIPSOID["WGS
> 84",6378137,298.257223563,LENGTHUNIT["metre",1]],ID["EPSG",6326]],PRIMEM["Gr
> eenwich",0,ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8901]],CS[ellips
> oidal,2],AXIS["longitude",east,ORDER[1],ANGLEUNIT["degree",0.017453292519943
> 3,ID["EPSG",9122]]],AXIS["latitude",north,ORDER[2],ANGLEUNIT["degree",0.0174
> 532925199433,ID["EPSG",9122]]]]'

There's no way in standard WKT to express a longitude of origin != 0 for a GeographicCRS. If 
you export to WKT1_GDAL, a EXTENSION[] node captures this however. Or if you re-export 
back to PROJ string the CRS object built from "+proj=latlong +lon_0=45", you'll get back the 
original string.

Even


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