[Geotiff] geos projection sweep parameter

[David Hoese]

Hi,

I work with data from the GOES-16 satellite ABI instrument. The official 
ABI Level 1B product consists of data mapped to the GEOS projection with 
a sweep angle axis parameter set to 'x' instead of the default 'y'. More 
information on this projection can be found here: 
http://proj4.org/projections/geos.html

As of GDAL v2.1+ the "sweep" parameter is handled internally and I can't 
remember what version of PROJ.4 added support for this. As far as I can 
tell there is no standard way to store the "sweep" parameter in a 
GeoTIFF file. I'm not sure if this is due to a limitation of the WKT 
specification or if it is something that only requires changes in the 
GeoTIFF library. Does anyone know what needs to happen to get "sweep" 
added to a GeoTIFF? If this requires changes to libgeotiff I am willing 
to add them or provide patches to add the functionality. Thanks for any 
information you can provide.

Dave