[CF-metadata] FGDC projection parameters not well documented

[John Caron]

I know that we decided to use FGDC names for projection parameters, but there doesnt seem to be any further documentation or references in the FGDC-STD-001-1998 doc to explain them.

For example:

Polar stereographic

Map parameters:

    * straight_vertical_longitude_from_pole
    * latitude_of_projection_origin - Either +90. or -90.
    * Either standard_parallel or scale_factor_at_projection_origin
    * false_easting
    * false_northing


The first parameter doesnt exactly match any proj4 names:

Name 	Polar Stereographic

Projection Parameters

Latitude of natural origin 	1 	NatOriginLat 	latitude_of_origin 	Angular 	
Longitude of natural origin 	2 	StraightVertPoleLong 	central_meridian 	Angular 	
Scale factor at natural origin 	5 	ScaleAtNatOrigin 	scale_factor 	Unitless 	Not in original GeoTIFF ... I added for similarity with EPSG. Defaults to 1.0.
False Easting 	6 	FalseEasting 	false_easting 	Linear 	
False Northing 	7 	FalseNorthing 	false_northing 	Linear 	


I would assume "straight_vertical_longitude_from_pole" = "Longitude of natural origin" but the FGDC name seems poorly chosen, whats a "vertical longitude?"

Also, one must choose "standard_parallel" or "scale_factor_at_projection_origin". I would assume that one finds the scale factor that makes the distortion  at the standard_parallel = 1.0, but theres no docs that would explain that. Someone else might make a different assumption.

Im thinking we need to start annotating the CF spec, unless someone knows where FGDC has documented all of this.