[CF-metadata] proposed UTM projection

[John Caron]

I would like to propose adding another "Grid mapping" to appendix F. 


The /UTM_Zone_Number/ parameter name is taken from FGDC, the optional 
ellipsoidal parameters follow Snyder.


        Universal Transverse Mercator

grid_mapping_name = universal_transverse_mercator

/Map parameters:/

    * UTM_Zone_Number - in range [1,60] (north) or [-1, -60] (south)
    * semimajor_axis - radius of the equatorial axis of the ellipsoid in
      meters
    * inverse_flattening - inverse flattening ratio (eg 297)

/Map coordinates:/ The x (abscissa) and y (ordinate) rectangular 
coordinates are identified by the standard_name attribute values 
projection_x_coordinate and projection_y_coordinate respectively.

/Notes:/

The Universal Transverse Mercator projection is a special case of the 
Transverse Mercator projection using an ellipsoidal earth. The scale 
factor (.9996), projection origin latitude (0), and false easting 
parameters (50000 m) are all fixed. The central meridian and 
false_northing parameters are specified by the UTM zone.

The ellipsoid parameters /semimajor_axis/ and /inverse_flattening /are 
optional. If not present, the standard ellipsoid for the given UTM zone 
should be used.

Formulas for the mapping and its inverse along with notes on using the 
PROJ.4 software package for doing the calcuations may be found at 
http://www.remotesensing.org/geotiff/proj_list/transverse_mercator.html