[Geotiff] undefined/unsupported Geotiff keys

[Chris McGlone]

While working with some foreign imagery, we realized that a number
of national coordinate systems enumerated in the EPSG database are
not supported by geotiff codes.  Do most people just write the 
national systems in terms of the underlying projection parameters, 
or do people just "roll their own" geotiff keys based on the EPSG 
keys?  

Thanks

Chris