[Geotiff] Polar Stereographic, GeoTIFF & ProjScaleAtCenterGeoKey

[Frank Warmerdam]

Moore, Dave M (UK Rochester) wrote:
> Hi All,
> I'm having 'fun' with GeoTIFF and Polar Stereographic projections. My 
> problem appears to be associated with the 'scale factor' attribute of 
> this projection. It appears that using the ProjScaleAtCenterGeoKey key 
> makes no difference to the output file irrespective of the value I give 
> it (at least when viewed in Global Mapper and other GeoTIFF viewers).
>  
> This issue has come about because my GeoTIFF output files for the polar 
> regions appear to have a small error in the scaling (of the order of 
> about 0.992) which just happens to be the average scaling factor for a 
> polar stereographic projection for over 80 degrees 
> (http://en.wikipedia.org/wiki/Universal_Polar_Stereographic_coordinate_system). 
> I thought I could apply this scaling through the ProjScaleAtCenterGeoKey 
> key on the geotiff library but irrespective of the value, it appears to 
> make no difference when I view the file. I have applied the scale factor 
> manually to the tiepoint and pixscale values which works but it would be 
> better to apply it through the key IMO.
>  
> Any ideas why the output file does not respond to the 
> ProjScaleAtCenterGeoKey?. An example would be nice. Thanks in advance 
> for any comments or suggestions.

Dave,

I have come to the conclusion that the proj.4 translation code in
libgeotiff and GDAL are setting lat_ts (latitude of true scale) in
the proj.4 string and this is causing proj.4 to ignore the scale factor.
Would you be willing to file a ticket on this issue - likely with libgeotiff
would be best?

I have determined that libgeotiff's normalize code looks first for
ProjScaleAtNatOriginGeoKey and if that is missing falls back to
ProjScaleAtCenterGeoKey.  So your formulation should be reasonable
though the promoted key for the scale is ProjScaleAtNatOriginGeoKey.

Best regards,
-- 
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