[Proj] Point Scale factor service

[Brent Fraser]

Lately I've run into issues regarding scale.  The need to:
   - measure a distance on a web map application
   - show layers based on map scale

In the past, I've carefully selected a good projection (and parameters) to 
minimize these problems.  These days, the standard is Spherical Mercator (thank 
you Google), and most applications ignore the map projection's point scale 
factor at the location/view of interest, or do their own hack to estimate it.

    I thought a more robust solution would be to have a function in proj to 
return the point scale factor at a location, something like:

   int pj_pointscale( ProjPJ prj, double lon, double y, double *pointscale );
       (return FALSE if not available for that projection?)

The calling applications could then do something useful with that information.

Thoughts?

Best Regards,
Brent Fraser