[Proj] Point Scale factor service

[Noel Zinn]

FYI, Google's "Web Mercator" is not truly a Mercator (nor is it spherical)
because it is not conformal.  Point scale factor varies as a function of
azimuth.  -Noel Zinn

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Brent Fraser
Sent: Wednesday, April 21, 2010 1:39 PM
To: PROJ.4 and general Projections Discussions
Subject: [Proj] Point Scale factor service

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


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