[Proj] Point Scale factor service

Noel Zinn ndzinn at comcast.net
Wed Apr 21 13:47:51 EST 2010

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
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

    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


Best Regards,
Brent Fraser

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