[Proj] Point Scale factor service

[Brent Fraser]

Yikes!  I've been using

+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 
+units=m +nadgrids=@null +no_defs

as the definition.  Should I be using something else?

Brent

Noel Zinn wrote:
> 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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