[Proj] Point Scale factor service

[Noel Zinn]

Someone more knowledgeable than I in proj4 will have to comment on your
syntax.  My point is that if WGS84 ellipsoidal coordinates are used with
spherical Mercator formulas as Google are doing (a=b=6378137), then the
result is a non-conformal projection (point scale factor not the same in all
directions).  If you plan to use scale factors, you need to compensate for
azimuth.  The scale factors (as functions of azimuth) have been quantified
in EPSG Guidance Note 7-2 (www.epsg.org).  -Noel

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Brent Fraser
Sent: Thursday, April 22, 2010 8:51 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Point Scale factor service

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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