[Proj] Point Scale factor service

[Brent Fraser]

Fair enough.  So how about:

int pj_pointscale( ProjPJ prj, double lon, double lat, double azimuth, double 
*pointscale );

Brent


Noel Zinn wrote:
> 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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