[Proj] Bit curious about +ellps

Emmanuel Séguin emmanuel.seguin at ign.fr
Thu Nov 27 10:25:08 EST 2008

Perfectly see your point ;) 



Frank Warmerdam wrote:
> Emmanuel Séguin wrote:
>> Hi,
>> I'm currently using proj registry file (epsg) for building a little
>> library.
>> Therefore i do have a question regarding the +ellps tag.
>> Some projections have a +a and +rf or +b tag (defining the ellipsoid) and
>> some projections are using the +ellps tag. I assume that projections
>> defined
>> with the +a and +rf/+b tags are relying on uncommon ellipsoid
>> definitions.
>> But what I find a bit peculiar considering the way proj is relying on
>> registry/dictionnary files is that ellipsoid definitions used by +ellps
>> are
>> actually stored into pj_ellps.c. 
>> Wouldn't it be more transparent for users to have an ellipsoid
>> registry/configuration file ? Or am i completely wrong here ;) ?
> Emmanuel,
> It would be more transparent to use a configuration file for units,
> ellipsoids,
> datums, and prime meridians.
> However, the practice in the past has been to hard code them into the
> library
> while providing mechanisms to users to parametrically define things when
> they
> want to.   This practice has these benefits:
> 1) avoid dependencies on support data files for "dll only" use of PROJ.4.
> 2) greater transportability of coordinate system definitions between
> different
> sites using PROJ.4 since there is a greater assurance that named
> components
> are the same everywhere.
> I'm not saying the current approach (built in names for components like
> ellipsoids, and dictionaries for "whole coordinate system definitions" is
> the best solution but it is quite workable and I'm not inclined to change
> it.
> I prefer an arguably sub-optimal solution to a bunch of churn.
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
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