[Proj] Bit curious about +ellps

Frank Warmerdam warmerdam at pobox.com
Thu Nov 27 09:52:49 EST 2008

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 ;) ?


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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