[Proj] "Double Ellipsoid" error, reproduction

Glynn Clements glynn at gclements.plus.com
Wed Dec 10 02:55:43 EST 2008

Richard Greenwood wrote:

> We have moved beyond paper maps and can no longer segregate
> cartography from geodesy - projections from datums. The addition of
> datum support was necessary and well intentioned. The implementation
> was probably imperfect.
> Perfecting the implementation of Proj.4 as a comprehensive _coordinate
> system_ library (not just a projection library) without a major
> rewrite is not likely. Restricting it to just a projection library
> would diminish its value to such an extent that it would lose its
> relevance.

All software is imperfect. But FWIW, the way that the current PROJ and
cs2cs handle the datum and ellipsoid was perfectly adequate for every
known use case until Google decided that they were going to use WGS84
for geographic coordinates but project them using spherical Mercator.

AFAICT, the simplest workaroud for the Google bug is to add another
projection, e.g. +proj=sphmerc, which claims to be an ellipsoidal
projection but implements spherical Mercator regardless of the actual
ellipsoid eccentricity.

Glynn Clements <glynn at gclements.plus.com>

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