[Proj] "Double Ellipsoid" error, reproduction
Gerald I. Evenden
geraldi.evenden at gmail.com
Wed Dec 10 11:13:00 EST 2008
On Wednesday 10 December 2008 2:55:43 am Glynn Clements wrote:
> 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.
And there was absolutely *nothing* wrong with what Google decided to do. In
fact, the only thing *I* would have done differently is use a better
projection than Mercator.
The problem is in cs2cs.
> 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.
Mercator is a legitimate projection whether using a sphere or an ellipsoid.
In any case of small scale usage I would use a sphere for mapping simply
because there is *no* reason to waste CPU clicks for an ellipsoid. In fact,
for computer screen usage you would have to go to rediculously low scales to
find any reason to use an ellipsoid in the projection.
Google should be given kudos for rational projection usage!
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
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