[Proj] proj4 string for perfect sphere

Fischer, Robert P. (GISS-6110)[COLUMBIA UNIVERSITY] robert.p.fischer-1 at nasa.gov
Tue Apr 17 09:08:34 EST 2012

I think it's hard to draw generalizations here, and these issues should be handled on a case-by-case basis.  The relevant issue is, WHAT ERRORS does working with a projection entail, and HOW BIG are they?  The answer varies for different problems.  In some cases, working with a projection is 100% accurate.  In that case, the projection can be seen as a way to derive computational geometry algorithms on a sphere.

As an aside, I was really surprised to see how little support CGAL offered for anything on a sphere.  Although the theory for all this is probably sitting in a geometry book somewhere (I even have an idea of where), the on-the-ground practice really lags behind.

-- Bob
From: proj-bounces at lists.maptools.org [proj-bounces at lists.maptools.org] On Behalf Of Mikael Rittri [Mikael.Rittri at carmenta.com]
Sent: Tuesday, April 17, 2012 7:12 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] proj4 string for perfect sphere

Janne wrote:

> never leave the sphere (or ellipsoid) just keep doing all calculations
> there - it is much easier (you don't need projections)

"Much easier" are not the words I would use. "Much harder" fits
better. That's one reason projections are useful.

Spherical geometry isn't too bad for simple problems (distance,
azimuth etc.), but just think of point-in-polygon test, area of polygon,
and so on.

Mikael Rittri

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