[Proj] proj4 string for perfect sphere

[support.mn at elisanet.fi]

if you need any accuracy and robustness you'll never leave the
sphere (or ellipsoid). It is much easier to spend a little more time
with the general spherical calculation than to fix it afterwards for
150+ projections for each one with maybe a different method.

If you are afraid of the ellipsoid just use the sphere and add some
correction tables or similar methods since the difference is
usually very very small. Another trick is to project it to a single
standard projection which works all over the world and then
do the plane calculations if they are too hard to be done on the
sphere.

Nobody likes the situation where the calculations are made on any
distorted projection plane .. the error can be huge in some cases!
At least the routine should be robust to detect such distortions.. but
this adds a lot of programming effort.. so it is still better not to leave
the sphere (or ellipsoid) which is the general case and works
everywhere and with every projection.

regards: Janne.

--------------------------

Mikael Rittri [Mikael.Rittri at carmenta.com] kirjoitti: 
> 
> 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. 
> 
> Regards,
> Mikael Rittri
> Carmenta
> Sweden
> http://www.carmenta.com
> 
> -----Original Message-----
> From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of support.mn at elisanet.fi
> Sent: den 17 april 2012 11:54
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] proj4 string for perfect sphere
> 
> Hello,
> 
> Demitri Muna [thatsanicehatyouhave at me.com] kirjoitti: 
> > Hi Bob,
> > 
> > Thanks very much for the extensive comments. The biggest thing I learned about GIS this week (starting from knowing nothing about it last week) is that calculations are primarily done on an XY plane which are projections of a curved surface. I had naively assumed going in that calculations and indices (e.g. spatialite) were optimized for the surface of a sphere.
> > 
> 
> A small notice for a beginner: If you want to stay accurate and avoid lots of trouble with projections generated errors never leave the sphere (or ellipsoid) just keep doing all calculations there - it is much easier (you don't need projections) and you will stay accurate for ever ;)
> 
> regards: Janne.
> 
> 
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