[Gdal-dev] Projected vs Geographic Coordinate Systems

Simon Perkins sy at perkins.net
Tue Mar 20 13:44:11 EDT 2007

Frank Warmerdam wrote:
>> I guess I've just been spoiled by playing with Google Earth too 
>> much... :)
> To the best of my knowledge Google Earth has one display project
> (some sort of Mercator) and it reprojects everything to that.  That
> approach is fine too if you are willing to reproject everything - either
> in advance or on the fly.  Many apps will allow you to do this too if you
> want.

I think that google maps does everything in Mercator, but google earth
allows you to view the entire globe, spin it around and view the poles,
so my feeling is it must be doing something more cunning.

According to wikipedia, GE uses a geographic coordinate system
internally, and projects everything to a general perspective projection
for display. I would guess though that it just texture maps
(non-rectangular) geographic tiles onto a 3D sphere and then lets OpenGL
take care of displaying things. Although it is possible to view the
poles in GE, there is definitely weird stuff happening there - there
appear to be very narrow triangular tiles. Anyway, it's all pretty cool...




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