[Gdal-dev] Projected vs Geographic Coordinate Systems

Simon Perkins sy at perkins.net
Mon Mar 19 23:58:08 EDT 2007

Frank Warmerdam wrote:
>> Just wondering if I can get away with avoiding having to handle 
>> geographic coordinate systems in my (mostly raster focused) 
>> application for the time being... Any advice much appreciated!
> I don't know your constraints or detailed objectives, but I'd be nervous
> about making this simplication.  You could likely get away with treating
> geographic coordinate systems similarly to projected coordinate systems.

Hi Frank,

Thanks for the advice. My main motivation for considering ignoring 
geographic coordinate systems was to avoid dealing with the pesky issue 
of angular units not being isotropic, i.e. a degree in the north-south 
direction might represent a very different distance to a degree in the 
east-west direction. If I have a bunch of objects stored in a geographic 
reference system, then when I display them I have to scale x and y 
coordinates differently. Similarly, when I'm computing the minimum 
visible dimension for rendering optimizations, I have to deal with 
different numbers in the x and y directions.

I haven't looked at the source code for a long time, but what does 
OpenEV do here? Do you convert geo-coordinates into some more isotropic 
coordinate system before rendering, or do you just deal with the funky 
anisotropy at all stages?



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