[Gdal-dev] Projected vs Geographic Coordinate Systems

Frank Warmerdam warmerdam at pobox.com
Tue Mar 20 01:10:14 EDT 2007

Simon Perkins wrote:
> Simon Perkins wrote:
>> 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?
> In a related question, if I have a vector layer in a geographic 
> coordinate system, does OGR provide some convenient way to compute the 
> relative scaling of degrees in the east-west and north-south directions? 
> I guess it comes down to some simple geometry (the size of latitude 
> degrees is uniform, the size of longitude degrees go like square of 
> cosine of latitude, I think) , but just wondered if there was some more 
> sophisticated procedure for what must be a common task.


OGR and it's SRS operations do not have anything specific for this.

I suppose there is some fairly straight forward math on the ellipse that
would transform the length of a degree of latitude at different latitudes
though I don't know it off hand.

Generally speaking though I assume if people want to work in a geographic
coordinate system it is because they don't care about the distortion.

If you were wanting to do mensuration (why do we need such a fancy term
for measuring?) you would do it in some locally reasonable projected
coordinate system.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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