[mapguide-dev] area calculation conversion to meters

[Jason Birch]

Not that I have any idea what the Right Thing would be in this case.  Perhaps returning a null or an exception because you can't convert reliably between decimal degrees and distance units?
 
My preferred solution would be to see something like GetAreaInMeters and GetDistanceInMeters functions :)
 
Jason
 
________________________________

From: Jason Birch
Sent: Sat 2006-12-02 2:06 PM
To: dev at mapguide.osgeo.org
Subject: RE: [mapguide-dev] area calculation conversion to meters


 
No, you're not over-engineering Paul.  I think it's non-trivial to accurately calculate large areas based on decimal degree coordinates because, as you've said, the vertical distance between decimal degrees varies with latitude.
 
For distance I think there are a couple methods (haversine, cosine law) you can use without having to rely on a projection library.  I don't think that the MapGuide ConvertCoordinateSystemUnitsToMeters function (in \Common\Geometry\CoordinateSystem\CoordinateSystem.cpp) is doing the right thing though.  For projected systems it's doing a simple multiplication by conversion factor, but it appears to be doing the same thing for geographic systems, multiplying by a constant:
 
        if(m_ogrSrs->IsGeographic())
        {
            meters = METERS_PER_DEGREE * units;
        }
 
For area, transforming to an equal-area projection would probably work better than a conformal projection like LCC, but I'm unsure of whether "equal area" applies across the entire projection or just within the "sweet spot".
 
Jason
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