[geotk] Circle Polygon

Martin Desruisseaux martin.desruisseaux at geomatys.fr
Tue Sep 15 05:08:21 EDT 2009

Hello Thuns

As Adrian said, its look like the deformation we would expect from a Mercator 
projection. A difference of 1° in latitude will appear larger and larger in 
metres as you approach a pole. At the pole, the value reach infinity.

If we put the raisoning in the opposite way, having a fixed length like 2 km 
converted to degrees will appear smaller and smaller as we approach a pole. At 
the pole, any circle will appear to have a radius of 0 degrees of latitude.

It is not possible to project a sphere on a flat surface while preserving every 
geometric properties. This is why there is so many projections. You need to 
choose which geometric properties you want to preserve (angles, surface, etc.).

If you need to preserve the circle radius while moving north-south, and if you 
don't need to move in the east-west direction more than 6°, try a UTM (Universal 
Transverse Mercator) projection.

If you prefer to stay with the classical Mercator projection, try to set the 
"Latitude of 1st standard parallel" parameter value to the central latitude of 
your area of interest. This the latitude where the scale is "true" and play the 
role of the correction factor you mentioned.


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