[Proj] Local Projection Selection

Patrick Mézard pmezard at gmail.com
Sun Aug 7 12:02:00 EDT 2005

Gerald I. Evenden wrote:

> My immediate question is why not use a geodesic program rather than 
> projecting
> the data and using a plane system with its inherent distortions?

- I did not know there was code in PROJ.4 to do that and I have not much 
time to test/integrate new stuff. Unfortunately, it seems the geodesic 
computations are again using global variables everywhere to configure 
the ellipsoid as well as to pass and retrieve parameters.
- I clearly overlooked the problem, and pure distance computations are 
not enough. Sorry, I made you lose your time with this, I keep your 
remarks about the geodesic calculations. The processing code really 
needs the points to be projected in a plane to perform all sorts of 
computations like nearest neighbour search using bounding box trees, and 
other things which are just easier to do in an euclidian space.

> If one insists on using a projection, then Stereographic is probably 
> most appropriate.
> Determining the center optimal central point is a problem as a 
> inverse/forward
> geodesic computation would be needed to determine optimal midpoint 
> between the
> points.

I understand it is not the optimal but I do think the mean center will 
do. I have looked at the documentation about the Stereographic 
projection in in PROJ.4 documentation and in 
I suppose I can ignore the False Easting and Northing. Latitude and 
longitude of natural origin would be the projection center point. 
Remains the scale factor. I could let it at 1.0. But maybe there is a 
way to generate it programmatically given a center point, and ellipsoid 
and a working area radius. It probably does not worth the pain however. 
Are the output coordinate units naturally in meters or have I missed 
other parameters ?

> I can see means of minimizing the use of floating point but 
> elimination would be quite
> difficult.

And premature optimization is evil. I will measure first. I listed this 
constraint in case there were several alternative projections to choose 

Thank you for your help.

Patrick Mézard

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