[Proj] Local Projection Selection

Noel Zinn ndzinn at houston.rr.com
Sun Aug 7 13:02:57 EDT 2005


Is this a 3D problem?  Is it important for your "nearest neighbour search 
using bounding box trees, and other things which are just easier to do in an 
euclidian space" that the vertical dimension be preserved?

If so, you might consider using Earth-Centered Earth-Fixed 3D coordinates or 
some local-horizontal or local-vertical flavor thereof.


----- Original Message ----- 
From: "Patrick Mézard" <pmezard at gmail.com>
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
Sent: Sunday, August 07, 2005 11:02 AM
Subject: Re: [Proj] Local Projection Selection

> 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?
> Because:
> - 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 
> <http://www.remotesensing.org/geotiff/proj_list/oblique_stereographic.html>. 
> 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 
> among.
> Thank you for your help.
> Patrick Mézard
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