[Proj] Local Projection Selection
pmezard at gmail.com
Sun Aug 7 15:38:22 EDT 2005
I am considering points on earth surface without elevation.
I have not thought about a full fledge 3D cartesian space. However, the
distances I have to compute are "geodesic" ones (or maybe "geoidic") not
euclidian distances. And I assumed that approximating in a 3D space
instead of a projective plane would be worse in term of accuracy, but I
may be wrong. It may be faster to computer however, but I am not even
sure since WGS84 => stereo-whatever has no reason to perform a cartesian
space rountrip as it is done when datum shifts are required.
Noel Zinn wrote:
> 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
> 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
>>> 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.
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