Paulm a écrit :
> Why does it take so much longer time when I change the ellipsoid? When using
> a Helmet 3D transtormation a rot.matrix is setup for fast converson of
> lat/long coord to another ellipsoid, or?
The matrix must be applied on geocentric coordinates, not on geographic
(long,lat) ones. A change of ellipsoid implies the following steps:
--- the point to transform ---------> (long,lat)
--- conversion to geocentric CRS ---> (X,Y,Z)
--- multiplication by the matrix ---> (X',Y',Z')
--- conversion to geographic CRS ---> (long',lat').
The conversions between geographic and geocentric CRS are costly.
A shortcut exists that can be applied directly on geographic coordinates,
without the conversion to geocentric CRS: this is the Molodenski transform,
which is an approximation (not an exact shortcut, only an approximation) of the
above steps. But this approximation is applicable only if the matrix doesn't
contains any rotation terms (only translation - maybe a scale, I would have to
check). Even when the Molodenski approximation is used, it still much more
costly than a simple matrix multiplication.
Martin
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