[Proj] grid shifting
hamish_nospam at yahoo.com
Sun Oct 22 21:58:53 EDT 2006
> Firts, excuse me for my poor english.
> Just a question to see if I have understood the mechanism of grid
> shifting in proj lib....
> The grid shifting is a correction of phi and lambda in a particular
> point on the ellipsoid : this corection is made by a matrix which is
> unique in a "step" of longitude and latitude. Isn't it ?
> So datum transformation can be schematized as :
> Plane Coordinates A => Geodetic Coordinates A => Grid Shifting =>
> Geodetic coordinates B => Plane Coordinates B
> So it is not possible (except by modifying code) to make a correction
> for these cases :
> 1) We have a grid of correction that represents differents values of
> 3D shifts for geocentric coordinates
> like this :
> Plane Coordinates A => Geotetic coordinate A => Geocentric cartesian
> coordinates A => Grid shifting => Geocentric cartesian coordinates B
> => Plane Coordinates B
> 2) We have a grid of correction that represents differents values of
> plane shifts like this
> Plane Coordinates A => Grid shifting (matrix : grid is based on planes
> coordinates and no more on geodesic coordinates) => Plane Coordinates
> Isn't it ?
> We have to do this because of increasing accuracy of transformation
> beetween systems ( the grids could be experimental calculations)
maybe this helps:
The deep IGN links seem to be broken now, but grid file for France and
instructions are there..
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