[Proj] grid shifting
ch.panel at free.fr
Fri Oct 20 16:10:33 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
Plane Coordinates A => Grid shifting (matrix : grid is based on planes
coordinates and no more on geodesic coordinates) => Plane Coordinates B
Isn't it ?
We have to do this because of increasing accuracy of transformation beetween
systems ( the grids could be experimental calculations)
thanks a lot for reply
More information about the Proj