[Proj] grid shifting

[Hamish]

cp wrote:
> 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
> B
> 
> 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:
  http://www.stjohnspoint.co.uk/gis/france.htm

The deep IGN links seem to be broken now, but grid file for France and
instructions are there..


good luck,
Hamish