[Proj] grid shifting
paul-grass at stjohnspoint.co.uk
Mon Oct 23 13:33:43 EDT 2006
On Mon, 23 Oct 2006, Hamish wrote:
> 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 ?
That is how the Canadian NTv2 gridshift file format (as used by PROJ.4)
works, yes. But it's not the only (nor necessarily the best) way.
>> So datum transformation can be schematized as :
>> Plane Coordinates A => Geodetic Coordinates A => Grid Shifting =>
>> Geodetic coordinates B => Plane Coordinates B
Yes, internal to PROJ.4 that is how datum shifting with +nadgrids=xxxxx
>> 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
Internal to PROJ.4 that is how datum shifting with +towgs84=x,x,x,...
Note that you can go straight from plane co-ordinates to Geocentric
cartesian co-ordinates in PROJ.4: use the "fake" projection +proj=geocent.
So it *is* possible to do a transformation this way.
>> 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 ?
You could convert the matrix into a grid in NTv2 format that can be read
by PROJ.4 - it needs some logical thinking and a bit of GIS work but it is
quite easily possible and with accurate results too - see the page on my
website below as Hamish has suggested.
>> 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..
They weren't working for a while but I fixed them to account for the
updated IGN site in July so all should be fine now... ;)
More information about the Proj