[GRASSLIST:852] Re: Ellipsoid Shift For GRASS Vector Map

[Paul Kelly]

Hello

On Tue, 29 Jul 2003, Vidya Kotamraju wrote:

...
> How do i go about converting the ellipsoid of the vector map from
> everest to WGS84 ? What command can i use ? I am not sure if i can us
> the "m.datum.shift" command for this. However, if i can go ahead with
> "m.datum.shift", what values do i need to provide in the lat and lon
> parameters of the command?

As the man page for m.datum.shift says, it is an experimental program and
in fact it has been superceded by other better solutions.
You need to use the latest CVS version (now to be called GRASS
5.3.x)---the CVS snapshot from the website will do for this and v.proj is
the command you need. (You may also like to look at the PROJ.4 software
http://remotesensing.org/proj/ )

However you need to provide some datum transformation parameters for
your everest location---these would be a set of either 3 or 7 numbers. Add
a line
towgs84: dx,dy,dz
(where dx dy dz are the 3 numbers) to the PROJ_INFO file in your everest
location. You can edit the file by hand and it is quite easy to do, or
run g.setproj and select a 'custom' datum when prompted (but you need to
re-enter the other paramters for your projection if you use g.setproj).
Then as long as you also have a datum defined for the wgs84 location, just
start GRASS in that location and run v.proj to re-project the map from the
everest location.

There are some links in the datum.table and datumtransform.table files (in
the etc subdirectory under the main GRASS installation directory) that
have hints on where to find appropriate datum transformation parameters.

If you have some information on the name of the datum and the parameters
that should be used in your area, let me know so I can add them to GRASS
for the future.

Paul Kelly