[GRASSLIST:936] Re: Ellipsoid Shift For GRASS Vector Map

[Vidya Kotamraju]

Hi paul,

Thanks for the earlier response.

As mentioned earlier, I have a GRASS vector map that has been converted 
from a toposheet.This vector map is based on the "everest" 
spheroid/ellipsoid.I have a few GRASS raster maps that are based on the 
"WGS84" ellipsoid.

I need to overlay the vector map on these raster maps.

The raster maps are of 1-kilometer resolution.

I wanted to know the order of error expected by making use of an 
everest-based vector map on a WGS84-based raster map.

what do u expect the order of error to be in this case ?

Regards,
Vidya Kotamraju.


Paul Kelly wrote:

> 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 in fact it has been superceded by other better solutions.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
> 
> 


-- 
---------------------------------------------------------
"Curiosity is one of the permanent and certain
characteristics of a rigorous intellect"
                                       - Samuel Johnson
---------------------------------------------------------
Kotamraju Vidya
Marine HydroDynamics Lab,
Physical Oceanography Division,
National Institute of Oceanography,
Dona Paula, Goa 403 004, India.
Ph.: +91-832-2450400 (O)
      +91-832-2541756 (R)
e-mail: vidyakr at darya.nio.org