[GRASS-user] DAtum Warning while using r.in.gdal

[Paul Kelly]

On Tue, 1 Jun 2010, Kim Besson wrote:

[...]
> Coordinate System is:
> PROJCS["WGS 84 / UTM zone 29N",
>    GEOGCS["WGS 84",
>        DATUM["WGS_1984",
                 ^^^^^^^^
Hi Kim,
The datum of the original datum is WGS84, which is almost the same as 
GRS80, so your data will be fine. So there's nothing to worry about in 
this case, but read on for a more detailed explanation...

[...]
> I just need to have this data in EPSG:3763 system so I thought that I would
> just need to define EPSG:3763 without anything else. Original is in WGS84
> and this one is GRS80.

As I said it is fine in this case because WGS84 and GRS80 are almost the 
same. But in general you always need to specify the datum you are 
reprojecting to. Your PROJ.4 string included "+ellps=GRS80" to specify the 
ellipsoid, but that is *not* enough information to do a datum 
transformation; you also need to specify the datum transformation to 
WGS84. For GRS80 you can do this by adding +towgs84=0,0,0 to the PROJ.4 
string (as well as +ellps=GRS80).

Specifying EPSG:3763 to gdalwarp instead should also have exactly the same 
effect, although personally I prefer the PROJ.4 strings as you can see all 
the parameters and see exactly what is going on.

> What should I do? use in gdalwarp EPSG:3763 or the PROJ4 extension?

It's up to you - there are several different ways of achieving the same 
thing!

Paul