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

[Paul Kelly]

On Tue, 1 Jun 2010, Kim Besson wrote:

> Thanks to Paul and Micha for the help.
> About this: I defined my location using this image but instead of being
> reprojected with PROJ4 extension I used EPSG:3763 and then r.in.gdal. Then,
> using the same original image (in UTM) I reprojected to this CRS but using
> PROj4 extension (showed in my original message).
>
> What is the correct one? the bellow expression or the EPSG:3763 in order to
> reproject an image?

It depends on what the original co-ordinate system of the image was. If it 
was in a different datum from ETRS89 or a different ellipsoid from 
GRS80/WGS84 (which is the datum/ellipsoid of your GRASS location), then 
there will be small errors in the reprojection because you didn't specify 
a datum for the target co-ordinate system.

You can check the co-ordinate system of the original image by running:
g.proj -p georef=L71203033_03320001023_B10.TIF

If the two datums are the same (for our purposes it is reasonable to say 
this if the two ellipsoids are the same, or the difference is very small 
(e.g. GRS80 and WGS84)) then there is no problem at all. But in general 
you should specify the datum when doing a reprojection or warping as it 
can be important.

> It was only a warning. It imported the image nevertheless, without using -o
> option.
>
>>  And when I run r.in.gdal I get this warning:
>>  Datum <unknown> not recognised by GRASS and no parameters found
>> Projection of input dataset and current location appear to match

Yes the first line is a warning coming from the GRASS gproj library as it 
is converting the co-ordinate system; the second is simply an 
informational message from r.in.gdal - nothing to worry about there.

Paul