[GRASS-user] Rectify an image

[Hamish]

Nikos Dumakis wrote:
> GreetingsI have an image without any
> GCP's but, teorethically it's in WGS84. I have all
> pixel positions outside that file. My question is, what
> should I do in GRASS to rectify/project this image? I
> thought in two solutions
> 1- import, using r.in.gdal in a XY projection and
> rectify. Is it possible to rectify a raster, with i.rectify,
> using a file outside GRASS (e.g. a text file?)2-
> use GDAL to add GCP's points to file and then import to
> grass already rectified..


I would use gdal_translate to add GCPs to the file and then
use gdalwarp to reproject it to what you want.

the other way is to create a group/$GROUPNAME/POINTS file by
hand. it is a very simple format, use one of the grass geo-ref
tools to create a couple of sample GCPs by hand and then edit
the file to place your known values in the right way. There
are column headings in the POINTS file.


be careful that gdal considers 1,1 to be the top left, while
grass considers 1,1 the bottom left pixel. so you might have to
apply rows-y at some point if swapping between the two.
(? or 0,0, so +/- 1 pixel ?)


Hamish