[GRASS-user] Help with Georeferencing

[Matt B]

On Mon, Nov 10, 2008 at 10:22 AM, Hamish <hamish_b at yahoo.com> wrote:

> Matt:
> > > At the moment I'm trying to georectify a stitched
> > > together google earth image
>
> often r.in.wms / r.in.onearth is just as good and totally free.
> (r.in.onearth from wiki addons)
>
> > > I've imported the raster into grass using gdal, it
> > > splits it up into 3 (RGB) files? and I can display it (in
> > > the wrong place at the wrong scale). I also have the
> > > coordinates of the 4 corner points of the image and those
> > > are what I'm using to georeference using the
> > > georeferencing tool from the file menu.
>
> in that case, avoid all the hassle and just use r.region.
>
>
> Moritz:
> > In which location are you running the georeferencing tool ?
> > You have to run it in the target location, not in the
> > location where you imported your original images.
> >
> > Workflow:
> >
> > - create two locations, one XY (unprojected) and one
> > projected in the reference system you want to georeference
> > the images into (AFAIR, Google Earth uses WGS84 lat-long,
> > alias EPSG code 4326
> > - import images with r.in.gdal into the XY location
> > - in the projected location, run georectifying tool
>
>
> wiki FAQ entry:
>
>  http://grass.osgeo.org/wiki/Georeferencing
>
>
> Hamish
>


Thanks Hamish & Moritz,
 I'm using this to figure out how to georeference. The tutorials / wiki are
useful but for someone new to GRASS there is a little detail lacking in the
documentation. I think my problem at the moment is that I've got a
theoretical understanding of how groups and locations work but applying this
to the menu structure/workflow within grass is another thing.

Locations: basically a place in the grass database with an area (ie region)
and a projection. Data within the location is stored in groups. ie, one
raster split into RGB would be a group, an imported shapefile or possibly
set of shapefiles would be another group.

Locations need to be created when starting grass.
Do I need to have a group created before I import data or, is there a group
created when I import data? I can import vector data easily enough and I'm
not prompted to create a group usually, however I am having trouble creating
a group using the i.group tool. If I start it form within the georectify
tool it won't accept any input until I close the georectify window and it
usually crashes. The man page doesn't have a cli example on how to create a
group within the location (that I can remember, I'm away from my grass pc
today).

Thanks
Matt
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