[GRASS-user] problem displaying imported, rectified raster images

Michael Barton michael.barton at asu.edu
Tue May 8 23:38:59 EDT 2007

Try the new georectifying module in GRASS 6.3 from William Kyngesbury¹s
Grass for Mac site (find from the GRASS downloads page).

Note: if using the new georectifying module, you START the georectifying
session in the location/mapset you want the image to end up in, rather than
the xy location/mapset where the unrectified image is stored (the procedure
for i.points).


On 5/8/07 11:39 AM, "pete davidson" <caitifty at gmail.com> wrote:

> Hi all
> I've been struggling with this problem for days..  I'm trying to import a
> scanned map into grass 6.1.0 on a mac.  All seems to go well, until I try and
> display the final raster.  Here's the steps I've taken, and thing's I've
> tried.  
> Many thanks in advance for any suggestions or advice for resolving what I
> suspect is a total newbie question
> Pete
> --
> - create a new xy region
> - import the image (a png scanned at 300dpi) using r.in.gdal input=[path to
> scanned file] output=TL0232 location=scanTL0232
> - exit grass, re-launch and use region scanTL0232/PERMANENT
> - i.target group=TL0232 location=[real_location] mapset=PERMANENT  (where
> 'real_location' is a pre-existing NAD83 region I want to use this raster in)
> - d.mon x0
> - d.rast TL0232_blue (as a quick test that the image imported correctly, and
> yes, I'm looking at a nice, high resolution image)
> - i.points TL0232  (note that contra several of the online guides, r.in.gdal
> automatically created a group, so I didn't need to run i.group first)
> - select a bunch of reference points, giving real-world eastings & northings,
> then quit i.points
> - i.rectify -a group=TL0232 extension=_1 order=1
> - exit grass, re-launch and use region [real_location]/PERMANENT
> - d.mon x0
> - d.rast TL0232.blue_1
> What I now get is a super-low res image: 6 or 7 giant pixels to a side.  If I
> display other pre-existing vector layers (eg a tiger line street map), the
> raster has clearly been located correctly within the region, with the
> orientation and relative size I'd expect.
> For several days I assumed I was making a mistake somewhere in the r.in.gdal /
> i.target / i.points / i.rectify process.  However, this morning I opened qgis
> to do something else, then on a whim created a new map, displayed a couple of
> pre-existing vector layers from the grass 'real_location' location, then added
> the grass raster layer TL0232.blue_1.  To my surprise it displayed perfectly.
> Correctly oriented and rectified and in a usable resolution and everything.
> Back in grass, if I do
> - g.region rast=TL0232.blue_1
> - d.mon x0
> - d.rast TL0232.blue_1
> I get a correctly displayed raster, however as soon as I do g.region -d to go
> back to the (larger) default region, any attempt to display the raster again
> produces an unusably low-res version..
> So.. What am I doing wrong?  Any suggestions?  Thanks again.

Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

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