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

pete davidson caitifty at gmail.com
Tue May 8 14:39:56 EDT 2007

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

Many thanks in advance for any suggestions or advice for resolving what I
suspect is a total newbie question


- 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.gdalautomatically 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.
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