[GRASS-dev] shifting maps created with r.mapcalc

Michael Barton michael.barton at asu.edu
Fri Jun 8 11:26:44 PDT 2012


I've run into an odd issue that could be a problem for anyone needing precise calculations from a landsat (or possibly other) image.

I have a landsat ETM image with a resolution of 28.5m for the bands 1-5, 7 and resolution of of 14.25m for band 8. If I set the region to exactly match one of the landsat channels (g.region rast=landsat_band8, for example) and copy the image with r.mapcalc (r.mapcalc expression='new_band8=landsat_band8'), GRASS produces an exact copy. 

However, if I set the region in other ways (e.g., via g.region setting the bounds, interactively in the GUI) any copy of the image made with r.mapcalc is shifted by half a cell. This happens whether or not I set the -a (align) flag. 

This does not seem to be a problem for another landsat with a 15m resolution (that is, not a fraction of a meter). 

Has anyone else run into this? Any ideas?

I'm working with GRASS 7 compiled a couple days ago.

Michael


On Jun 8, 2012, at 1:26 AM, Markus Neteler wrote:

> This is definitely for grass-dev. Perhaps an issue in the Python interface?
> But it would be important to know what " if I set the region any other way"
> exactly means.
> 
> Markus
> 
> On Fri, Jun 8, 2012 at 6:54 AM, Michael Barton <michael.barton at asu.edu> wrote:
>> In trying to figure out why I was getting some odd results in the sharpening
>> algorithms, I discovered a weird problem.
>> 
>> I've zoomed in on a landsat to run the pan sharpening routines faster (to
>> test various changes in the code) and see in better detail the results. When
>> I make a copy of the panchromatic map, I find that the copy is shifted by
>> 1/2 cell from the original. If I set region to exactly match all of the pan
>> map (g.region rast=pan), there is no shift. But if I set the region any
>> other way, there is a 1/2 cell shift. This means that every map I make will
>> not align with my original landsats. This causes problems in the sharpening
>> algorithms in different places. The worst seems to be in the PCA
>> sharpening. I've quit GRASS and restarted and this shift is still there. It
>> doesn't seem to matter whether I use the -a flag or not in g.region.
>> 
>> Is this a known issue? Is there some way around it?
>> 
>> Michael
>> _____________________
>> C. Michael Barton
>> Visiting Scientist, Integrated Science Program
>> National Center for Atmospheric Research &
>> University Consortium for Atmospheric Research
>> 303-497-2889 (voice)
>> 
>> Director, Center for Social Dynamics & Complexity
>> Professor of Anthropology, School of Human Evolution & Social Change
>> Arizona State University
>> www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu
>> 
>> 
>> 
>> 
>> 

_____________________
C. Michael Barton
Visiting Scientist, Integrated Science Program
National Center for Atmospheric Research &
University Corporation for Atmospheric Research
303-497-2889 (voice)

Director, Center for Social Dynamics & Complexity 
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University
www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu



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