On 24/10/2012 22:54, Luciano La Sala wrote:
Hola!
I am using GRASS inside QGIS and I need to perform some very basic
arithmetic on raster map layers using "r.mapcalc". Once I have created my
mapset in GRASS and imported all three rasters into GRASS from QGIS view, I
define my algorithm by simply doing "Raster_A + Raster_B + Raster_C".
However, the output gives the message "Unable to open raster map. Finished
with error".
The r.mapcalc model builder is, in my opinion, tricky to use. Can you
try your mapcalc expression in the GRASS shell instead, and report back
what result you get? i.e.
r.mapcalc ABC="Raster_A+Raster_B+RasterC"
I am not sure what the problema is, but I suspect that it may be related to
the extent of each layer, which is different for each raster:
Raster_A
Layer Extent (layer original source projection):
-264465.6428050000104122,3892490.3601970002055168:
1442088.1851800000295043,7588100.0655570002272725
Raster_B
Layer Extent (layer original source projection):
-231150.3030560000042897,3942814.9593090000562370:
1421853.4221910000778735,7568606.9837530003860593
Raster_C
Layer Extent (layer original source projection):
-302992.6578800000133924,3856086.4785620002076030:
1162350.0904449999798089,7559207.0393540002405643
Q1: What do these long numbers represent?
These are the upper left and lower right corners of each raster
Q2: Should every raster layer have the exact same layer extent for the
mathematical calculation to come out right?
No, you can use r.mapcalc on rasters with different extents. However you
should be aware that wherever any of the maps is NULL, the result will
be NULL. In other words, for each pixel, if either of your three maps
has a NULL value, that pixel will become NULL also.
Note that there's also r.patch which was designed to combine maps which
do not overlap, and fill in NULL values from one map with values from
the others.
Thank you so much in advance!
Luciano
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Micha Silver
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