[Qgis-user] Same layer extent?

[Luciano La Sala]

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". 

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?

Q2: Should every raster layer have the exact same layer extent for the
mathematical calculation to come out right? 

Thank you so much in advance! 

Luciano