[Qgis-developer] Problem with GRASS Region Extent and Region Cellsize on Processing

gene martin.laloux at gmail.com
Sat Mar 12 01:46:39 PST 2016

To keep the raster resolution.

In the book "OPEN SOURCE GIS A GRASS GIS Approach Third Edition",   p.57

"It may happen, that region boundaries lead to a modified, non-integer
raster resolution. If this is not desired, the -a flag can be used to align
the region to resolution. It adjusts all four boundaries to be even
multiples of the resolution by slightly enlarging the current region. To see
the effect, compare:
g.region res=15 -dp
g.region res=15 -adp" 

Look also:

-  suggestion for g.region -a
(Michael Barton)

"Currently, if you use the -a flag with g.region it will align the region
exactly to match the resolution of a map (e.g. g.region -a rast=myrast will
set the region to exactly the resolution of myrast regardless of the
extents) or the resolution specified in the res or ewres/nwres fields (e.g.
g.region -a n=523 s=15 res=10 will adjust the extents a little to make sure
the resolution is exactly 10)"

- or in  proposal: make -a flag the default behaviour for g.region
<https://trac.osgeo.org/grass/ticket/1745>  (Moritz Lennert)

"In my experience, when working with raster, having a precise resolution is
the most important issue, while exact extension is less important. I often
use existing vector data or bounds coordinates to determine the extent, but
then have to use the -a flag to make sure that the desired resolution is
I would like to propose that in grass7, the behaviour is switched, i.e. to
make the -a flag (Align region to resolution) the default behaviour, and
"align to bounds" the option"

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