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 <http://osgeo-org.1560.x6.nabble.com/suggestion-for-g-region-a-td4014517.html> (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 kept. 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" -- View this message in context: http://osgeo-org.1560.x6.nabble.com/Problem-with-GRASS-Region-Extent-and-Region-Cellsize-on-Processing-tp5255693p5255960.html Sent from the Quantum GIS - Developer mailing list archive at Nabble.com. _______________________________________________ Qgis-developer mailing list [email protected] List info: http://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-developer
