Thank you for your answer. I read on the pages you mention : "If the point falls exactly upon a grid line, the exact result will be determined by the direction of any rounding error. One consequence of this is that downsampling by a factor which is an even integer will always sample exactly on the boundary between cells, meaning that the result is ill-defined"
This should be the source of my problem ! Regards ________________________________________ De : Moritz Lennert [[email protected]] Envoyé : lundi 31 mars 2014 15:54 À : BLANDENIER Lucien; [email protected] Objet : Re: [GRASS-user] r.mapcalc and g.region On 31/03/14 15:33, BLANDENIER Lucien wrote: > Hello, > > I'm wondering how does r.mapcalc compute the value of a new pixcel if the > resolution have been reduced before (i.e. the pixcel is larger). Is it an > average ? > > I did a simple test with a 10*10 pixcel raster. I reduced the resolution to > 5*5 and made a new raster with r.mapcalc "newrast=oldrast*1" but the result > was not an average but the higher value of the four underlying pixcels. > > I did then the same operation with a more coarser resulution and it seemed to > be ok. > > Could someone explain this? Short answer: GRASS uses nearest neighbor interpolation as the default method for resampling. For more details: http://grass.osgeo.org/grass64/manuals/rasterintro.html "Resampling methods and interpolation methods" Moritz _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
