2010/9/29, Markus Metz <markus.metz.gisw...@googlemail.com>: > > In GRASS, you have full control of the computational region, > consequently you must be able to explain (to the client/supervisor) > every aspect of the computational region. Why is the horizontal > resolution not only slightly off from exactly one meter, but also > different for north-south and east-west? If these are the settings of > the input DEM, why was the DEM created with these settings? If these > resolution settings are different from the DEM's settings, there is > again lots of explaining and justification to do. Life is much easier > with, in this case, exactly 1m resolution which might need to be set > when preprocessing and creating the high-res DEM. The resolution > settings are just slightly different from 1m which can cause rounding > errors later on (export, import, file formats storing resolution in > single and not double precision, other GIS software using single and > not double precision for resolution, etc.)
Thanks for the extensive comments. I don't really know why the resolution in the x and y directions are not the same. Even when I set them explicitly, I get a slight difference: ----- g.region -p vect=c83a_bor...@permanent nsres=25 ewres=25 projection: 99 (Transverse Mercator) zone: 0 datum: ** unknown (default: WGS84) ** ellipsoid: wgs84 north: -3122229.69982072 south: -3155672.30728643 west: -66483.27873573 east: -33474.90929218 nsres: 24.99447494 ewres: 25.00634049 rows: 1338 cols: 1320 cells: 1766160 ----- The original resolution of the DEM was at 25m resolution. I just assumend that the differences in x and y resolutions are a result of the projected surface being skew with respect to the cartesian axes. I say this because vectors that looked square in a latlong GCS display slightly skewed when projected to the Transverse Mercator PCS that I use in this location. My reason for running r.watershed at a much finer resolution was that I would get a smoother boundary when I then convert to vector. Obviously not a good idea. > > Markus M > _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user