Problem solved, please ignore my previous post: g.region's a flag does the job.
Christoph On Feb 21, 2011, at 3:40 PM, Christoph Heibl wrote: > Dear list, > > I am puzzled by a phenomenon in GRASS that I cannot really explain and that > spoils some of my downstream applications after preparing my data in GRASS. > > Problem: > > I have a set of vector points and some raster layers with environmental data > . For each point, the environmental data of the raster cell it lies within > has be to be sampled (either with v.what.rast or by the downstream > application [Maxent]). So far, so good and no problem. Now imagine only a > subset of the vector points is taken to sample the raster data and this is > achieved by laying a buffer around this subset of points and then setting the > region to this buffer. What happens now is that the vector points are shifted > with respect to the raster grid (or vice versa?). This pushes those of my > vector points, which lie near the coastline, into the sea and the downstream > application skips them, because it cannot link them to environmental data. > > These pictures may clarify the point: > > http://www.christophheibl.de/points1.png: This is the default setting. The > three red points represent my set of vector points. > > http://www.christophheibl.de/points2.png: Now only one of the three red > points is choosen as a subset and displayed, but without changing the region > setting. All points keep their position relative to each other and the raster > grid as in points1.png. > > http://www.christophheibl.de/points3.png: The same as in points2.png, but > this time a buffer was laid around the red point and this buffer was used to > update the region settings. Now, as described above, the vector points are > shifted with regard to the raster grid and the whole grid seems to be > centered in a way that the red point comes to lie at the corner where four > raster cells meet. > > Question: > > 1. Why does this shifting occur? This is somewhat frightening for me, because > I thought that the position of the grid would be fixed and given by the > projection. > > 2. How can I can make sure that the relationship between the raster grid and > my vector points "stays the same", when I "zoom in" on subsets of the study > region? > > Best wishes > -- Christoph > > ________________________________________________________ > > Christoph Heibl > > Systematic Botany > Ludwig-Maximilians-Universität München > Menzinger Str. 67 > D-80638 München > GERMANY > > > > > > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
