On Fri, Jul 9, 2010 at 12:50 PM, Hamish <[email protected]> wrote: > > David wrote: > > I have a couple of questions about DEMs, and how they work in GRASS. > > My application is hydrological and hydraulic analysis, and at the moment > > I'm investigating using r.sim.water to generate floodlines. > > > > Usually, for this sort of analysis, I would have 5m contours for the > > catchment and a detailed survey (i.e. spot heights, sometimes with > > triangles) for sections of river. So my first question is, what is the > > best way to generate a DEM from this information? > > take your pick: > http://grass.osgeo.org/wiki/Contour_lines_to_DEM > > > I know that I can use r.surf.contour to create a DEM from the contours, > > and I'd assume that something like v.surf.rst would be best for the > > points, but I need a single DEM for the analysis. > > you will see in the r.surf.contour help page that adding points is well > recommended. Use v.to.rast then r.patch to combine them with the rasterized > contour lines. (make sure points get priority) > > > The second problem is that I don't need a highly detailed DEM for the > > catchment, I only need detail along the watercourses - i.e. where I have > > the detailed survey. If I create two DEMs at different resolutions, can > > I patch them without losing detail? > > sure. I believe that Helena has an example of that in her & Markus's GRASS > book. Data is resampled to the current resolution on-the-fly though, so > it really doesn't cost you too much to just make the original data source > hi-res everywhere. > > > My third question is about creating a mask. If I'm working along the > > coast, or if I'm creating a detailed DEM along the watercourse, I only > > want raster data where there is base data (i.e. contours or points). Is > > there a way to quickly generate a mask for this? > > see the r.mask module. or use r.mapcalc to create a MASK map where > elevation > 0. (the MASK map has value in areas to show, and is 0 or > NULL in areas to hide; and is only applied when reading raster data from > disk). If you have a vector polygon of the area/coastline it's easy to > do 'v.to.rast output=MASK'. > > > Hamish >
Thanks for the reply. I don't entirely understand what you mean by on-the-fly resampling. Surely the raster needs to be created at a high resolution to start with, which means quite a lot of processing? I'm considering areas of several hundred square kilometers at less than 5m res. Would the best approach, then, be to patch the contours and points, and create a single hi-res raster? I understand how to create the mask from a polygon, the problem is that I don't have the area or coastline. What I'd like to do is to automatically generate a polygon covering, for example, all the contours, but without extending far past them. David _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
