Chris Carleton wrote: > Okay thanks. I was using a 'trial-and-error' method for determining the > threshold, but I think I ought to start large and proceed toward smaller > values. I haven't come across any analytical method for determining the best > threshold value for stream extraction.
There are conceptually different approaches to determine a reasonable threshold. One would be to use the DEM only and estimate what threshold provides a reasonable stream network. Obviously you can get vastly different stream networks from a 1m LiDAR DEM and a 90m SRTM DEM. Such an approach gives an answer to what is possible with the DEM, which does not necessarily match reality (most of the time I guess it does not match reality). A different approach would be to look at rainfall patterns, soil infiltration rates etc, and decide whether you are interested in ephemeral, intermediate or permanent streams. For permanent streams, a decision must be made whether you want to get all, also very small permanent streams, or only major streams. Markus M Some smart person needs to develop a > statistically sound method for assessing the uncertainty associated with > geophysical feature extraction so that multiple results can be compared > meaningfully and we're not left to assess the validity of a result on the > basis of whether or not it 'looks okay' and preferably that doesn't involve > going into the field to count streams. I'm going to compile GRASS from > source with the latest stable release and configure for 64bit support. I > think the current install was done through the repository so it's behind and > not optimized. On that note, I'm trying to figure out what the appropriate > configure flag(s) is(are) for HDF4 and HDF5 support in GDAL so that I can > open HDF files in GRASS. The only like that I can find that seems to be > discussing it formally is here: > > http://trac.osgeo.org/gdal/wiki/HDF > > Does anyone have the answer handy or another link with instructions that are > more clear? Thanks, > > Chris > > On 24 August 2010 05:39, Markus Metz <[email protected]> > wrote: >> >> Martin Landa: >> > >> > Chris Carleton: >> >> I haven't been able to find something in the mailing lists about this, >> >> but >> >> if you know of somewhere that I can find a solution I'd appreciate the >> >> link. >> >> I'm running GRASS 6.4 RC5 on Ubuntu LTS 10.04 with a Dell Precision >> >> T7500 >> >> Workstation. The region settings are as follows; >> > >> > RC5 is quite old - please try RC6 or better fresh SVN. >> > >> The Ubuntu version and grass version used are both true 64bit? >> Processing over 130 million cells with r.watershed in memory and 12GB >> is possible, but only if grass is compiled as a 64bit application. >> >> SECTION 4: Watershed determination is pretty much unchanged since RC5, >> the segfault might also happen in current 6.4. >> >> The combination of threshold=10 and over 130 million cells is a bit >> unusual, producing a very large number of stream segments, but AFAICT >> not large enough to trigger a segfault. >> >> Markus M >> > > _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
