I very much agree with Hamish:
>it is really nice to have two independent methods to use & race against
>each other, and compare the results of. ie apply the scientific method.
>Each will have its strength and weaknesses and now we can quantify more
>what those are.
I am a big proponent and user of r.watershed, especially for stream extraction and watershed delineation at lower resolutions (30-90m), mostly due to its unique algorithm for overflowing depressions, which is among the most robust and accurate because it is not doing the standard pit filling. However, its D8 flow direction algorithm makes it less suitable for modeling at higher resolutions where it creates artificial pattern of parallel lines as seen in examples posted by Hamish (btw some of the parallel line features may also be due to the flats created by use of integers in r.watershed). r.watershed works well with SRTM data which are rather noisy, helping to avoid the parallel lines to some extent.
Another example where r.watershed does not produce usable results is here:
http://www.grassbook.org/gallery/chapters/s050404f070_flow.jpg
a) is by r.watershed
Adding one of the MFD algorithms with an optional switch to SFD for given accumulation threshold might help to make the results of r.watershed useful to those applications that require decent representation of flow dispersion on convex hillslope forms and avoid some of the artifacts.
One of the reason we have included DEMs ranging from 30m resolution to 2m resolution in the NC data set was to provide examples of how the different flow routing modules available in GRASS behave and how to select which one to use based on the data and task at hand.
Helena
P.S. and a small addition to the color table discussion - uniform interval indeed rarely works
unless you want to have all your streams and rivers colored a single color - the color table from Hamish is quite clever and provides a good way how to display more info abut the size of streams.
I found that both -e and log color tables worked pretty well for the nc_spm data, -e was good for applications where distribution on hillslopes was of interest, log helped to highlight the streams:
see slides 18 and 19 here
http://skagit.meas.ncsu.edu/~helena/lteachtest/GIS_anal_lecture/GIS_Anal_Lvisual.ppt
On Dec 2, 2008, at 2:23 PM, Isaac Ullah wrote:
Markus Metz wrote:
In my personal opinion, flow accumulation of r.watershed is also more
realistic than flow accumulation of r.terraflow (SFD), but I have
admittedly not tested it in detail.
I just wanted to add that I have, in fact, compared the results of the r.watershed with r.terraflow, as well as with r.flow. In my opinion, r.watershed consistently gives more accurate results than either of the other two. r.flow gives the worst results (for the kind of process modeling that I am interested in at least), while r.terraflow provides somewhat better results. Both modules produce results with "spikes" of high values that do not correspond perfectly with real topographic changes (or perhaps it is better to say that they overestimate changes in accumulation at certain places on the landscape), but r.watershed does not do that. The only reason I was not using the old r.watershed in my scripts was the time vs. map size issue. It was impossible to build it in as a dependant if it would make the modle hang for days if the input map was too big for it. However, I am please to say that initial tests running my r.landscape.evol with new r.watershed are VERY encouraging! Not only does the whole thing run faster, the results are MUCH more realistic! I may even be able to remove a time consuming "smoothing" routine from the module since it seems that I no longer am creating the anomolous "spikes" in elevation change do to problems in the flowacc map. This, I think, is a very good reason for focusing more development "sweat" to perfecting r.watershed. For future reference, I am more than happy to test out any changes to the module....
Cheers
--
Isaac I Ullah, M.A.
Archaeology PhD Student,
ASU School of Evolution and Social Change
Research Assistant,
Mediterranean Landscape Dynamics Project
***************************************************
isaac.ullah@asu.edu
ullah@archaeologist.com
http://www.public.asu.edu/~iullah
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