<p style="MARGIN: 0cm 0cm 0pt"><span lang="EN-US"><font face="Arial Narrow">I suppose that the accuracy of the algorithm you use depends on the type of terrain you want to model. Anyway, sometimes the resolution of the DEM used implies more influence on the final result than the algorithm used to generate the friction surface (which in the case of Tobler's hiking function represents a velocity map).
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<div style="MARGIN: 0cm 0cm 0pt"><font face="Arial Narrow"><span lang="EN-US">The region that I'm trying to model it's quite rocky (although it is not Everest… I ensure you). By the other hand, Tobler's function is far from been perfect (just like all these hiking functions, Naismith's, the one used in
r.walk, it's far from being perfect too…). Anyway, it seems that for this particular region in northwest </span><span lang="EN-US">Spain</span><span lang="EN-US"> , Tobler's hiking function and a 25x25m DEM is sufficient to produce reliable results. Obviously it will never produce perfect results (which is impossible, I suppose), but it's a good starting point.
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<p style="MARGIN: 0cm 0cm 0pt"><span lang="EN-US"><font face="Arial Narrow">It would be very interesting if we could use this forum to keep discussing how Grass can model these questions, don't you think? Yesterday I wrote some things that are wrong and I'm working on what I think it might be the solution. Let's keep talking about this. How did you resolve this problem with hydrology?
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<p style="MARGIN: 0cm 0cm 0pt"><span lang="EN-US"><font face="Arial Narrow">Miguel.</font></span></p><br><br>
<div><span class="gmail_quote">2007/2/2, Glynn Clements <<a href="mailto:glynn@gclements.plus.com">glynn@gclements.plus.com</a>>:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid"><br>Michael Barton wrote:<br><br>> If you are calculating round-trip, simply create a slope map and use the
<br>> original Tobler algorithm to make a positive (i.e. uphill) cost map and a<br>> negative (downhill) cost map. Then add them together and use the summed<br>> result as the basis for a round-trip cost surface. If you are doing one-way
<br>> only you¹ll need an anisotropic algorithm. Perhaps you can modify the<br>> coefficients in r.walk to simulate a Tobler result. If not you can create<br>> the whole thing in the map calculator (e.g., using neighborhood functions)
<br>> or the best way would be to work with Markus on r.walk to add the Tobler<br>> algorithm as an option if it is widely used and generally desirable.<br><br>You can't implement r.cost/r.walk purely with r.mapcalc
, as such an<br>algorithm is inherently stateful, while r.mapcalc is stateless.<br><br>You would probably want to modify r.walk or r.cost. One relatively<br>simple option would be to modify r.cost to use 8 different cost maps:
<br>one for each of the possible directions (I'm ignoring -k here). You<br>could generate the cost maps from a DEM using r.mapcalc.<br><br>--<br>Glynn Clements <<a href="mailto:glynn@gclements.plus.com">glynn@gclements.plus.com
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