[GRASS-user] negative slope values and movement costs

Colin Nielsen colin.nielsen at mail.mcgill.ca
Tue Feb 6 22:24:56 EST 2007

Perhaps a good idea to discuss the possible ways forward. My scenario
was a prehistoric one, so I used a hydrology network to develop a
friction surface that represented the travel cost of travelling by
canoe vs. hiking vs. portaging, etc. The cost values are relative
rather than actual time or energy spent but by experimenting with the
ratio of water to land travel for different scenarios I was able to
produce very interesting results. Perhaps a specific solution to a
specific problem only. In this case there is only on water or on land,
only two options. Two possibilities are a lot easier to model than the
90 or 180 different degrees of slope in a DEM. But perhaps if people
have generally had so little success with DEM-based path modeling,
with the exception of high relief regions, some alternative methods
are in order.

-Colin Nielsen

On 2/2/07, Miguel Correia <miguellage.rc at gmail.com> wrote:
> 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).
> 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 Spain , 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.
> 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?
> Miguel.
> 2007/2/2, Glynn Clements <glynn at gclements.plus.com>:
> >
> > Michael Barton wrote:
> >
> > > If you are calculating round-trip, simply create a slope map and use the
> > > original Tobler algorithm to make a positive (i.e. uphill) cost map and
> a
> > > negative (downhill) cost map. Then add them together and use the summed
> > > result as the basis for a round-trip cost surface. If you are doing
> one-way
> > > only you¹ll need an anisotropic algorithm. Perhaps you can modify the
> > > coefficients in r.walk to simulate a Tobler result. If not you can
> create
> > > the whole thing in the map calculator (e.g., using neighborhood
> functions)
> > > or the best way would be to work with Markus on r.walk to add the Tobler
> > > algorithm as an option if it is widely used and generally desirable.
> >
> > You can't implement r.cost/r.walk purely with r.mapcalc , as such an
> > algorithm is inherently stateful, while r.mapcalc is stateless.
> >
> > You would probably want to modify r.walk or r.cost. One relatively
> > simple option would be to modify r.cost to use 8 different cost maps:
> > one for each of the possible directions (I'm ignoring -k here). You
> > could generate the cost maps from a DEM using r.mapcalc.
> >
> > --
> > Glynn Clements <glynn at gclements.plus.com >
> >
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