[GRASS-user] negative slope values and movement costs

Colin Nielsen colin.nielsen at mail.mcgill.ca
Thu Feb 1 22:32:05 EST 2007

I guess I will weigh in on this question since I have tried it out.
First let me say that depending on the terrain you may want to
validate Tobler's function. I used a GPS to validate it in a wooded
terrain and found no difference between uphill and downhill movement.
Therefore remove the 0.05 and the neg v pos slope values are not

Secondly, I experimented with creating a time surface just the way
described but found the results to be unsatisfying. I found that
unless the terrain relief was extremely high (ie. Rocky Mountains)
and/or the resolution of the DEM is very fine, the effect on time was
small and therefore not a good indicator of cost or deriving travel
routes (as I'm guessing you're trying to do).

I advise trying to work out other possible sources of friction (I had
success with hydrology). Let me know if you'd like some references on

Good luck.


Anthropology Department
McGill University

On 2/1/07, Hamish <hamish_nospam at yahoo.com> wrote:
> mlage at usal.es wrote:
> > In the slope maps computed through r.slope.aspect, all slope values are
> > always positive? Is there any way to transform positive slope values into
> > negative values, to estimate movement costs in a slope map?
> r.slope.aspect has the format= option for slope.
>           format   Format for reporting the slope
>                     options: degrees,percent
>                     default: degrees
> if you want something different use r.mapcalc, the map calculator.
> Hamish
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