[GRASS-user] negative slope values and movement costs

[Colin Nielsen]

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
important.

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
this.

Good luck.

-Colin

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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