[GRASS-user] Time and slope: a hiking problem

Dylan Beaudette dylan.beaudette at gmail.com
Mon Feb 5 16:52:38 EST 2007

On Thursday 01 February 2007 02:28, Miguel Correia wrote:
> Hello.
> I want to estimate off-path travelling with Tobler's hiking function. This
> function says that, for example, in a slope value of 10º the travelling
> velocity is 3Km/hr and in a slope value of -10º the velocity is 4km/hr.
> This generates a friction surface, in which the cost value is the cost of
> traversing that cell in Km/hr. Then, through r.mapcalc I divide the
> distance (cell resolution) by this friction surface to generate another
> friction surface with the corresponding time values.
> The aim of all this is to generate an accumulative cost surface (using
> r.cost with –k flag) from a given point and then get the isochrones (of 1
> hour, 2 hours, etc.).
> The problem is that the function needs negative slope values if the hiker
> goes downhill or positive values is the hiker goes uphill. Has you might
> imagine, If the slope surface only have positive values is the same that
> hiker would always go uphill and never downhill!
> How can Grass resolve this?
> Miguel.

This might be a good opportunity to modifiy r.walk - with the addition of 
Tobler's hiking function. Have a look at the source code, and chat with 
Markus for the details. 


Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis

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