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

Daniel Victoria daniel.victoria at gmail.com
Thu Feb 1 06:24:59 EST 2007

```The way I'd do it is calculate the aspect (r.slope.aspect). From the manual:

--------------------
The aspect categories represent the number degrees of east and they
increase counterclockwise: 90deg is North, 180 is West, 270 is South
360 is East, and the aspect value 0 is used to indicate undefined
aspect in flat areas with slope=0.
--------------------

Then you just have to know the general orientation of the trail and
you can come up with a rule in r.mapcalc and multiply all uphill areas
of the trail by -1. Of course, if it's a circular trail or a very
curvy one, then this might not work...

Cheers
Daniel

On 2/1/07, Miguel Correia <miguellage.rc at gmail.com> 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.
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```