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

Miguel Correia miguellage.rc at gmail.com
Thu Feb 1 06:50:04 EST 2007

```Hi Daniel.
You say

"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"

It seams to me that you might be right, or at least what you say makes
sense. But I have some doubts:
How can I get to know this general orientation of the trail?
Could you tell me more about that r.mapcalc rule to multiply all uphill
areas of the trail by -1?

The only thing that I don't understand is that if you multiply all uphill
areas by -1, than it's like having all hills downhill, and the problem is
that when a hiker goes uphill there must be a positive slope value, and when
the hiker goes downhill I must have negative slope values.

Thanks a lot!

2007/2/1, Daniel Victoria <daniel.victoria at gmail.com>:
>
> 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.
> > _______________________________________________
> > grassuser mailing list
> > grassuser at grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
> >
> >
>
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