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

[Miguel Correia]

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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