[GRASS-user] Tobler's hiking function

[Glynn Clements]

mlage at usal.es wrote:

> I am trying to express Tobler’s Hiking Function with r.mapcalc.
> This is the expression that I’m trying to apply:
> 
> T = D / (6 exp (-3.5) * abs (S+0.05)))

What do you mean by "6 exp (-3.5)"? "6 ^ (-3.5)" or "6 * e^(-3.5)"?

> T= time to cross the cell
> D=resolution of the cell (in my case 25m)
> S=slope in percent
> 
> I’m using it to calculate the time needed to cross a cell through the
> relation between the traveling speed (in Km/hr) and the slope to generate
> a friction surface.
> 
> After creating the slope map, I “translated” it like this:
> 
> r.mapcalc newmap=“(6 exp(-3.5*(abs(slopemap)+0.05)))/0.025”

This isn't valid r.mapcalc syntax; if the 6 is supposed to be
multiplied by the result of exp(), you need a "*" in there.

-- 
Glynn Clements <glynn at gclements.plus.com>