[GRASS-user] Tobler's hiking function

[mlage at usal.es]

Thank you very much, Glynn.
The original Hiking Function is:

W= 6 exp {-3.5*abs(Slope + 0.05)}

Through this I want to estimate anisotropic cost movements in friction
surfaces.
Maybe the problem is that I haven't understood the r.mapcalc syntax... how
would you express this in the command line? Do you have any idea?
Thank you very much for your help.
Miguel Lage.


----- Original Message -----
From: "Glynn Clements" <glynn at gclements.plus.com>
To: <mlage at usal.es>
Cc: <grassuser at grass.itc.it>
Sent: Thursday, January 25, 2007 9:30 PM
Subject: Re: [GRASS-user] Tobler's hiking function



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>