[GRASS-user] Tobler's hiking function

GRASS GRASS user
1

Hello
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)))

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”

slopemap it’s for S and 0.025 it’s for D.

What do you think about this? Am I "translating" it well?I would
appreciate you expert opinion.

Thank you!!!
Miguel Lage.

2

mlage@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@gclements.plus.com>

3

I am trying to express Tobler's Hiking Function with r.mapcalc.

see also "r.walk":
"Outputs a raster map layer showing the anisotropic cumulative cost of
moving between different geographic locations on an input elevation
raster map layer whose cell category values represent elevation combined
with an input raster map layer whose cell values represent friction
cost."

Hamish