# [GRASS-user] negative slope values and movement costs

Gerald Nelson gnelson at uiuc.edu
Thu Feb 1 05:15:53 EST 2007

```Well, given my experience hiking (which includes multiple trips to the Grand Canyon) hiking down and hiking up take the same amount of time, just different muscles get sore. ;-)

But I see your issue now. You need some kind of directional slope. That's beyond me.

Jerry

---- Original message ----
>Date: Thu, 1 Feb 2007 10:53:48 +0100
>From: "Miguel Correia" <miguellage.rc at gmail.com>
>Subject: Re: [GRASS-user] negative slope values and movement costs
>To: grassuser at grass.itc.it
>
>   This is my problem:
>   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!
>
>   Miguel.
>________________
>_______________________________________________
>grassuser mailing list
>grassuser at grass.itc.it
>http://grass.itc.it/mailman/listinfo/grassuser
Gerald Nelson
Professor, Dept. of Agricultural and Consumer Economics
University of Illinois, Urbana-Champaign
office: 217-333-6465
cell: 217-390-7888
315 Mumford Hall
1301 W. Gregory
Urbana, IL 61801

```