[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.


---- 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
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

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