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

Michael Barton michael.barton at asu.edu
Thu Feb 1 10:11:23 EST 2007

R.cost creates an isotropic cost surface. If a slope map is used to create a
cost surface using r.cost, All slopes of the same magnitude have the same
costs (i.e., the cost is the same whether you are going uphill or downhill).
This can be OK for 2-way travel (out and back), but can be problematic when
calculating 1-way costs.

As I understand it, r.walk will calculate an anisotropic cost surface--i.e.,
such that uphill and downhill costs are calculated differently. It also
allows for inclusion of an additional friction surface to weight costs and
input of coefficients to automatically calculate walking (or theoretically
other) energy costs.

Multiplying a slope map by -1 would only make all slopes negative rather
than leaving some positive and some negative. You could multiply by a
function of the aspect to make upslope positive, downslope negative, and
'aroundslope' in between.

However, what is uphill and what is downhill will depend on the specific
route. That is, the uphill direction will vary depending on the direction of
travel outward from a point of origin. I believe that r.walk takes this into
account.

Michael

On 2/1/07 1:40 AM, "Gerald Nelson" <gnelson at uiuc.edu> wrote:

> Why do you want negative slope values?
>
> What r.cost does is calculate the smallest cost of moving over a friction
> surface, which could be slope, to one or more destinations.
>
> The manual for the command is here.
>
> http://grass.itc.it/gdp/html_grass63/r.cost.html
>
> Jerry
>
> ---- Original message ----
>> Date: Thu, 1 Feb 2007 09:05:33 +0100 (CET)
>> From: mlage at usal.es
>> Subject: Re: [GRASS-user] negative slope values and movement costs
>> To: grassuser at grass.itc.it
>>
>> Thank you Jerry.
>> How does r.cost produce negative slope values?
>>
>> Miguel
>>
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> 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
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>

__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton