<div>Hi Daniel.</div>
<div>You say</div>
<div> </div>
<div>"Then you just have to know the general orientation of the trail and<br>you can come up with a rule in r.mapcalc and multiply all uphill areas<br>of the trail by -1"</div>
<div> </div>
<div>It seams to me that you might be right, or at least what you say makes sense. But I have some doubts:</div>
<div>How can I get to know this general orientation of the trail?</div>
<div>Could you tell me more about that r.mapcalc rule to multiply all uphill areas of the trail by -1?</div>
<div> </div>
<div>The only thing that I don't understand is that if you multiply all uphill areas by -1, than it's like having all hills downhill, and the problem is that when a hiker goes uphill there must be a positive slope value, and when the hiker goes downhill I must have negative slope values.
</div>
<div> </div>
<div>Thanks a lot!</div>
<div><br><br> </div>
<div><span class="gmail_quote">2007/2/1, Daniel Victoria <<a href="mailto:daniel.victoria@gmail.com">daniel.victoria@gmail.com</a>>:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">The way I'd do it is calculate the aspect (r.slope.aspect). From the manual:<br><br>--------------------
<br>The aspect categories represent the number degrees of east and they<br>increase counterclockwise: 90deg is North, 180 is West, 270 is South<br>360 is East, and the aspect value 0 is used to indicate undefined<br>aspect in flat areas with slope=0.
<br>--------------------<br><br>Then you just have to know the general orientation of the trail and<br>you can come up with a rule in r.mapcalc and multiply all uphill areas<br>of the trail by -1. Of course, if it's a circular trail or a very
<br>curvy one, then this might not work...<br><br>Cheers<br>Daniel<br><br>On 2/1/07, Miguel Correia <<a href="mailto:miguellage.rc@gmail.com">miguellage.rc@gmail.com</a>> wrote:<br>> Hello.<br>><br>> I want to estimate off-path travelling with Tobler's hiking function. This
<br>> function says that, for example, in a slope value of 10º the travelling<br>> velocity is 3Km/hr and in a slope value of -10º the velocity is 4km/hr. This<br>> generates a friction surface, in which the cost value is the cost of
<br>> traversing that cell in Km/hr. Then, through r.mapcalc I divide the distance<br>> (cell resolution) by this friction surface to generate another friction<br>> surface with the corresponding time values.<br>
> The aim of all this is to generate an accumulative cost surface (using<br>> r.cost with –k flag) from a given point and then get the isochrones (of 1<br>> hour, 2 hours, etc.).<br>> The problem is that the function needs negative slope values if the hiker
<br>> goes downhill or positive values is the hiker goes uphill. Has you might<br>> imagine, If the slope surface only have positive values is the same that<br>> hiker would always go uphill and never downhill!<br>
> How can Grass resolve this?<br>><br>> Miguel.<br>> _______________________________________________<br>> grassuser mailing list<br>> <a href="mailto:grassuser@grass.itc.it">grassuser@grass.itc.it</a><br>
> <a href="http://grass.itc.it/mailman/listinfo/grassuser">http://grass.itc.it/mailman/listinfo/grassuser</a><br>><br>><br></blockquote></div><br>