[GRASSLIST:9457] Re: r.walk and r.drain, and some other questions

[Markus Neteler]

On Thu, Dec 08, 2005 at 10:12:35PM -0700, Tom Russo wrote:
> Funny, just today I was starting to think about how to develop a friction
> cost map using an elevation model and other data to compute a least-cost
> path for walking in wilderness areas (i.e. with no established roads or
> trails -- just finding the easiest way to get from here to there given
> the terrain, vegetation, hydrography, etc.).  Tonight I updated CVS and found 
> a new module called r.walk that appears (from what info there is so far) to be 
> very much related to exactly what I was pondering.  Marcus, get out of 
> my head!

:-)
I also added the description now.
 
> Anyhow, even without the mysterious and timely arrival of this new module,

strange things happen on this planet...

> I still had a question about how to accomplish something that I'd just 
> read about today as a related task in ArcGIS.  Perhaps someone can 
> help me?

I BCC to a colleague here who is probably thinking about the same
thing. Maybe he'll answer you or to the list.
 
> The paper I was reading was about computing paths through wilderness areas
> to minimize the work needed to visit a small number (N) of sites starting from 
> the N+1th site.  They did it by computing a friction surface for the region, 
> and generating N+1 cost surfaces (each one the cost surface with a given site 
> at origin).  They used the N cost surfaces to compute N(N+1) least-cost 
> paths from each point to each of the others.
> 
> So far so good --- I see that in GRASS I'd have used r.cost if I were using
> their particular friction surface (which I think is far too simplistic as 
> it is isotropic), and r.drain to compute a raster version of the least cost 
> path.  And now that there's an r.walk (if it does what it seems to say it does) I won't have to figure out how to use r.spread and r.spreadpath to do this
> with anisotropic costs due to the terrain.
> 
> But *then* they generated a vector network of paths and used network analysis
> to compute the optimum route that minimized the work needed to visit the
> various sites --- a travelling salesman problem, basically, but not only 
> trying to pick the right order of things to visit, but the best path
> through the terrain to use to get there with the least effort and time.
> 
> I can see how I might generate such a vector network, by using r.to.vect
> on each of the r.drain output rasters and then patching together the 
> vectors --- but how in the world could one attach cost columns to the vectors?
> r.drain appears to be able to produce a raster where the pixels in the 
> least-cost path are the cost to get from the origin to that pixel, but
> it doesn't seem like there's any way to get that information into a 
> vector created from that raster.  Hints?

I'm too tired now to clearly read above but I will submit a patch tomorrow
to make r.drain accept a vector path. Maybe it's what you need, maybe not.

> Assuming that there is in fact a way to convert the r.drain raster into
> a vector with costs attached to the line segments, it shouldn't be that
> difficult to do what these folks had done.  Thoughts?
> 
> -- 
> Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
> Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 
>  "The only thing you can do easily is be wrong, and that's hardly
>   worth the effort." -- Norton Juster
>

Markus

-- 
Markus Neteler     <neteler itc it>       http://mpa.itc.it
ITC-irst -  Centro per la Ricerca Scientifica e Tecnologica
MPBA - Predictive Models for Biol. & Environ. Data Analysis
Via Sommarive, 18        -       38050 Povo (Trento), Italy