[GRASS-user] movement direction surface in r.drain

[Margherita Di Leo]

Markus,

Thank you for replying and sorry for delay,

On Fri, Dec 28, 2012 at 4:37 PM, Markus Metz
<markus.metz.giswork at gmail.com>wrote:

> On Wed, Dec 19, 2012 at 3:25 PM, Margherita Di Leo <diregola at gmail.com>
> wrote:
> > Hi All,
> >
> > I have a friction map, a start polygon and an end polygon and would like
> to
> > find the least cost path, meaning not a corridor but a thin line. To now
> > I've tried both r.cost and r.walk and r.drain eventually, but the result
> of
> > this latter
> >
> > r.drain input=friction at PERMANENT output=rdrain_out
> > vector_points=start_line at PERMANENT
> >
> > is a path calculated *within* the start polygon (a closed line).
> > I suppose this is because I don't provide a direction map to r.drain (I'm
> > using GRASS 7). I read:
> >
> > If the input surface (a raster map layer) is a cumulative cost map
> generated
> > by the r.walk or r.cost modules, the -d flag and a movement direction
> > surface "indir" must be specified.
> >
> > I assume it's a kind of flow direction or aspect, but what is the
> physical
> > meaning, and how can I calculate it? Could anyone please provide some
> > further info.
>
> The physical meaning is movement direction in degrees CCW from East,
> similar to e.g. flow direction output of r.watershed. The direction
> map is an optional output of r.cost and r.walk, and should be used
> with the indir option for r.drain.
>
> The workflow would thus be:
>
> 1) convert the first polygon to a raster map
> 2) use all cells of the first polygon as start points for r.walk,
> include the friction map
> 3) convert the second polygon to a raster map
>

quite plain up to this point, but:


> 4) use the second polygon as a MASK and get the coordinates of the
> cell with the lowest cost
>

The cost map obtained by r.walk happens to be all 0 within the ending
polygon. How should I choose my coordinates in this case?

Thanks in advance.

Best,
madi

-- 
Margherita DI LEO
Postdoctoral Researcher

European Commission - DG JRC
Institute for Environment and Sustainability (IES). Unit H03 – FRC
Via Fermi, 2749
I-21027 Ispra (VA) - Italy - TP 261

Tel. +39 0332 78 3600
margherita.di-leo at jrc.ec.europa.eu

Disclaimer: The views expressed are purely those of the writer and may not
in any circumstance be regarded as stating an official position of the
European Commission.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/grass-user/attachments/20130108/161db5ec/attachment.html>