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

Margherita Di Leo diregola at gmail.com
Wed Dec 19 06:25:57 PST 2012

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<http://grass.osgeo.org/grass70/manuals/r.walk.html>
* or *r.cost <http://grass.osgeo.org/grass70/manuals/r.cost.html>* 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.

Thank you in advance


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.
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