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

[Margherita Di Leo]

Oops!
I've made another tentative, I've used r.mask to mask out the area within
the start polygon, but surprisingly I got the same result, as it seems that
the mask does not affect the result at all. IMHO this could be a bug of
r.mask (ok, separate topic), or is there any reason why r.mask shouldn't
affect it?

Thanks

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<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
>
> 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.
>



-- 
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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