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

[Markus Metz]

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
4) use the second polygon as a MASK and get the coordinates of the
cell with the lowest cost
5) use these coordinates as start coordinates for r.drain, together
with the movement direction map and the cumulative cost map produced
by r.drain

The result should be a line connecting the two polygons

HTH,

Markus M

>
> 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.
>
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