[GRASS-user] What algorithm does r.drain use?

GRASS GRASS user
1

Hello. I'm sorry by my poor English.

I have analyzed distribution of wild animals using r.drain now
(http://grass.osgeo.org/grass65/manuals/r.drain.html).

Some examples have been described to "EXAMPLES" in an
online manual. In "EXAMPLES", there are two ways of cost
value though the Cost value of fifith step is 10. In this case,
I do not think that there is a problem because there is a
smaller cost value since six steps even if the fifth step
selects either.
  However, by what algorithm has the cell been selected
when there are two or more end points in route
(There is a minimum same cost value as surroundings of cell. )?

Thanks

Takenori K.

2

On Wed, Jan 7, 2015 at 7:27 AM, Takenori KANAI <kanai@ube-c.ac.jp> wrote:

Hello. I'm sorry by my poor English.

I have analyzed distribution of wild animals using r.drain now
(http://grass.osgeo.org/grass65/manuals/r.drain.html).

Some examples have been described to "EXAMPLES" in an
online manual. In "EXAMPLES", there are two ways of cost
value though the Cost value of fifith step is 10. In this case,
I do not think that there is a problem because there is a
smaller cost value since six steps even if the fifth step
selects either.
However, by what algorithm has the cell been selected
when there are two or more end points in route
(There is a minimum same cost value as surroundings of cell. )?

To my knowledge there is no scientific paper available related to the
r.drain algorithm.
It has been written by Roger S. Miller in 2001 (replacing an older and
less functional version).

You may see the internals here in the code base:
http://trac.osgeo.org/grass/browser/grass/trunk/raster/r.drain

Maybe other users know more about this algorithm family?

Best
Markus

3

On Wed, Jan 7, 2015 at 7:27 AM, Takenori KANAI <kanai@ube-c.ac.jp> wrote:

Hello. I'm sorry by my poor English.

I have analyzed distribution of wild animals using r.drain now
(http://grass.osgeo.org/grass65/manuals/r.drain.html).

Some examples have been described to "EXAMPLES" in an
online manual. In "EXAMPLES", there are two ways of cost
value though the Cost value of fifith step is 10. In this case,
I do not think that there is a problem because there is a
smaller cost value since six steps even if the fifth step
selects either.
However, by what algorithm has the cell been selected
when there are two or more end points in route
(There is a minimum same cost value as surroundings of cell. )?

r.drain simply follows the lowest cost in the surrounding cells (must
be lower than the current cell). If more surrounding cells have the
same minimum cost, the first cell encountered is used.

Markus M

Thanks

Takenori K.
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4

You might also take a look at the r.connectivity.* tools:

http://grass.osgeo.org/grass65/manuals/addons/r.connectivity.corridors.html

It looks like it uses the igraph package in R, which has good documentation.

Regards,

Will

···

On Thu, Jan 8, 2015 at 6:03 PM, Markus Metz <markus.metz.giswork@gmail.com> wrote:

On Wed, Jan 7, 2015 at 7:27 AM, Takenori KANAI <kanai@ube-c.ac.jp> wrote:

Hello. I’m sorry by my poor English.

I have analyzed distribution of wild animals using r.drain now
(http://grass.osgeo.org/grass65/manuals/r.drain.html).

Some examples have been described to “EXAMPLES” in an
online manual. In “EXAMPLES”, there are two ways of cost
value though the Cost value of fifith step is 10. In this case,
I do not think that there is a problem because there is a
smaller cost value since six steps even if the fifth step
selects either.
However, by what algorithm has the cell been selected
when there are two or more end points in route
(There is a minimum same cost value as surroundings of cell. )?

r.drain simply follows the lowest cost in the surrounding cells (must
be lower than the current cell). If more surrounding cells have the
same minimum cost, the first cell encountered is used.

Markus M

Thanks

Takenori K.

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