[GRASS-user] r.drain problem when flow reach the border

Markus Metz markus.metz.giswork at gmail.com
Thu Dec 27 08:13:20 PST 2012

On Wed, Dec 26, 2012 at 10:01 PM, Milton Cezar Ribeiro
<miltinho.astronauta at gmail.com> wrote:
> Hi Mohamed and Markus
> Mohamed, thanks for your reply, I will wait for a solution within GRASS :-)
> Markus, thanks for your reply,. Sorry  but I disagree a tiny bit,
> because is expected that between a source and target I will find a
> solution with the Least Cost path. Do you think that is there any
> situation where a least cost between source and target will not be
> found? Maybe if other software provide the soluction, although is not
> the bestest option, I prefere receive one complete output than a
> broken one.

Ah, you are right, if the input is output from r.cost, the cost
directions should be used. Depending on the input data and the region
extents, the least cost path found may not be the real least cost
path, but there should be one solution.

There are situations where there is no connection between source and
target if source and target are separated by NULL cells and no NULL
cost is specified to r.cost.

Markus M
> Happy new year!
> miltinho
> 2012/12/23, Markus Metz <markus.metz.giswork at gmail.com>:
>> On Sun, Dec 23, 2012 at 7:07 PM, Milton Cezar Ribeiro
>> <miltinho.astronauta at gmail.com> wrote:
>>> Dear all,
>>> I am running a least cost path analysis combining r.cost and r.drain.
>>> But on some cases, when the least cost path between source and target
>>> reach the border of my map, r.drain stop on that position, and target
>>> point isn´t reached.
>> This is correct because at the borders, the direction can not be
>> determined, the path could lead out of the current region, you can not
>> tell.
>> Running on other softwares the algorithms the
>>> source and target is connected using edges without problem.
>> If in doubt, I would call it a bug in that other software.
>>> Any hint are welcome.
>> Try to expand the current region
>> Markus M
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