[GRASS-user] v.water.outlet

[Markus Metz]

On Fri, Jan 6, 2012 at 9:42 AM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 06/01/12 09:23, Markus Metz wrote:
>>
>> On Thu, Jan 5, 2012 at 12:12 PM, Massimiliano Cannata
>> <massimiliano.cannata at supsi.ch>  wrote:
>>>
>>> Moreover,
>>> check that your dem has sinks filled (r.fill.dir)
>>
>>
>> Since the drainage map has been created by r.watershed, sink filling
>> is not only not needed but does more harm than good and should not be
>> used to prepare a DEM for r.watershed.
>
>
> for r.water.outlet I suppose...
>
> Could you explain why ?

Because r.water.outlet generates a watershed basin from a drainage
direction map from r.watershed (says the manual), more generally,
r.water.outlet uses as input map only a drainage map and not a DEM,
thus it does not matter where the drainage map comes from, it could in
theory also be properly encoded moving directions from a cost surface
analysis. As pointed out by Micha and you, care must be taken with the
coordinates, otherwise an unexpectedly small basin will be delineated.
Additionally, r.water.outlet will most probably not work properly if
the current region does not match exactly the drainage map.

r.watershed has been designed from the very beginning by Chuck
Ehlschlaeger in 1989 to work with raw DEMs and to produce realistic
output without sink filling (see references in the manual).

Markus M