[GRASS-user] r.watershed: pretty flat areas

[MS]

Great news.  It sounds like this will be helpful with the very flat  
and wetland rich terrain in Florida, US.

Mark

On May 12, 2010, at 5:51 AM, Markus Metz <markus.metz.giswork at googlemail.com 
 > wrote:

> Hi all,
>
> r.watershed in trunk r42236 has a new option to beautify flat areas,
> activated with a new -b flag, works with both SFD and MFD. The method
> is adapted from Garbrecht & Martz (1997).
>
> The justification for the new option is that some regard straight flow
> lines in flat areas as an issue, e.g.
>
> Nardi et al (2008), Hydrogeomorphic properties of simulated drainage
> patterns using digital elevation models: the flat area issue
> http://dx.doi.org/10.1623/hysj.53.6.1176
>
> For r.watershed, this is not that much of an issue because it does not
> create flat areas by sink filling. For naturally flat areas like lakes
> and broad rivers, it is not possible to determine the "correct"
> drainage direction only from a DEM, information about river and lake
> bottom would be needed to more accurately determine flow direction,
> but DEMs only show water surface (apart from bathymetry). Therefore I
> chose to describe the new option as "Beautify flat areas" and not as
> "correct flat areas". The new option is currently only implemented for
> the all in memory mode, and it requires more memory and more time.
> Contrary to other implementations, this new one modifies elevation
> values only temporarily to adjust drainage direction, flow is
> accumulated using the original elevation values. I'm not really
> convinced about this option, but since methods to modify flat areas
> appear regularly in the literature, I added one. If there is no strong
> support for the new option, I would remove it again. I guess it's
> mostly useful to produce pretty maps.
>
> Markus M
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