[GRASS-user] "ladders" in watershed delineation

[Micha Silver]

Markus Metz wrote:

>
> r.watershed in 6.5+ can do MFD which reduces these single-cell strings 
> substantially. I would recommend MFD over SFD for anything with 30m 
> resolution or higher, not much gained for 90m or coarser. Results are 
> generally more accurate with MFD for high resolution (<= 30m) DEMs.
>
> Markus M
>

I finally got around to compiling GRASS 6.5, with the r.watershed MFD 
options, and I must say... wow!

First, the memory handling must be much better. I'm working on a region 
with about 80 million cells. Theoretically I should need 2.5 GB to run 
the module in memory. There are many null cells surrounding the actual 
basin, but still, On my aging Pentium 4 with 1 GB memory, using the 
older r.watershed (GRASS 6.4) I would run out of memory, and I'd have to 
use the "seg" mode, which of course took hours and hours.
Now the newer module whips thru the same dataset in about 20 minutes.

And more importantly, using the -f flag (Multi flow direction) the 
original problem of "ladders" or strings of single cells in the 
catchment raster has almost disappeared!
The only remaining problem I noticed is streams that split and 
reconnect. Although this was quite minor.

Best regards,
Micha