[GRASS-user] r.watershed: output basin map

[Margherita Di Leo]

On Thu, Apr 19, 2012 at 5:58 PM, Rich Shepard <rshepard at appl-ecosys.com>wrote:

> On Thu, 19 Apr 2012, Margherita Di Leo wrote:
>
>  Please post information about the region, r.info of the map and the
>> command line of r.watershed that you use. Also, do you use a mask? Pankaj
>> Kr Sharma, please avoid to reply attaching the whole digest.
>>
>
> madi, Micha, Pankaj:
>
>  The Level 6 HUC basins (a.k.a. HUC-12 or sub-watersheds) are defined by
> the USGS to enclose areas of 10k-40k acres (15.625-62.5 square miles;
> 40.5-161.9 square kilometers). These are a bit too large, but vector
> boundaries for 14- or 16-digit hydrologic units are not available for this
> area (or for my other project area which is approximately 125 square miles,
> 323.75 square kilometers) so I tried different threshold values for
> r.watershed and 150000 seems to provide additional resolution without
> generating a separate drainage for each individual stream segment. Yes,
> it's
> arbitrary. But for the purposes of these two projects it is a practical
> balance between too coarse and too fine.
>
>  The region was set by the base, 10m DEM:
>
> projection: 99 (Lambert Conformal Conic; units in International Feet)
> zone:       0
> datum:      nad83
> ellipsoid:  grs80
> north:      735780
> south:      655650
> west:       2042100
> east:       2138580
> nsres:      30
> ewres:      30
> rows:       2671
> cols:       3216
> cells:      8589936
>
>  The r.watershed command line (all on one line but wrapped in the MUA):
>
> r.watershed -f elev=dem10m.avg acc=dem.acc drain=dem.drain bas=dem.basin
> stream=dem_str.seg thresh=150000 --o
>
>  And r.info for dem.basin:
>
>   Type of Map:  raster               Number of Categories: 94
>   Data Type:    CELL
>   Rows:         2671
>   Columns:      3216
>   Total Cells:  8589936
>        Projection: Lambert Conformal Conic
>            N:     735780    S:     655650   Res:    30
>            E:    2138580    W:    2042100   Res:    30
>   Range of data:    min = 2  max = 94
>   Data Source:
>    dem10m.avg
>   Data Description:
>    generated by r.watershed
>   Comments:
>    Processing mode: MFD
>    Memory mode: All in RAM
>    r.watershed -f elevation="dem10m.avg" accumulation="dem.acc" drainag\
>    e="dem.drain" basin="dem.basin" stream="dem_str.seg" threshold=15000\
>    0 convergence=5 memory=300
>
>  If the white areas with vector streams (the blue lines) represent basins,
> then that's OK. What puzzles me is why a couple of basins would be white
> while the others are different colors and the small white areas without
> mapped streams are appently external to the basins of interest.
>

You can query the white cells and discover that they are null values. This
is because r.watershed delineates only the basins that fall entirely into
the computational region. You didn't provide information about any mask,
but you probably either set a too small mask or should enlarge the
computational region in order to delineate also the other basins. In
addiction, consider to provide at least a small buffer along the ridge line
in order to allow the algorithm to work properly.

Regards,
madi

>
>  As far as I know, there's no attribute table for raster maps that hold
> descriptions of the generated basins, only information on the contents of
> each cell.
>
> Rich
>
>
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>



-- 
Ing. Margherita Di Leo, Ph.D.
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