[GRASS-user] r.watershed: Interpreting Visual Output Map

[Markus Metz]

Rich Shepard wrote:
> On Tue, 23 Feb 2010, Markus Metz wrote:
>
>> Almost right. ...with surface flow accumulation between 0 and the 
>> threshold value.
>
> Markus,
>
>   Now I understand, yet still miss the difference between the 
> accumulation
> map and the visual map.
>
>> All flow accumulation values larger than the threshold value are set 
>> to the threshold value.
>
>   Got that from the man page.
I forgot, the difference between flow accumulation and visual output is 
not only that all flow accumulation values larger than the threshold 
value are set to the threshold value, but also that all negative values 
are converted to positive values.
>
>> Number of cells draining through a given cell. Converted to squared map
>> units by multiplying with nsres * ewres, of the flow accumulation map of
>> course, and first setting the computational region to match the flow
>> accumulation map with g.region -p rast=<my_flow_acc>. -p to check that
>> everything is all right.
>
>   I assume that the above calculation is left for me to do; that the
> module does not do the math before displaying the results. Am I 
> correct in
> my assumption?
Yes.
>
>   One last point: why no color in the legend? There is color on the 
> visual
> map but not on the accompanying legend. Strange and I don't know how 
> to fix
> this.
Must be a problem of d.rast.leg, no quick help from my side, sorry!

Markus M