[GRASS-user] Interpreting r.watershed accumulation map

[Anna Petrášová]

On Mon, Oct 10, 2016 at 3:03 PM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Mon, 10 Oct 2016, Laurent C. wrote:
>
>> According to the r.watershed manual: "Output accumulation map contains the
>> absolute value of each cell in this output map is the amount of overland
>> flow that traverses the cell. This value will be the number of upland
>> cells plus one if no overland flow map is given. If the overland flow map
>> is given, the value will be in overland flow units. Negative numbers
>> indicate that those cells possibly have surface runoff from outside of the
>> current geographic region. Thus, any cells with negative values cannot
>> have their surface runoff and sedimentation yields calculated accurately.
>
>
>   Yep, that's what I read, too.
>
>> So basically the darker the cell, the larger the drained area. Darker
>> cells therefore show the flow courses according to the topography.
>
>
>   I thought this to be the case, but the legend doesn't reflect it.

the color table is logarithmic, so try to use logarithmic option in
d.legend (GRASS 7.2 and higher only)


Anna

>
>> Note that Itzï does not need an output to run. If you set a simulation
>> with only the DEM, an uniform friction coefficient and an uniform
>> rainfall, the water will simply accumulate in the depressions, giving you
>> an insight of the flow patterns.
>
>
>   Now, this is very useful information that I did not pick up from reading
> the manual page.
>
> Thanks, Laurent,
>
>
> Rich
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user