[GRASS-user] flow accumulation (GRASS and Esri ArcGIS)

[Markus Metz]

On Wed, May 2, 2018 at 8:29 PM, Anna Petrášová <kratochanna at gmail.com>
wrote:
>
> On Wed, May 2, 2018 at 2:21 PM, Markus Neteler <neteler at osgeo.org> wrote:
> > On Wed, May 2, 2018 at 7:13 PM, Martin Landa <landa.martin at gmail.com>
wrote:
> >> Hi,
> >>
> >> one of my colleague is dealing with flow accumulation calculation in
> >> GRASS, and comparing results with Esri ArcGIS (builtin Flow
> >> Accumulation [1]). Tool in ArcGIS to me knowledge uses D8, so we run
> >> r.watershed with -s flag and also -a flag to avoid negative values.
> >> Resultant values are very different.
> >>
> >> * Esri ArcGIS (range: 0 - 20741)
> >> * r.watershed -s (range: -34068 - 5697)
> >> * r.watershed -sa (1 - 34068)
> >
> > I'd suggest to look at the univariate statistics first (r.univar -e)
> > and compare that instead of min/max only.
>
> Also, r.watershed uses least-cost path, unlike the arcmap tool. Did
> you fill the depressions before running the arcmap tool?

If you want to get nearly identical results, use r.hydrodem to remove sinks
from the DEM, then use this conditioned DEM as input for both ArcGIS and
r.watershed. Note that with D8 there is some randomness about where surface
flow will go to in case of several neighbors with the same elevation. In
any case, if surface accumulation of ArcGIS and r.watershed differ,
literature cited in the corresponding manuals should allow to decide which
method is more appropriate for the current task.

Markus M

>
> Anna
>
> >
> > markusN
> >
> >> Do you have idea why results are so different? Thanks for any pointers
> >> in advance! Martin
> >>
> >> [1]
http://desktop.arcgis.com/en/arcmap/10.5/tools/spatial-analyst-toolbox/flow-accumulation.htm
> >>
> >> --
> >> Martin Landa
> >> http://geo.fsv.cvut.cz/gwiki/Landa
> >> http://gismentors.cz/mentors/landa
> >> _______________________________________________
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> >
> >
> >
> > --
> > Markus Neteler, PhD
> > http://www.mundialis.de - free data with free software
> > http://grass.osgeo.org
> > http://courses.neteler.org/blog
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