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

[Radek Novotný]

 Thanks all for your suggestions!

I created a comparison of methods [1] which were mentioned. But
unfortunately, the results are still very different (at least from my point
of view). What do you think about it? What approach should be chosen?

[1] https://docs.google.com/document/d/15MyV-l2BH5agjN1T3LtQBotH7RJsIL_
5K7exSZypTnQ/edit?usp=sharing


2018-05-02 22:17 GMT+02:00 Markus Metz <markus.metz.giswork at gmail.com>:

>
>
> 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
> > >> _______________________________________________
> > >> grass-user mailing list
> > >> grass-user at lists.osgeo.org
> > >> https://lists.osgeo.org/mailman/listinfo/grass-user
> > >
> > >
> > >
> > > --
> > > 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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>
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