[GRASS-user] Watershed delineation result problem

[ming han]

Hi Rich

    Thanks for sharing, I see, I will continue with GRASS version I guess.

Cheers
Ming

Rich Shepard <rshepard at appl-ecosys.com> 于2020年10月7日周三 下午1:42写道：

> On Wed, 7 Oct 2020, ming han wrote:
>
> >   Hope this email find you safe and healthy
> >
> >   I tried watershed delineation with both ArcGIS and GRASS. but I got
> > different results.
> >
> >   I was using D8 flow direction method in both ArcGIS and GRASS. My study
> > area is very flat. Is there any tool in GRASS will generate the same
> result
> > with ArcGIS
>
> Ming,
>
> I did wetland determinations and delineations for almost a decade but
> dropped it when it became a commodity. I offer two points for your
> consideration:
>
> 1) The ArcGIS may not be any more 'accurate' than the GRASS results. Never
> having used the former I cannot comment on how they do this.
>
> 2) Of greater importance is that you can pick either one; it doesn't matter
> in the real world. In 1994 I was the first environmental consultant
> authorized by Oregon's Department of State Lands to use GPS receivers to
> delineate wetland boundaries. They had insisted that only professional land
> surveyors could do this and they set a 2cm accuracy standard. Really?
> Wetland boundaries are transistion zones that can be several meters wide,
> depending on topography, soils, and antecedent precipitation conditions
> when
> the boundary is flagged. A stream bank is an exception to this broad
> transition area. When I made the case that there is no sharp line of
> demarkation between wetland and upland they accepted my delineations.
>
> Of similar disconnect between engineering and natural ecosystems, I worked
> for a brief time for a water management district in the 1980s. They decided
> to digitize the 7.5 min (1:24000) topographic maps covering the District's
> area and contracted with a company in India to do the work. The contract
> specified that the digitized lines had to aline withine 1/2 the width of
> roads and other boundaries on the maps. When I pointed out to my Division
> Director that the maps themselves said "this map is accurate to +/- 24
> feet"
> so they were trying to be more accurate than the maps themselves it was not
> well received. :-) (That's one reason I left a government position.)
>
> Anyway, draw your boundary and in most cases you'll be within that
> transition zone.
>
> HTH,
>
> Rich
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