[GRASS-user] Watershed delineation result problem

[Rich Shepard]

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