Date:         Wed, 12 Mar 2003 14:05:39 -0800
Reply-To:     Dave Hockman-Wert <[log in to unmask]>
Sender:       Biological Conservation and GIS <[log in to unmask]>
From:         Dave Hockman-Wert <[log in to unmask]>
Subject:      Re: hydrologic modeling extension
In-Reply-To:  <[log in to unmask]>
Content-Type: text/plain; charset=us-ascii

Some of you may remember my post a few months ago that
expressed frustration with the ESRI ArcGIS Hydrology
Modeling tool.  In particular, I found the Watershed
button to be most unhelpful (see original post below).

Well, I'm happy to inform this list that Kevin
Johnston and others at ESRI have updated the Hydrology
Modeling tool so that it now includes the option to
accept the actual point that is clicked on.  It can
still "snap" to a point of higher Flow Accumulation as
well, but it doesn't force that as the only option.

Documentation describing the tool and its uses is also
available with the updated version.  You can download
the HydrologyModeling.zip file with all of the
necessary Visual Basic scripts at the ArcObjects
website -- http://arconline.esri.com/ArcObjectsOnline/

Navigate to Samples/Spatial Analyst/Hydrology Modeling
by clicking on the aforementioned folders in the
left-hand column.

ESRI did not update the tool because of my concerns,
but I certainly appreciate the serendipity.

Dave

--- Dave Hockman-Wert <[log in to unmask]> wrote:
> Hi all,
>
> While the hydrologic modeling extension can be very
> helpful, beware the "Watershed" button in ArcGIS.
> This is the one that is set up by entering the Flow
> Direction and Flow Accumulation grids in the
> "Properties" section.  Then, when the "Watershed"
> button is selected, you can click on a point and it
> will produce the "watershed" from that pour point.
>
> Only it doesn't start from the point you actually
> click.  I don't know Visual Basic very well, but I
> can
> read the code well enough to see that it actually
> snaps to the point of highest flow accumulation
> within
> a radius of a certain number of pixels.  (I don't
> know
> how many pixels.)
>
> So what happens, for example, is that you click on a
> point at the downstream end of a tributary, just
> before the junction, and it actually produces a
> watershed starting on a point along the main stem a
> few pixels downstream of the tributary junction.
> This
> is very unhelpful.
>
> In my experience, none of the other commands are a
> problem.  But if you want to generate watershed(s)
> from specific points, use the “watershed” command in
> Map/Raster Calculator.  (Create a grid of pour
> points
> and use that as the source grid.)
>
> If anyone else has had a different experience with
> this button, I’d be happy to hear about it.
>
> Regards,
> Dave Hockman-Wert


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