# RE: [OpenMap Users] watershed area, dem

Date: Tue, 30 Nov 2004 13:47:19 -0000

Hi,
Thank you for all your information.
The usual algorithm it is used to calculate slope and flow path is the 8
point method.
http://www.gishydro.umd.edu/documents/train_manual/old/dems.pdf

You've just to Loop though each Point in the grid to calculate first the
slope and then, the direction of the flow (from your point to the lowest of
the eighth surrounding it). Usually you give a value to each point of the
grid according to the flow path direction 1,2,5,8,16,32,64,128 from East to
North East. The watershed is made by all the point that flow trough the
others to the main outlet point. I guess that stating from the outlet point
you go backwards to the points that flow directly or indirectly in it.
The problem in the algorithm is when there is a sink (or horizontal area),
and you can not know the real flow path, then there are quite few models for
correction of the horizontal areas, like TOPAZ
http://gcmd.nasa.gov/records/TOPAZ.html (also free).
In the Grass project they have alredy implemented a similar algorithm, but I
would like to try to do it in OpenMap, to have a java based one.
As you suggest to start I'll try to write a DEM driver for OpenMap, but I'm
not a very experienced programmer, so it will take a while.
Please, let me know if you find out something else.

Cheers,
Andrea

-----Original Message-----
From: Chapman, Martin [mailto:MChapman_at_sanz.com]
Sent: 30 November 2004 06:35
To: Andrea Leone; openmap-users_at_bbn.com
Subject: RE: [OpenMap Users] watershed area

Andrea,

Now that I think about it harder, the strategy I gave you was more of a
flood plane solution I guess. In order to calculate slope and a descending
path you will need more of a mesh object or scene graph class. Once it was
in a structure like that you could traverse the mesh and run tests on the
edge of each triangulated face.

Martin

-----Original Message-----
Sent: Mon 11/29/2004 5:12 PM
To: openmap-users_at_bbn.com
Cc:
Subject: [OpenMap Users] watershed area
Hi Don,
The algorithms to calculate watershed area are based on elevation data. I
would need a grid of elevation data to perform slope, flow paths and
draining area calculations.

Cheers,
Andrea

-----Messaggio originale-----
Da: owner-openmap-users_at_bbn.com [mailto:owner-openmap-users_at_bbn.com] Per
conto di Don Dietrick
Inviato: luned́ 29 novembre 2004 22.03
A: Andrea Leone
Cc: openmap-users_at_bbn.com
Oggetto: Re: [OpenMap Users] FW: distance along set of lines

Hi Andrea,

What's your criteria for designating a watershed area based on
elevation data?

Just so you know, the TerrainLayer uses DTED data instead of DEM, but
that should be OK, too.

- Don

On Nov 28, 2004, at 11:45 AM, Andrea Leone wrote:

> Hi people,
>
> Does somebody have some idea on how to delineate watershed areas using
> DEM data in OpenMap.
>
> I guess that the Terrain layer could be a good starting point.
>
>
>
>
>
> Thank you,
>
> Cheers, Andrea.
>
>
>
>
>
>
>

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Don Dietrick, dietrick_at_bbn.com
BBN Technologies, Cambridge, MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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Received on Tue Nov 30 2004 - 08:47:47 EST

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