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| Subject: | Re: GISList: flow convergence and grids |
| Date: |
04/07/2003 03:55:01 PM |
| From: |
Peter Shary |
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Jennifer,
An interesting task, but not for ESRI's software. Wind
magnitude and direction are components of some
two-dimensional vector. You may wish to interpret it as
gradient and to restore its potential function (something like
atmospheric pressure that causes wind). Strictly speaking,
this is incorrect theoretically (because your vector field may
not be a potential field), but you have finite number of
grid points, and you can use this approach to study flow
convergence and divergence.
After doing so, calculate plan curvature for your new grid
of this "potential" using even ESRI's software, because
flowline divergence is equal to plan curvature as proven
by Theorem 4 of this paper:
Shary, P.A., 1995. Land surface in gravity points
classification by a complete system of curvatures.
Mathematical Geology 27(3): 373-390.
Plan curvature probably will not satisfy you, because it does
not sum flows, is too sensitive to errors in data, and does not
take into account important depressions in your "potential"
(pressure). To have much better solution, you should change
this approach by another one using so-called catchment area,
but this is out of ESRI's competence. To my knowledge, they
use only some "alchemy" in this direction on Silicon Graphics.
This is why I've mentioned that your task is not for ESRI's
software. You may learn the sense of catchment area and its
scale-free features in the following paper:
Shary, P.A., Sharaya, L.S., Mitusov, A.V., 2002.
Fundamental quantitative methods of land surface
analysis. Geoderma 107(1-2): 1-32.
Together with catchment area, you will necessarily calculate
depressions in your "potential". They are of great
importance in land surface (Great Lakes, large deserts,...),
why not in atmospheric pressure ?
This is the solution to your task, as I see it. You may
wish to see map images at my website
http://members.fortunecity.com/eco4/giseco/
or download free full-featured DEMO
http://www.rockware.com/catalog/pages/giseco.html
that performs what you are interesting in.
By the way, you're trying to study divergence, why not
rotor? Its effects are essential in atmosphere.
I'm doing this, it doesn't matter for which surface:
land, human skin, pressure.
Hope this helps.
Peter
************************
Peter A. Shary,
scientific researcher and GIS developer
Institute of physical, chemical and biological
problems of soil science
of the Russian Academy of Sciences
142290 Poushchino
Moscow region
Russian Federation
Phone: +7 0967 733604
Fax: +7 0967 790595
************************
>From: "Jennifer Horsman" <jenh@gust.sr.unh.edu>
>Reply-To: "Jennifer Horsman" <jenh@gust.sr.unh.edu>
>To: "GISlist" <gislist@geocomm.com>
>Subject: GISList: flow convergence and grids
>Date: Thu, 3 Apr 2003 13:55:44 -0500
>
>Does anyone know of a utility which allows calculation of flow convergence
>and/or divergence from a grid with azimuth (angle of direction) and
>magnitude of flow values at each cell (I am working with wind data)? I have
>ESRI ArcGIS.
>
>Thanks,
>Jennifer
>
>
>
>
>
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