| Re: triangulating over undefined space in irregular grids [message #59442 is a reply to message #59440] |
Wed, 26 March 2008 09:48   |
Kenneth Bowman
Messages: 86
Registered: November 2006
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Member |
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In article <419932f8-47d6-4822-aa67-6f6e235ef003@n77g2000hse.googlegroups.com>,
bjelley@worldwindsinc.com wrote:
> The setup: I am trying to contour plot a number of variables for a
> storm surge grid that has up to 10 km resolution in the open Atlantic,
> yet has 8 meter resolution in New Orleans area waterways. The grid
> obviously includes most anything at or below sea level for the
> Northwest Atlantic domain, but also includes many (200k or so) land-
> based grid points (or it would be useless as a storm surge model). I
> mention the land-based nodes to express that a coastline mask will not
> fix the problem.
>
> The problem: When plotting the results using the triangles returned,
> the result includes vast areas that are outside of the model domain.
> For example, the plot shows data in trangles running from central
> Louisiana to New England, yet the grid does not include any land with
> an elevation greater than 20 meters. So the grid does include some
> land for a range of distances from the coast, from ~5 to ~100 km
> inland based on elevation.
I think your main problem is that TRIANGULATE computes the convex hull of
the set of points. That is, there are no "bays" or concave regions in the
resulting set of triangles. This produces the long narrow triangles that
you see across the SE U.S.
> Also of importance is that the grid is numbered and ordered in a
> counter-clockwise fashion, not in any order of south to north and the
> like. This is native to the model which uses a finite element method
> for calculations allowing us to successfully resolve conservation of
> momentum, velocity, etc at very high resolutions in areas of interest
> while maintaining the ability to carry out computation at lower
> resolutions over larger areas where mass conservation is
> required...such as the Gulf of Mexico. Could I reorder for purposes of
> plotting? Sure, but do I need to and where would it get me?
>
> I have been through Dr. Bowman's book, Liam Gumley's book, David
> Fanning's site, the astro site, the online help, and the German
> library of IDL routines with no light shed on the solution.
>
> I have fiddled with the tolerance variable passed to triangulate which
> has not changed anything until I make it too large at which time...
> % TRIANGULATE: Points are co-linear, no solution.
You might also have some round-off problems, as your grid spacing spans
4 orders of magnitude 10 to 10^4 m. Are your inputs double precision?
Your best bet may be to learn how to use object graphics polygon objects,
but I'm afraid I can't help you there.
Ken Bowman
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