| Re: Random problem with Delaunay triangulation - Correction [message #55812 is a reply to message #55811] |
Wed, 12 September 2007 05:14   |
Haje Korth
Messages: 651
Registered: May 1997
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Senior Member |
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Bill,
what you describe seems to pretty much the story of my life with spherical
interpolation in IDL. Basically, I can find after some playing around a fix
for a particular data set only to find that the fix was not universal enough
and the next interpolation with another data set would blow up again. There
must be a certain set of underlying rules for the IDL spherical
interpolation routines. If one would know them, the data set could be
massaged appropriately to avoid the problem. My experience is though that
this goes beyond the $64,000 question.So any enlightenment is appreciated
here.
Haje
"Bill Gallery" <wgallery@aer.com> wrote in message
news:1189547798.290652.299080@b32g2000prf.googlegroups.com.. .
> On Sep 11, 5:15 pm, Bill Gallery <wgall...@aer.com> wrote:
>> On Sep 11, 3:27 pm, "Haje Korth" <haje.ko...@nospam.jhuapl.edu> wrote:
>>
>>> Bill, here a practical tip: Have you tried to randomly reshuffle your
>>> input
>>> data? Does this make a difference? H.
>>
>>> "Haje Korth" <haje.ko...@nospam.jhuapl.edu> wrote in message
>>
>>> news:fc6oi5$ei3$1@aplnetnews.jhuapl.edu...
>>
>>>> Bill,
>>>> you just hit a nerve with me. I thought I was the only one having
>>>> those
>>
>> Haje,
>>
>> I did not try reshuffling the input data, but did find a fix.
>> Originally, the input data had a minimum latitude of 30.0 deg N and
>> the specified regular grid also had a minimum latitude of 30.0. When
>> I expanded the input data to have a minimum latitude of 20 deg N, the
>> error message went away. Apparently you need data outside the area of
>> interest for the interpolation to be robust.
>>
>> Further experimentation showed that when the minimum latitude of the
>> input data was 30. deg, the interpolated values at 30 deg for the
>> cases that did not fail showed large excursions from expected values.
>> This may be partially due to the nature of the input data, which is
>> poorly sampled below ~40 deg N (temperature data from the SABER
>> instrument on the TIMED satellite.)
>>
>> Does anyone has any experience with the relative merits of the
>> following routines for interpolation on a sphere?
>>
>> 1. qhull and griddata
>>
>> 2. sph_scat.pro
>>
>> 3. triangulate and trigrid
>>
>> Bill
>
> Correction, expanding the input data did not correct the problem: I
> got the same message from a different case.
>
> Puzzled
>
> Bill Gallery
>
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