| Irregular data interpolation problem [message #17185] |
Fri, 24 September 1999 00:00  |
Philippe Peeters
Messages: 14
Registered: April 1996
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Junior Member |
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Hi all,
Sorry if this has already been answered but I didn't find a solution in
the FAQ, various web site etc...
The problem is quite simple: I have a set of measurements from all over
the Earth. The measurements are irregularly spaced (satellite
measurements) and I want to interpolate the data to a regular grid (1
degree x 1 degree).
I have experimented with TRIANGULATE and TRIGRID with de SPHERE keyword
but the resulting interpolated grid becomes very strange as I increase
de number of measurements.
It seems that, with SPHERE keyword, TRIGRID does not make a linear
interpolation but a quintic polynomial interpolation (even though the
QUINTIC keyword is not used). Second, let's say I feed the
TRIANGULATE+TRIGRID with 10 points, the resulting grid seems reasonable.
With 100 points, it is still ok. With 200, I have very strange
interpolated value (peaks and negative value although the input value is
strictly positive). With higher number of observation points, the result
becomes more and more dramatic (I can have as much as 25000 points).
Maybe I'm doing something wrong, maybe TRIANGULATE+TRIGRID is not the
right tool for the job
--
Philippe Peeters
------------------------------------------------------------ ------------
Institut d'Aeronomie Spatiale de Belgique Tel: +32-2-373.03.81
Belgisch Instituut voor Ruimte-A�ronomie Fax: +32-2-374.84.23
3 Avenue Circulaire Email: philp@oma.be
B-1180 Brussels, Belgium http://www.oma.be/BIRA-IASB/
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| Re: Irregular data interpolation problem [message #17303 is a reply to message #17185] |
Tue, 28 September 1999 00:00  |
Jonathan Joseph
Messages: 69
Registered: September 1998
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Member |
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Philippe,
I can recall having some problems with this sort of thing in the
past, though I can't remember the cause. Have you looked
at sph_scat? You can look at the IDL code for this. It should
be similar (or identical) to what you want to do.
If you post the relevant portion of your code and the
list of 200 points which causes the faulty behavior as
well as the list of 100 points that yeilds correct behavior, I'm
sure someone will be able to help you out.
-Jonathan
Philippe Peeters wrote:
>
> Hi all,
>
> Sorry if this has already been answered but I didn't find a solution in
> the FAQ, various web site etc...
>
> The problem is quite simple: I have a set of measurements from all over
> the Earth. The measurements are irregularly spaced (satellite
> measurements) and I want to interpolate the data to a regular grid (1
> degree x 1 degree).
> I have experimented with TRIANGULATE and TRIGRID with de SPHERE keyword
> but the resulting interpolated grid becomes very strange as I increase
> de number of measurements.
>
> It seems that, with SPHERE keyword, TRIGRID does not make a linear
> interpolation but a quintic polynomial interpolation (even though the
> QUINTIC keyword is not used). Second, let's say I feed the
> TRIANGULATE+TRIGRID with 10 points, the resulting grid seems reasonable.
> With 100 points, it is still ok. With 200, I have very strange
> interpolated value (peaks and negative value although the input value is
> strictly positive). With higher number of observation points, the result
> becomes more and more dramatic (I can have as much as 25000 points).
>
> Maybe I'm doing something wrong, maybe TRIANGULATE+TRIGRID is not the
> right tool for the job
>
> --
> Philippe Peeters
> ------------------------------------------------------------ ------------
> Institut d'Aeronomie Spatiale de Belgique Tel: +32-2-373.03.81
> Belgisch Instituut voor Ruimte-A�ronomie Fax: +32-2-374.84.23
> 3 Avenue Circulaire Email: philp@oma.be
> B-1180 Brussels, Belgium http://www.oma.be/BIRA-IASB/
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