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Re: spherical gridding problem

Jonathan Joseph <jj21@cornell.edu> writes:

> Help!
> I'm seeing a spherical gridding problem, and I swear I've seen
> something similar before - but only when I've been working with
> large datasets, so I'm afraid I don't have a simple
> small example to show it - only a great whopping example.
> Well, here's the description:
> I have a random collection of data of the sort
> [longitude, latitude, value] and I would like
> to create a uniform cylindrical map of this data,
> so I use sph_scat (equivalent to triangulate followed by trigrid).
> When I do this, I sometimes see anomolous features (big positive
> and negative spikes) in the map.  Please see http://baritone.tn.cornell.edu/~jj/idl
> for a zoomed picutre an anomaly (a 6x6 degree sample).
> The anomaly seems to be more closely tied to the
> lat/lon locations of the data than to the data values themselves,

Hi Jonathon--

The results of the output are definitely not right.  I can get the
artifact to disappear or at least decrease by shifting that center
point about 0.1 degrees in any direction.

However I think this may come back to a problem some people have been
seeing regarding TRIGRID.  [ TRIGRID is the underlying routine of
SPH_SCAT. ]  When passed data points that are colinear then TRIGRID
actually crashes.  In spherical coordinates the problem must manifest
when points lie on a great circle.  I can imagine that if points are
very *nearly* colinear then some kind of cancellation error occurs,
which might give you the blow-ups you are seeing.

That center point appears to be at a crossing of two sets of nearly
colinear points, so that may indeed be a problem.  However, I have to
admit that there are a lot of other points like that.

How to deal with it?  Beats me.  This is really something that the RSI
people should try to fix.  You could test for the error by putting
some random scatter in your input lat/lon points and looking for major
deviations in the result.

Sorry I can't help more,

Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response