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*Subject*: Re: spherical gridding problem*From*: Jonathan Joseph <jj21(at)cornell.edu>*Date*: Fri, 06 Apr 2001 15:19:58 -0400*Newsgroups*: comp.lang.idl-pvwave*Organization*: Cornell University*References*: <3ACCD53E.F2D23394@cornell.edu> <onwv8zx793.fsf@cow.physics.wisc.edu>*Sender*: verified_for_usenet(at)cornell.edu (jj21 on scorpio.tn.cornell.edu)*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:24429

Does anyone know exactly how the spherical gridding in IDL really works? There is mention under QUINTIC of a paper by Renka, and JD was kind enough to find me an different article by Renka in the ACM titled "Interpolation of data on the surface of a sphere" (http://www.acm.org/pubs/citations/journals/toms/1984-10-4/p417-renka/) though I can't be sure that is the method used by IDL. If it is, I would suspect that the problem is somehow in the calculation of the gradients at the corners of the triangles. Though to tell the truth, I only really followed the idea and not the actual details. Not that knowing the reason would solve my problem. I guess for now, I'll take your idea Craig and try adding some random scatter in several passes, and try comparing the results to find problem areas. I think I need several passes, because one random scattering is bound to turn up new problems in other places. -Jonathan Craig Markwardt wrote: > > 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 > > -- > -------------------------------------------------------------------------- > Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu > Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response > -------------------------------------------------------------------------- -- -Jonathan

**References**:**spherical gridding problem***From:*Jonathan Joseph

**Re: spherical gridding problem***From:*Craig Markwardt

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