[Met_help] More MET questions

[John Halley Gotway]

Dan,

I looked more closely at the bug you're seeing when running Grid-Stat over the 1830x918 domain.

I narrowed it down to a GSL library routine that's choking on a the large number of grid points.  For whatever reason, the GSL routine "gsl_cdf_chisq_Pinv", which MET uses in computing normal
confidence intervals, is choking when the degrees of freedom passed to it are greater than 1,263,131.  For your data, we're passing it a value of 1,679,940.

I tried downloading and linking to the latest version of GSL, version 1.11, released 3/30/2008.  But the bug is still present in that version.  So I've submitted a bug report to GSL.

As n -> infinity, the value returned by this routine -> infinity as well.  As a crude workaround, I've added a check to the wrapper for this GSL routine to check for n being too large.  If it is, the
routine now returns a value of n*0.998.  With n so large anyway, this should have a negligible affect on the confidence intervals computed.

I tried this using your sample data files, and it does now work.

The attached file "gsl_cdf.cc" which includes the workaround replaces the file "METv1.1/lib/vx_gsl_prob/gsl_cdf.cc".  After replacing that file, please rebuild MET being sure to do a "make clean" first.

Thanks,
John

Daniel Schaffer wrote:
> Hey John
> 
> FYI.  Given the difficulties I ran into as we have been discussing and a short time fuse I face, I have gone ahead and written my own code that computes counts for various neighborhood widths.  So at this moment it is not super-critical to resolve these issues.  However, it would be interesting to compare the counts I generate with MET's answers.
> 
> I've attached the files that you requested.  Note that I only attached a sample forecast file since the behavior is the same if the forecast and obs files are identical.
> 
> As for your response below; I had already seen that.  I'm not clear how it's relevant to the problem I'm having with the GSL gamma function.
> 
>>> Regarding GSL, please see the entry about GSL on the known issue page of the MET website:
>>> http://www.dtcenter.org/met/users/support/known_issues/METv1.1/index.php
>>> Look in the section on Grid-Stat.
>>>
> 
> Thanks for your continued support.
> Dan
> 
> 
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