Negative kriging weights can occur when you have a so-called "screening effect", that is points close to the location at which an estimation is needed "mask" points that are further appart. The problem is thus the topology of your sampling locations.
Solution: reduce the neighborhood of your estimator (e.g. use 1 or 2 points in each sector of your serch ellipse to avoid searching too far) A reference explaining the maths behind the weights is: Clayton V. Deutsch, (1996) Correcting for negative weights in ordinary kriging, Computers & Geosciences, Volume 22, Issue 7,Pages 765-773. An excellent (free!) tool for visualising this problem is E{Z}-Kriging (see FAQ section of AI-GEOSTATS) written by Denis Walvoort. Hope this helps, Gregoire __________________________________________ Gregoire Dubois (Ph.D.) European Commission Tel. +39 (0)332 78 6360 Fax. +39 (0)332 78 5466 WWW: http://rem.jrc.cec.eu.int WWW: http://www.ai-geostats.org -----Original Message----- From: Abhijith Titus D'souza [mailto:[EMAIL PROTECTED] Sent: 11 November 2005 21:59 To: ai-geostats@unil.ch Subject: [ai-geostats] why do negative kriging values occur Hello List: I'm new to this list and just beginning to get into geostatistics. I tried searching for possible answers on the mailing list, but had no luck. So here I am with my question: My dataset consists of 149 samples(too less ???, but that is all I have !!) from an offshore area and I am trying to estimate the grade of a mineral. I used the software ISATIS for my work. 40% of my data is between 0 to 5% with the maximum being 99 %. The data displays a uniform distribution if we ignore the 40% low values.I tried using gaussian transformation, but to no avail and so stuck with the original data. The variogram model did fit well (at least globally)and as I proceeded towards ordinary kriging I got quite a few percentages of negative values (3% of the estimated values were negative), with the lowest being -6%. I contacted the ISATIS technical support team and they told me to play around with the neighbourhood distance and number of samples in the neighbourhood etc. After many trial and error runs I finally got a nice kriging map but it sill had some negative values (less than 1% of the estimated values) with the lowest being -0.02.I'm curious as to what could be the reasons behind the negative values. I do get some negative weights, but is that only reason. Could someone give me a mathematical and/or intuitive meaning to the negative estimates? Thank you Titus __________________________________ Yahoo! FareChase: Search multiple travel sites in one click. http://farechase.yahoo.com
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