Ooops sorry, I have put a almost usless code. I will try to find something more usefull on an old PC. It will be on the web before the end of the day.
Sorry again Gregoire -----Original Message----- From: Gregoire Dubois [mailto:[EMAIL PROTECTED] Sent: 01 September 2004 10:57 To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Subject: RE: [ai-geostats] XValid script Hi Mark, 1) At http://www.ai-geostats.org/software/Codes_Softfaq.htm you will find a few codes for Surfer. I have just added crossvalidation.bas , a code I derived from Aleksey Amantov's jacknife.bas You will need to code a bit yourself to select the correct interpolation function (Inverse Distance Weighted is the default) as well as the proper parameters. Surfers manual is sufficient to learn to program with Scripter (quick basic). Sorry, but I will not provide any support. Note that crossvalidations are implemented in Surfer 8. 2) In short: Geostatistics are not appropriate when you don't have a spatial correlation (even this is questionable :) ). Frequently, people who do not find any spatial correlation in their data apply other functions like Inverse Weighted Distance. I believe such a solution is completelt wrong. Either you have a spatial correlation and you can interpolate/predict values, or you don't and only models based on sound physics (e.g. Temperature T decreases with latitude and altitude, thus you can predict T) can be used. Outside of these two solutions, I would recommend only to use Thiessen polygons for displaying purposes (not for real estimations!): they do not look "natural" when applied to continuous variables and can thus not fool the persoon looking at the output. Moreover, Thiessen polygons provides useful information on the weight of each sample. Maps of proportional symbols would also do. Cheers, Gregoire -----Original Message----- From: Mark Dowdall [mailto:[EMAIL PROTECTED] Sent: 01 September 2004 10:02 To: [EMAIL PROTECTED] Subject: [ai-geostats] XValid script Hello It may be cheeky, but I have two questions: 1. Where can I get I get the Surfer v7.0 script for crossvalidation? I have been through the archives and although its mentioned I cannot find a download site. Nor could I find it and the site of Aleksey Amantov. Some emails mentioned it is in the samples folder of the surfer install but I could not find it. I know its in Surfer 8 as standard but thats not available to me at the moment. 2. I have been following the discussion on extreme values in a data set. And my question is: is there any context in which geostatistical methods are absolutely not appropriate? If so, how can this appropriateness be tested? Is there a point at which a user should know that it would be better to try something else? Any help with these two is very much welcome mdowdall
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