[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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
RE: [ai-geostats] XValid script
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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
[ai-geostats] spatial relationships
Mark I could not agree more with Gregoire (with one proviso, see below). Both geostatistics and any weighted average estimators are based on the same assumptions -- that relationship between values at two locations depends on the distance between them and (possibly) their relative orientation. If you cannot get a decent semi-variogram after trying every type of graph [normal, robust, relative] and every transformation and/or interpretation of your data [logarithm, indicator, rank transforms, Normal scores, mixed populations], you do not have a distance-based relationship. This conclusion also rules out: inverse distance weighting of any kind; Delaunay triangles; Thiessen polygons and so on. My proviso: there are other forms of spatial relationship than pure distance/direction types. The simplest example of this is data with a trend, where the value at a specified point will depend on its absolute position. There may be an added component for the 'residuals' which turns out to be distance/direction based. There are also many examples where, for example, flow characteristics, connectivity and so on play a large part in the structure of your variable. In short: no decent semi-variogram does NOT mean no spatial relationship. It means no simple second-order stationary geostatistical type spatial relationship. Isobel http://geoecosse.bizland.com/whatsnew.htm ___ALL-NEW Yahoo! Messenger - all new features - even more fun! http://uk.messenger.yahoo.com * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
