Re: AI-GEOSTATS: Declustering
Hi Oliver, I don't know which algorithm you are using to compute these declustering weights but there is something wrong in your rescaling procedure. These declustering weights are proportional to the size of the polygon of influence of each observation and they can not be negative. Regards, Pierre Goovaerts Dr. Pierre Goovaerts President of PGeostat, LLC Chief Scientist with Biomedware Inc. 710 Ridgemont Lane Ann Arbor, Michigan, 48103-1535, U.S.A. E-mail: [EMAIL PROTECTED] Phone: (734) 668-9900 Fax: (734) 668-7788 http://alumni.engin.umich.edu/~goovaert/ On Sat, 5 Jul 2003 [EMAIL PROTECTED] wrote: hello list, to get the global mean of my data set (484 observation wells) and as a prerequisite for Normal Score Transformation with GSLIB, i declustered my data with polygonal declustering. When standardizing the weights to 1, so that the weights sum up to the number of data, I receive some negative weights. this results in negative values for the observations (nitrate concentration). how do i have to interpret this ? does this require a special treatment? skip them? as I mentioned before, i want to normal score transform the data set prior to simple kriging. many many thanx for some help in advance. Regards, Oliver -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and unsubscribe ai-geostats followed by end on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and unsubscribe ai-geostats followed by end on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
RE: AI-GEOSTATS: Declustering
I am slightly confused: ... if sampling is clustered preferentially in e.g. higher values areas, would this not bias the semi-variogram for the first few lags?...at least if, as it can happen, the variance is related to the mean. The semi-variogram is calculated on the difference between the two sample values. If the basic assumptions for semi-variogram construction are correct, differences are unrelated to the actual value of each sample or to the actual absolute location of the pair. Therefore, clustering does not influence the semi-variogram. If you have a situation where variance is related to the mean, e.g. with highly skewed data, you need to transform these values in some way before constructing a semi-variogram. This is true whether or not you have clustered sampling. Absolutely regular sampling will not give you a valid semi-variogram if you violate the assumptions upon which it is based. What about the effects of the possible over-estimation of the global mean due to clustering? If you are estimating the global mean based on a distribution model, you need to decluster. If you are estimating the global meaning on the basis of a kriged grid, you do not need to decluster as the kriging system does that for you. You can experiment with these questions using our totally free unlimited kriging game. This can be found in my 'briefcase' at http://uk.geocities.com/drisobelclark/briefcase.html Does this help? Isobel Clark __ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and unsubscribe ai-geostats followed by end on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
Re: AI-GEOSTATS: Declustering
I would like to know what is the declustering, why we used this method and how we proceed to decluster a set of data? If your data is clustered spatially (in location) this may bias any histograms or probability plots which you draw and, therefore, any conclusions you make about what kind of distribution the values come from. For example, in mining projects geologists tend to drill a lot more holes in the good bits than the bad bits. This means that a histogram contains a lot more samples than it should in the higher end. If you try to fit a model to such data, or use a transform or 'anamorphosis' it will not really reflect the values in the whole of the area. Backtransforms will be biassed like the original samples. Declustering is one way to get rid of the bias. There are various ways to decluster but the most common ones revolve around laying a grid of squares over your map area and either (a) selecting one sample per square or (b) averaging all the samples in each square. (b) is not very sensible given what we are trying to do with the data, but is very common (again) in mining. If you use (a) it is a good idea to choose which sample to 'keep' in the histogram at random. You may still use all of the clustered sampling for geostatistical analysis, of course. The semi-variogram and kriging techniques are not affected by clustering. In fact, one of the main reasons for inventing kriging was to make full use of every one of clustered and/or preferentially sited sampling. The term 'declustering' became popular around 1982/83 and is first used widely in the proceedings of the geostatistical congress held at Lake Tahoe in September 1983. Isobel Clark http://uk.geocities.com/drisobelclark __ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and unsubscribe ai-geostats followed by end on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
