Digby The original derivation of the kriging equations does not demand any specific distribution. However, it only makes sense if the 'increments' - difference in value - have a fairly stable variance.
Kriging works much better IN PRACTICE if those differences are Normal, hence the various transformations offerred such as lognormal kriging, hermitian polynomials, Normal scores, rank transforms and so on. Normalising a mixture of distributions will result in specious answers unless the distributions are mixed in the same proportions all over the study area. Usually a mixed distribution is due to a mixture of real populations. For example, in geology oxide and sulphide samples may show different behaviour. There may be separate phases of deposition. In short, a mixture of distributions generally indicates a violation of the assumption of homogeneity (or, if your prefer, stationarity) which is needed for the proper application of any geostatistics. If possible the populations should be separated and the analysis carried out. If not, you may be able to cope with the data using indicator kriging such as suggested in my 1993 paper. There is also an example in my Cardiff paper of 2000, I think. Isobel http://geocities.com/drisobelclark/resume/Publications.html __________________________________________________ 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
