> 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
