Meng Your question sounds very complicated, so forgive me if I give a simplistic answer. Read our 1987 paper called "a novel approach to co-kriging" which explains what is now known as the "non-co-located" cross semi-variogram. You can download a copy from my personal website at:
http://drisobelclark.kriging.com/resume/ Follow Publications link. Noel Cressie's book on Statistics for Spatial Data is probably a good definitive reference for co-kriging of both co-located and non-co-located types, although it is heavily mathematical. Computationally, it uses all observations on both variables and is faster than calculating the ordinary semi-variogram on the larger data set. I cannot speak for the software you are using, but that is certainly how ours behaves. You are correct that the sill will be biassed downwards if the overall mean is estimated. However, the bias should be equal to the variance of the error on the estimation of that mean, which should be minimal compared to the variance between individual samples -- even for a small data set. Isobel + + To post a message to the list, send it to ai-geostats@jrc.it + To unsubscribe, send email to majordomo@ jrc.it with no subject and "unsubscribe ai-geostats" in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list + As a general service to list users, please remember to post a summary of any useful responses to your questions. + Support to the forum can be found at http://www.ai-geostats.org/
