Hi Valquirio, This issue has been discussed in the past and you might find interesting information in the archives.
I would suggest the use of indicator kriging to deal with the presence of a large number of zero values. Use zero as the 1st threshold and pick up a few other thresholds, then apply indicator kriging to derive the local distributions of probability the mean of which can be used for estimation. Note that in presence of a large proportion of zeros (say more than 90%) your indicator variogram will more likely still look erratic. Pierre <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><> Dr. Pierre Goovaerts Consultant in (Geo)statistics President of PGeostat, LLC and Senior 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 Thu, 23 Jan 2003, [iso-8859-1] Valquiria Ferraz Quirino wrote: > > Dear All, > > I am working modeling the distribution of tree parameters (Basal area per ha and >Number of trees per ha) for an adult strata of a tropical forest. The modeling is >being done for (1) all species present in the area, and (2) the seven economically >most important species in the area. The final intention is to use the range to >improve the sampling technique (if possible less plots for and approximately same >precision). As the area has been intensively explored, same species are just present >in less than 10 plots (from a total of 357). For the plots were they are not present, >I used the number zero to represent a measured 0 m�/ha of basal area (in the first >case), or 0 trees/ha (in the second case) on the plot. My questions are: > > (1) How should I deal with these zeros while modeling the semivariogram? I am asking >because I tried using them and the semivariograms look strange (small lags presenting >sometimes higher semivariances than large lags). In this case, I also tried to >interpolate (using kriging) for values between my plots. Cross validation (Jack >knife) shows also an unsatisfatory result (line below the x axis). On my second try, >I took the zeros out. The semivariogram looks much better. But the kriging is >unsatisfatory estimating very high values for plots were there aren't trees of the >studied species at all! Another problem is the number of observations that I used in >this case: sometimes just 8. Can anyone give me a help? > > (2) Can anyone recommend literature that deals with the use of geostatistics to help >the planning of number and location of sampling units in forests? > > Thank you very much!! > > Valquiria > > > Forst-Ing. Valquiria Ferraz Quirino > > Kappler Stra�e 57, Zi. 2121, 79117 Freiburg i. Br. Deutschland > Tel.: +49 761 6806-6204 > > > --------------------------------- > Busca Yahoo! > O servi�o de busca mais completo da Internet. O que voc� pensar o Yahoo! encontra. -- * 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
