Hi, A lot of people in our institute use ArcGIS. But I always advise them not to use ArcGIS for kriging etc. Mainly because kriging in ArcGIS is a black box tool. You only know the input and the output, but not how things are calculated. More over people tend to try the different options without really knowing what (and why) they are doing. The just stick with the interpolated map that "looks" the best.
For kriging I promote R and gstat. HTH, Thierry ------------------------------------------------------------------------ ---- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey -----Oorspronkelijk bericht----- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens D G Rossiter Verzonden: zaterdag 6 september 2008 16:07 Aan: r-sig-geo@stat.math.ethz.ch Onderwerp: [R-sig-Geo] ArcGIS Geostatistical Analyst -- how does it display /fit variograms? Hi, I know this is mostly an R-spatial list but this is where the most computational geostats experts hang out, so please forgive me for asking an ArcGIS question. I use R almost exclusively for my own work, but have been asked to supervise the development of an introductory geostats course for our partner at the University of Rwanda. They have standardized on ArcGIS for all of their GIS work (and SPSS for non-spatial stats), and the prospective students (mostly centre workers and collaborating researchers) are familiar with it. The decision was taken by their administration not to use my R/gstat material from the ITC distance education course, rather to develop the course with ArcGIS. My counterpart is now with me developing the course. The deficiencies of ESRI documentation are well-known. I have dug around quite a bit both within the ESRI docs (on-line and with the program) and through various mailing lists and the web and can not find out some basic information. I hope you can shed some light, 1. What exactly is the display of the empirical variogram? The doc. implies there is one average semivariance per bin (as is usual) but the display often has several at the same bin. The variogram can be exported as a table, where it shows multiple (2 - 6 or so) semivariances for each bin; the table also shows a "weight" for each of these, but they do not add to 1 or 100 or anything I can recognize! The close-range bins usually have one, then the number increases. So I guess each dot represents some number of point-pairs. 2. How is the variogram being fit? What weighting, what solver? If the user changes the cutoff/bin width, the solution changes (as it should); but I can't see how it's solving, and I can't find any option to change the weighting (as in e.g. gstat). 3. When fitting direct and cross-variograms for co-kriging, it seems that a linear model of co-regionalization is being enforced (i.e. same range). Again, how is the fit being done? Like fit.lmc in gstat? Naturally we want the students to understand what the program is doing for them! Although ESRI promotes "press the button and look at the cross-validation". I do like their disclaimer in the ArcGIS Desktop 9.3 help: "Kriging is a complex procedure that requires greater knowledge about spatial statistics than can be conveyed in this command reference". They then ref. Burrough (1986! not even the revised book), Heine (1986), McBratney & Webster Journal of Soil Sci. 37:317 (1986), Oliver IJGIS 4 (1990), Press etc. Numerical Recipes, and Royle et al. Geoprocessing 1 (1981). Not exactly the most up to date or accessible reference list (no offrence to the fine authors mentioned). Thanks for your help. D. G. Rossiter Senior University Lecturer Department of Earth Systems Analysis International Institute for Geo-Information Science and Earth Observation (ITC) PO Box 6, 7500 AA Enschede, The Netherlands Internet: http://www.itc.nl/personal/rossiter/pubs/list.html#pubs_m_R International Institute for Geo-Information Science and Earth Observation (ITC) Chamber of Commerce: 410 27 560 E-mail disclaimer The information in this e-mail, including any attachments, is intended for the addressee only. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution or action in relation to the content of this information is strictly prohibited. If you have received this e-mail by mistake, please delete the message and any attachment and inform the sender by return e-mail. ITC accepts no liability for any error or omission in the message content or for damage of any kind that may arise as a result of e-mail transmission. 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