Hello,
I have what are probably some fairly basic indicator simulation questions, but I've checked the list archive as well as Dr. Pebesma's articles and Dr. Cressie's book on geostatistics and some questions remain- no doubt due more to my lacking in geostatistical knowledge than anything else. I have presence/absence data of vegetation alliances in the Mojave Desert and want to generate vegetation models based on environmental variables (e.g., elevation, climate, landform...) using generalized linear models and classification trees. As these models will be used more for prediction than explanation, I am interested in incorporating spatial dependence as an additional explanatory variable (even if it just represents some unmodeled, unobserved spatially dependent environmental variable rather than an actual biological process resulting in spatial clustering of the vegetation). Using sample data (coded 1 and 0), I want to generate a surface of alliance presence using simulation and estimation (indicator kriging), add this variable to the modeling process, and compare the results. I started using GSLIB for this, but as a non-computer programmer I found it a little unwieldy, and also had problems creating a surface from the output. I've started using GSTAT through the new version of IDRISI and have been very pleased with it so far but had a few questions about some of the results I've been getting: 1. My understanding of conditional simulation is that it uses input sample data and the resulting simulations follow the mean, variogram and pdf of the sample data. When I used GSLIB, one of the parameters the user gives is the global pdf values and I found that the resulting simulation had this exact proportion of value 1 to value 0; however with GSTAT, I've found that the resulting proportions have quite a bit of range. Following a post in the GSTAT archives, Dr. Pebesma said that the simple kriging mean is approximately equal to the sample mean value. My data has a sample value of 0.305 and I use that for the simple kriging mean but get simulations with proportions ranging from 0.24 to 0.37. I saw in one of the user manuals that pdf violations could be attributed to insufficient data across all the thresholds and that's possibly my problem, but I wanted to check and see if I was doing something wrong. 2. Related to the conditional data, my sample data are not fully "honored". About 10% of the sample points are on a grid cell with the incorrect value (e.g., a "1" on a "0" grid, or a "0" on a "1" grid). Also in an archived post I saw that this could be attributed to a nugget effect in the variogram (which I have: 0.06 NUG(0)), but wanted to see if something else might be wrong as well. Sorry for the length of this post and thanks very much for your help. Jenniifer Miller J e n n i f e r M i l l e r Geography Department San Diego State University San Diego, CA 92182-4493 [EMAIL PROTECTED] http://www.rohan.sdsu.edu/~jmiller
