AI-GEOSTATS: Risk Assessment with Gaussian Simulation?

[Chaosheng Zhang]

Dear list,   First, I would like to say thank you to Gregoire for keeping this list alive.   I'm trying to do "risk assessment", and I have some questions about risk assessment with Gaussian Simulation:   (1) How to produce a probability map?   With Gaussian simulation, we can produce many maps/realisations, e.g., 100. Based on the 100 maps, a probability map of higher than a threshold can be produced. I wonder how to produce such a probability map? My understanding is that for each pixel, we just count how many values out of the 100 are >threshold, and the number is regarded as the "probability". Am I right? It seems that this is a time consuming procedure with GIS map algebra. Are there any suggestions for a quick calculation?   (2) Is a probability map better than a Kriging interpolated map for the purpose of risk assessment?   (3) Is "PCLASS" function in IDRISI 32 Release 2 better/easier than the probability map from Gaussian simulation?   From the online help of IDRISI 32 R2, Section "Kriging and Simulation Notes", it says "If the final goal of simulated surfaces will be to directly reclassify the surfaces by a threshold value, and calculate a probability of occurrence for a process based on that threshold, conditional simulation may be unnecessary. Instead kriging and variance images may be created and then used together with PCLASS." Any comments?   (4) How to carry out "PCLASS"?   Following the above question, I have a problem in doing PCLASS. I cannot input the file name of Kriging variance to the field of "Value error" of the documentation file. It seems that this field only accepts a "value", not an "image file name" or anything in text. Anyone has the experience?   Cheers,   Chaosheng Zhang ================================================= Dr. Chaosheng Zhang Lecturer in GIS Department of Geography National University of Ireland Galway IRELAND   Tel: +353-91-524411 ext. 2375 Fax: +353-91-525700 Email: [EMAIL PROTECTED]        [EMAIL PROTECTED] Web: http://www.nuigalway.ie/geography/zhang.html =================================================