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607Re: AI-GEOSTATS: Risk Assessment with Gaussian Simulation?

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  • Chaosheng Zhang
    Apr 30, 2002
      Dear all,

      Thanks for so many interesting replies and thoughtful discussion. This is
      not a summary yet, as I am expecting more to come.

      Just to express my feeling about Indicator Kriging. To produce a probability
      map, IK might be one of the choices. However, I always feel that too much
      information is lost when doing the indicator transformation. When I see so
      many "0"s in a dataset, I just feel the data quality is too poor.

      Well, the other method of combination of Kriging and Kriging variance for
      risk assessment has not been well discussed yet, and I would like to read
      more comments.

      My last question "(4) how to carry out "PCLASS" " is now answered by the
      developer of Idrisi. The fact that the file name of Kriging variance cannot
      be entered (with Metadata command) is a bug of the program, which will be
      corrected soon. At present time, a text editor may be used to modify the
      image documentation file.

      Now, let me discuss how I would like to make a probability map based on
      Kriging and Kriging variance. For each pixel of the Kriging interpolated
      map, there is a value of Kriging variance. The Kriging variance is a measure
      of uncertainty (which is related to sampling density and spatial variation,
      etc.???). If we assume that the value of the Kriging pixel follow a normal
      distribution and the standard deviation is equal to the SQRT of Kriging
      variance, the probability of any threshold can be calculated. Furthermore,
      to make the risk assessment more realistic, I would like to include other
      errors, such as sampling error and laboratory analysis error into risk
      assessment. These errors can hardly be quantified, but if we say 10% or 20%
      of the pixel value (for soil samples), perhaps there is no objection.
      Therefore, the standard deviation of the pixel is increased by adding this
      kind of errors.

      I am not clear how to calculate the total standard deviation of the two
      sources, is it:
      Total standard deviation =
      SQRT (Kriging Variance + SQUARE (Sampling Errors) ) ?

      Any ideas and comments on this method?

      Chaosheng Zhang

      > On Sat, 27 Apr 2002, Chaosheng Zhang wrote:
      > Dear list,
      > First, I would like to say thank you to Gregoire for keeping this list
      > 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
      > 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".
      > 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
      > reclassify the surfaces by a threshold value, and calculate a probability
      > occurrence for a process based on that threshold, conditional simulation
      > be unnecessary. Instead kriging and variance images may be created and
      > 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
      > documentation file. It seems that this field only accepts a "value", not
      > "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: Chaosheng.Zhang@...
      > ChaoshengZhang@...
      > Web: http://www.nuigalway.ie/geography/zhang.html
      > =================================================

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