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603Re: AI-GEOSTATS: Risk Assessment with Gaussian Simulation?

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  • Pierre Goovaerts
    Apr 29, 2002
      Hi Brian,

      One hundred realizations are typically generated
      mainly for CPU reasons.
      You are perfectly right that this number is
      too small when looking at small probabilities
      like 0.05 or 0.01. It's why I wouldn't recommend
      using stochastic simulation to derive probability of occurrence
      of events at pixel locations. Just use kriging to build
      your local probability distributions.
      Use simulation if you have a transfer function, such as flow
      simulator, that requires a model of spatial uncertainty,
      or if you need to derive block probability distributions
      (upscaling or aggregation problems).

      More generally, there is more research to be done on the
      use of stochastic simulation for probabilistic assessment,
      including the question of equally-probability of realizatiuons
      being generated.

      Pierre
      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

      ________ ________
      | \ / | Pierre Goovaerts
      |_ \ / _| Assistant professor
      __|________\/________|__ Dept of Civil & Environmental Engineering
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      On Mon, 29 Apr 2002, Brian R Gray wrote:

      >
      > I am curious about the use of 100 realizations to generate a probability
      > map. is this a standard approach? if so, is a "small" p-value (such as
      > .05) used? if so, it would seem like 100 iterations might be a smallish
      > sample size for distinguishing, say, .05 (ie 5 outcomes out of 100) from,
      > say, .01. is 100 used because it seems like it is a reasonable number or
      > because of the computer time restrictions?
      >
      > do geostat folks treat these as realizations or as pseudo-realizations?
      >
      > brian
      >
      > ****************************************************************
      > Brian Gray
      > USGS Upper Midwest Environmental Sciences Center
      > 575 Lester Avenue, Onalaska, WI 54650
      > ph 608-783-7550 ext 19, FAX 608-783-8058
      > brgray@...
      > *****************************************************************
      >
      >
      >
      > Chaosheng Zhang
      > <Chaosheng.Zhang@nui To: ai-geostats@...
      > galway.ie> cc: Dave McGrath <dmcgrath@...>
      > Sent by: Subject: AI-GEOSTATS: Risk Assessment with Gaussian Simulation?
      > ai-geostats-list@uni
      > l.ch
      >
      >
      > 04/27/2002 10:25 AM
      > Please respond to
      > 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: Chaosheng.Zhang@...
      > ChaoshengZhang@...
      > Web: http://www.nuigalway.ie/geography/zhang.html
      > =================================================
      >
      >
      >
      >
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