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[ai-geostats] Uncertainty of Conditional Stochastic Simulated results

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  • Feng Liu
    Dear List: I got a question about the uncertainty associated with the Conditional Stochastic Simulated results, the question is: I have soil C data for a
    Message 1 of 2 , Oct 10, 2004
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      Dear List:

      I got a question about the uncertainty associated with the Conditional
      Stochastic Simulated results, the question is:

      I have soil C data for a sample area, within that area, there are woodland
      and grassland. The variance of C data in woodland is higher than that in the
      grassland. Now I want to use Conditional Stochastic Simulation to simulate a
      bunch of images of soil C on the area. I will use one single variogram model
      to do the simulation. The questions is: Do the estimated C data more
      variable under woodland than those under grassland (simulated soil C data in
      woodland have a larger variance than those in grassland)? or a single
      variogram model means uniform or random variance of simulated data?

      Thank you very much

      Feng Liu
      Graduate Student
      Texas A&M University
    • Pierre Goovaerts
      Hi Feng, You are in the ideal situation where it would make sense to separate the two populations and do the variography and simulation separately for each
      Message 2 of 2 , Oct 10, 2004
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        Hi Feng,

        You are in the ideal situation where it would make sense
        to separate the two populations and do the variography
        and simulation separately for each stratum, since clearly
        the pattern of variability of soil carbon is expected to be
        different under these two landcovers (I am sure that besides the
        variance, the range and nugget effect must be pretty different)
        and also the average C concentration should vary a lot .
        In most situations it is difficult to consider different populations since:
        1. we might not have enough data within each stratum to compute a
        reliable variogram,
        2. the boundaries of the different strata might be fuzzy,
        3. the multi strata approach creates discontinuities in the map
        that are not physically realistic.
        I don't know your data but I am pretty sure that none of these limitations
        apply.

        Now to answer your question, the use of a single semivariogram implies
        an assumption of stationarity, hence the fact that the
        variance/covariance does not depend on the location within the study area,
        which is clearly not your case. Even wen using a single variogram,
        the variability in your simulated map will be influenced to some
        extent by your data (conditioning observations). Hence, I suspect
        that more variability will appear on woodland anyways.
        By analogy, the same happens when you use an isotropic semivariogram
        in simulation/estimation, while the data display strong anisotropy.
        The simulated/estimated map will display some anisotropy, which is
        always reassuring.

        Cheers,

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

        Dr. Pierre Goovaerts
        President of PGeostat, LLC
        Chief Scientist with Biomedware Inc.
        710 Ridgemont Lane
        Ann Arbor, Michigan, 48103-1535, U.S.A.

        E-mail: goovaert@...
        Phone: (734) 668-9900
        Fax: (734) 668-7788
        http://alumni.engin.umich.edu/~goovaert/

        <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

        On Sun, 10 Oct 2004, Feng Liu wrote:

        > Dear List:
        >
        > I got a question about the uncertainty associated with the Conditional
        > Stochastic Simulated results, the question is:
        >
        > I have soil C data for a sample area, within that area, there are woodland
        > and grassland. The variance of C data in woodland is higher than that in the
        > grassland. Now I want to use Conditional Stochastic Simulation to simulate a
        > bunch of images of soil C on the area. I will use one single variogram model
        > to do the simulation. The questions is: Do the estimated C data more
        > variable under woodland than those under grassland (simulated soil C data in
        > woodland have a larger variance than those in grassland)? or a single
        > variogram model means uniform or random variance of simulated data?
        >
        > Thank you very much
        >
        > Feng Liu
        > Graduate Student
        > Texas A&M University
        >
        >
        >
        >
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