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Re: GEOSTATS: colocated samples in cokriging

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  • Pierre Goovaerts
    Stefan, The fact that some of the primary and secondary data must be collocated is not a prerequisite for the application of cokriging itself. It is needed to
    Message 1 of 2 , Sep 13, 1998
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      Stefan,

      The fact that some of the primary and secondary data
      must be collocated is not a prerequisite for the
      application of cokriging itself. It is needed to
      assess and model the spatial dependence between the
      two variables, usually through the computation and modeling
      of direct and cross semivariograms. One way to avoid
      this problem is to work in terms of auto- and
      cross-covariance functions, which can be computed from
      non-colocated sets of primary and secondary data. Note
      that for the application of cokriging, you still have
      to estimate the covariance at zero lag distance,
      which could be done by extrapolating the behavior of
      the first few covariance values to the vertical axis,
      as for the estimation of the nugget effect for semivariogram.

      One situation where cokriging has been applied to non-colocated
      primary and secondary variables is when the two variables
      actually refer to the same attribute measured at two different
      times and the destructive nature of the sampling
      precludes multiple measurements at the same location, see
      the following paper for more details.


      Papritz, A., Fluhler, H., 1994. Temporal change of spatially
      autocorrelated soil properties: optimal estimation by cokriging.
      Geoderma 62, 29--43.

      Regards,

      Pierre


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      On Sat, 12 Sep 1998, Stefan Falke wrote:

      > When applying cokriging, is it required that there be secondary samples
      > colocated with at least some of the primary samples?
      > I have looked at multiple references and none explicitly answer this
      > question but all of the examples I've found use secondary data sets that
      > are, at least partially, colocated with the primary data set. Is it
      > possible to use cokriging on data sets that have no colocated samples?
      >
      > Thanks,
      > Stefan
      >
      > ---------------------------------------
      > Stefan Falke
      > Graduate Research Assistant
      > Center for Air Pollution Impact and Trend Analysis
      > Washington University in St. Louis
      >
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