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GEOSTATS: Kriging Median Polish detrended data?

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  • Laura Lengnick
    Hi folks, Well, I m back again, a little wiser (thanks to you all) and a little farther along in my analysis, but I ve hit a new snag.....namely, how do you
    Message 1 of 1 , Aug 29, 1997
      Hi folks,

      Well, I'm back again, a little wiser (thanks to you all) and a little
      farther along in my analysis, but I've hit a new snag.....namely, how do
      you create surface maps of data that have been detrended using median polish?

      I'm following the advice of several papers published in soil science
      literature, and they all suggest: 1) detrend 2) fit a semivariogram model
      to the residuals 3) develop kriged estimates of the residuals at unsampled
      locations AND 4) Add these estimated values back to the trend to create
      surfaces. Funny, but that crucial last step is never demonstrated, and
      neither Cressie nor Issaks/Srivastava address the issue directly.

      After reading the literature, I'm convinced that the best way to detrend my
      data is median polish, although I have the software (SPLUS) to support
      polynomial detrending. So, I choose to do median polish, and now I don't
      know how to create estimates of the *trend* at the unsampled locations for
      which I've estimated the residual values. Can anybody help?

      My questions:

      1) can I create a trend surface from the results of a median polish? I
      have row, column and all values from the median polish - are these
      expressing some kind of trend along the rows and columns that I can use to
      estimate trend at unsampled locations?

      2) what do you think of the relative value of median polish vs. polynomial
      detrending - should I be concerned about the claims that polynomial
      detrending is invalid, because spatially correlated data do not meet the
      assumptions required for valid regression?

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