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2227[ai-geostats] Re: Unusual Ordinary Kriging Results

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  • Isobel Clark
    Oct 4, 2005
      You seem to have two problems:
      (1) the Vulcan answer does not match your hand calculation for the same weights and values.
      (2) you have negative weights.
      I would think that (1) was of far more concern than two simply because it suggests that the software is not performing the correct calculations. If this block is wrong how about all the rest of them? Maybe you could try a few reasonable looking blocks and see if you can reproduce those answers. If not, Vulcan and all users need to know that the software has a bug! 
      It is possible you have something like 'affine transformation' or recoverability factor switched on. Does Vulcan do stuff like that? This would produce results outside the range of the data and not equal to weight times grade calculation.
      On (2), opinions differ about negative weights. With your well behaved data and very small negative weights you are unlikely to get values substantially outside the range of the samples. Witness your own calculation. If negative weights worry you, reduce your search radius until the negative weights become negligible.
      Does this help?

      "Reid, David W" <David.W.Reid@...> wrote:
      After running ordinary kriging estimations using Vulcan mine planning software it was noticed there were some unusual estimated grades. I was hoping that someone can confirm that I on the right path + not heading up the yellow brick road to Oz.
      The estimated value reported/calculated by Vulcan for one block was 80.77.  I thought this unusual as the grade of the 16 samples selected for the estimation range from 51.2 to 65.9 (mean 59.6).   I calculated the estimated grade by summing the products of sample grade and sample weight (given by the software) and got a value of 60.13 which seems far more reasonable.
      Maptek the software vendor's response was to suggest that negative weights were responsible for the high estimation.
      Details of the samples are below.
      Have I overlooked something in my calculation or is there some other explanation for the result?
      David Reid
      NumberXYZGradeDistanceweightweight * grade
      Ave59.60031block estimate 80.77

      David Reid

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