Hello List:
I'm new to this list and just beginning to get into
geostatistics. I tried searching for possible answers
on the mailing list, but had no luck. So here I am
with my question:
My dataset consists of 149 samples(too less ???, but
that is all I have !!) from an offshore area and I am
trying to estimate the grade of a mineral. I used the
software ISATIS for my work. 40% of my data is between
0 to 5% with the maximum being 99 %. The data displays
a uniform distribution if we ignore the 40% low
values.I tried using gaussian transformation, but to
no avail and so stuck with the original data. The
variogram model did fit well (at least globally)and as
I proceeded towards ordinary kriging I got quite a few
percentages of negative values (3% of the estimated
values were negative), with the lowest being -6%. I
contacted the ISATIS technical support team and they
told me to play around with the neighbourhood distance
and number of samples in the neighbourhood etc. After
many trial and error runs I finally got a nice kriging
map but it sill had some negative values (less than 1%
of the estimated values) with the lowest being
-0.02.I'm curious as to what could be the reasons
behind the negative values. I do get some negative
weights, but is that only reason. Could someone give
me a mathematical and/or intuitive meaning to the
negative estimates?
Thank you
Titus
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