## 2489RE: [ai-geostats] A novice question

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• Mar 23, 2006
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On 23-Mar-06 West, Nancy \(DOH\) wrote:
> Hi all,
>
> I am trying to do a simple interpolation of just 17 values
> with a very wide range from about 0.12 to 6500 over about
> a city block area. Being a novice, I decided to use a simple
> method without assumptions and few parameters to set. So, I
> chose local polynomial interpolation. It turned out about the
> way we expected. I set it to just four classes for easy
> identification. BTW, I am using ESRI's Geostatistical Analyst.
>
> I was asked to overlay the sample points with the values labeled.
> It was then pointed out to me that there were sampled values such
> as 14 that are in the interpolated class range of 900 to 6500.
>
> My questions are "Can this be correct?" and "If so, how?" Also,
> is there a reference anyone can point me to that explains this?
>
> Kriging course. Hi, Isobel!
>
> Thanks for any guidance you can give me on this.
>
> Nancy

It *could* be "correct", *if* the observed data include a very
wide random scatter relative to the values they are supposed
to measure.

However, if that is not a plausible interpretation, then one
is tempted to conclude that your polynomial interpolation is
not a good model. (And, by the way, I take it that when you
say "interpolation" you really mean "smoothing", since true
interpolation exactly fits the observed data where the data
points occur, and the mismatch you report should not happen).

Since there are only 17 data points (presumably each with
[x,y] coordinates and a z value), provided it is acceptable
from the point of view of whatever confidentiality may apply
to your investigation, it might be possible to offer more
considred advice if you showed us the data.

An alternative view of your data might be that, while they
are good measurements of the values at the points where the
variable is measured, that variable varies substantially
from place to place, so that neighbouring measurements can
be very different from each other.

If you want to preserve the point values, but interpolate
between them to make estimates of values at points where
you have not made measurements, then that is one thing;
but if you regard the measured values at one time as a
realisation of some random field, and you want to estimate
the underlying intensity of the random field, then that is
another thing!

Hoping this helps,
Ted.

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E-Mail: (Ted Harding) <Ted.Harding@...>
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Date: 23-Mar-06 Time: 19:18:54
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