Hi Juliann,
This type of feature is frequently observed when
the variogram is being computed for lags larger
than half the maximum dimension of the study area.
In your case it might also correspond to the
separation distance between the two most densely sampled
subareas.
The most important questions is "What do you want
to do with this variogram". If the next step is to apply
kriging and this rise is not spurious (e.g. may
reflect some trend in the data), the issue is whether you
need to model this second part of the variogram.
If the radius of the kriging search window is smaller
than the lag at which the second rise occurs, I wouldn't
bother modeling it.
Pierre
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On Wed, 28 Nov 2001, Juliann Aukema wrote:
> Hi,
>
> I have a question about interpretation and
> robustness of a variogram. My variogram rises then
> plateaus and then rises again. I interpret this as
> meaning that there are two scales at which there is
> spatial dependence of the values. However, the number
> of pairwise comparisons is quite different for each of
> these stages and I am afraid I may just be seeing an
> artifact of the sampling. I used variowin and when I
> fit a model for just the first rise and plateau, the
> sill was at about 1500 meters. The first point has 70
> pairs and then from right before the plateau (1000 m)
> to the beginning of the second rise, there are 324-396
> pairs and finally the points in the second rise have
> between 512-534 pairs. I have a total of 66 sample
> points, but they are not evenly spaced with two areas
> more heavily sampled than intervening areas.
> Additional sampling is not feasible. Do I have a
> problem? Is there a way to test the robustness of this
> variogram (I don't know how to fit a model to a
> variogram with two rises, so I couldn�t' do cross
> validation)?
>
> (Additional information - other data and analyzing the
> same data with nested ANOVA, looking at smaller scales
> within the data set, support the first rise. Taking
> the residuals of a correlated variable - elevation -
> removes the second rise but maintains the first rise -
> my interpretation is that there are different
> processes at the two scales).
>
> I would appreciate any suggestions,
>
> Thanks a lot,
> Juliann Aukema
> [EMAIL PROTECTED]
>
>
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