Geography Reference
In-Depth Information
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Figure 7.5
PP1: quadrats.
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2
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1111
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Figure 7.6
PP2: quadrats.
while for queen's case (again, see Section 4.8) it was 0.0887. For rook's case contiguity,
I
for the quadrat counts in Figure 7.6 was -0.0395 while for queen's case it was -0.1053.
For the quadrats used in the example, PP1 appears clustered, i.e. it is positively
spatially autocorrelated (albeit not to a marked degree), while PP2 appears regular or
dispersed, i.e. it is negatively autocorrelated, thus coni rming the impression gained
from visual inspection. h e
I
coei cient was computed using the GeoDa sot ware
(Anselin
et al.
, 2006). h e value of
I
is, of course, partly a function of the size of the
quadrats.
A simple means of assessing the degree of clustering or regularity in a point pattern
is the variance/mean ratio (VMR). h e VMR indicates the degree to which a point
pattern departs from that predicted by the Poisson distribution, which is ot en used
to express the probability of events occurring in a given area. h e Poisson distribution
can be given by:
k
e
-
l
l
(7.3)
Pk
()
=
k
!
where
l
is the mean average intensity of the point pattern,
k
is the number of events,
and
e
is the base of the natural logarithm (= 2.71828…). In words, the equation
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