Geography Reference
In-Depth Information
referred to as the 'population at risk. Methods exist that control for the population at
risk but this chapter focuses on the standard unmodii ed measures. Nevertheless, it is
an issue that should be taken into account when considering the approaches presented.
Basic measures
7.2
Analysis of spatial point patterns is, like any analysis of any form of spatial data, likely
to begin with visual inspection. Again, as with any form of spatial analysis, a second
step may be to compute one of a variety of descriptive measures. Two quite widely
used summary measures are the mean centre and the standard distance. h e mean
centre of a point pattern indicates the central tendency of the points. It is simply the
mean of the x and y coordinates:
Ê
n
n
ˆ
ÂÂ
x
y
i
i
x
=
(,
mm
)
= Á
Á
i
=
1
,
i
=
1
˜
(7.1)
x
y
n
n
˜
Ë
¯
h e bold letter x indicates a vector representing location (i.e. x and y coordinates) and
the bar above it indicates that it is the mean average value: m x is the mean of the x values
and m x is the mean of the y values. h e number of events is given by n . Dispersion
around the mean centre can be measured with the standard distance, d s :
Â
n
2
2
1 (
x
-+-
m
)
(
y
m
)
i
x
i
y
d
=
i
=
(7.2)
s
n
h e mean centres and standard distances for the two point patterns illustrated in
Figures 7.1 (PP1) and 7.2 (PP2) are given in Figures 7.3 and 7.4.
h e mean centres and standard distances for the two examples are similar with respect
to the common boundary box. However, the characteristics of the two point patterns
are very dif erent in other ways. For example, the intensity of points in Figure 7.1 is
Figure 7.3 PP1: mean centre (large point) and standard distance (circle).
 
Search WWH ::




Custom Search