Analysing spatial point patterns in R - CSIRO

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9 Manipulating point patterns
Before proceeding, we need to know more about how to manipulate and interrogate point pattern
data.
9.1 Point pattern objects
A point pattern is represented in spatstat by an object of the class "ppp". This contains the
coordinates of the points, optional ‘mark’ values attached to the points, and a description of the
study region or spatial ‘window’.
9.1.1 Internal Format
WARNING: It is strongly advisable NOT to directly access or modify
the internal components of an object.
It is a “beginner’s mistake” to modify the internal components of a structured object such
as a point pattern (object of class "ppp"). The internal structure of objects in a package can
change from one version of the package to another. It is much safer to use operators defined in
the package to extract and modify information.
However, in the spirit of “open source”, here is a description of the internal format.
A point pattern object P has the following components:
P$n is the number of points (which may be zero).
P$x is a numeric vector containing the x coordinates of the points. Its length equals P$n
(and may be zero).
P$y is a numeric vector containing the y coordinates of the points. Its length also equals
P$n.
P$marks contains the marks. It is either NULL, or a vector of length P$n containing the
mark values, or a data frame with P$n rows containing the mark values. The entries of
P$marks may be of any atomic type (character, numeric, integer, logical, complex) or
factor.
P$window is an object of class "owin" (“observation window”) determining the study
region or spatial ‘window’.
It is possible to extract these components individually; for example, to make a histogram of
the x coordinates you could just type hist(P$x). However, do not assign values to these
components directly, or you may create inconsistencies in the data which cause spatstat to
crash.
To extract or manipulate the data in a point pattern object, use the functions provided in
the package. Important ones are:
npoints(X) number of points in X
marks(X) marks of X
coords(X) coordinates of points in X
as.owin(X) window of X
as.data.frame(X) coordinates and marks of X
marks(X)