| Jcross {spatstat} | R Documentation |
For a multitype point pattern, estimate the multitype J function summarising the interpoint dependence between points of type i and of type j.
Jcross(X, i=1, j=2, eps=NULL, r=NULL, breaks=NULL, ...)
X |
The observed point pattern, from which an estimate of the multitype J function Jij(r) will be computed. It must be a multitype point pattern (a marked point pattern whose marks are a factor). See under Details. |
i |
Number or character string identifying the type (mark value)
of the points in X from which distances are measured.
|
j |
Number or character string identifying the type (mark value)
of the points in X to which distances are measured.
|
eps |
A positive number. The resolution of the discrete approximation to Euclidean distance (see below). There is a sensible default. |
r |
Optional. Numeric vector. The values of the argument r at which the function Jij(r) should be evaluated. There is a sensible default. First-time users are strongly advised not to specify this argument. See below for important conditions on r. |
breaks |
An alternative to the argument r.
Not normally invoked by the user. See the Details section.
|
... |
Ignored. |
This function Jcross and its companions
Jdot and Jmulti
are generalisations of the function Jest
to multitype point patterns.
A multitype point pattern is a spatial pattern of points classified into a finite number of possible ``colours'' or ``types''. In the spatstat package, a multitype pattern is represented as a single point pattern object in which the points carry marks, and the mark value attached to each point determines the type of that point.
The argument X must be a point pattern (object of class
"ppp") or any data that are acceptable to as.ppp.
It must be a marked point pattern, and the mark vector
X$marks must be a factor.
The argument i will be interpreted as a
level of the factor X$marks. (Warning: this means that
an integer value i=3 will be interpreted as the 3rd smallest level,
not the number 3).
The ``type i to type j'' multitype J function of a stationary multitype point process X was introduced by Van lieshout and Baddeley (1999). It is defined by
Jij(r) = (1 - Gij(r))/(1-Fj(r))
where Gij(r) is the distribution function of the distance from a type i point to the nearest point of type j, and Fj(r) is the distribution function of the distance from a fixed point in space to the nearest point of type j in the pattern.
An estimate of Jij(r) is a useful summary statistic in exploratory data analysis of a multitype point pattern. If the subprocess of type i points is independent of the subprocess of points of type j, then Jij(r) = 1. Hence deviations of the empirical estimate of Jij from the value 1 may suggest dependence between types.
This algorithm estimates Jij(r)
from the point pattern X. It assumes that X can be treated
as a realisation of a stationary (spatially homogeneous)
random spatial point process in the plane, observed through
a bounded window.
The window (which is specified in X as X$window)
may have arbitrary shape.
Biases due to edge effects are
treated in the same manner as in Jest,
using the Kaplan-Meier and border corrections.
The main work is done by Gmulti and Fest.
The argument r is the vector of values for the
distance r at which Jij(r) should be evaluated.
The values of r must be increasing nonnegative numbers
and the maximum r value must exceed the radius of the
largest disc contained in the window.
An object of class "fv" (see fv.object).
Essentially a data frame containing six numeric columns
J |
the recommended estimator of Jij(r), currently the Kaplan-Meier estimator. |
r |
the values of the argument r at which the function Jij(r) has been estimated |
km |
the Kaplan-Meier estimator of Jij(r) |
rs |
the ``reduced sample'' or ``border correction'' estimator of Jij(r) |
un |
the ``uncorrected''
estimator of Jij(r)
formed by taking the ratio of uncorrected empirical estimators
of 1 - Gij(r)
and 1 - Fj(r), see
Gdot and Fest.
|
theo |
the theoretical value of Jij(r) for a marked Poisson process, namely 1. |
The result also has two attributes "G" and "F"
which are respectively the outputs of Gcross
and Fest for the point pattern.
The argument i is interpreted as
a level of the factor X$marks. Beware of the usual
trap with factors: numerical values are not
interpreted in the same way as character values. See the first example.
Adrian Baddeley adrian@maths.uwa.edu.au http://www.maths.uwa.edu.au/~adrian/ and Rolf Turner rolf@math.unb.ca http://www.math.unb.ca/~rolf
Van Lieshout, M.N.M. and Baddeley, A.J. (1996) A nonparametric measure of spatial interaction in point patterns. Statistica Neerlandica 50, 344–361.
Van Lieshout, M.N.M. and Baddeley, A.J. (1999) Indices of dependence between types in multivariate point patterns. Scandinavian Journal of Statistics 26, 511–532.
# Lansing woods data: 6 types of trees
data(lansing)
Jhm <- Jcross(lansing, "hickory", "maple")
# diagnostic plot for independence between hickories and maples
plot(Jhm)
# synthetic example with two marks "a" and "b"
pp <- runifpoispp(50)
pp <- pp %mark% sample(c("a","b"), pp$n, replace=TRUE)
J <- Jcross(pp, "a", "b")