Gmulti: Marked Nearest Neighbour Distance Function

Description

For a marked point pattern, estimate the distribution of the distance from a typical point in subset I to the nearest point of subset $J$.

Usage

Gmulti(X, I, J, r=NULL, breaks=NULL, ..., disjoint=NULL)

Arguments

The observed point pattern, from which an estimate of the multitype distance distribution function $G_{IJ}(r)$ will be computed. It must be a marked point pattern. See under Details.

Subset of points of X from which distances are measured.

Subset of points in X to which distances are measured.

Optional. Numeric vector. The values of the argument $r$ at which the distribution function $G_{IJ}(r)$ should be evaluated. There is a sensible default. First-time users are strongly advised not to specify this argument. See below for

breaks

An alternative to the argument r. Not normally invoked by the user. See the Details section.

...

Ignored.

disjoint

Optional flag indicating whether the subsets I and J are disjoint. If missing, this value will be computed by inspecting the vectors I and J

Value

An object of class "fv" (see fv.object). Essentially a data frame containing six numeric columns
rthe values of the argument $r$ at which the function $G_{IJ}(r)$ has been estimated
rsthe ``reduced sample'' or ``border correction'' estimator of $G_{IJ}(r)$
kmthe spatial Kaplan-Meier estimator of $G_{IJ}(r)$
hazardthe hazard rate $\lambda(r)$ of $G_{IJ}(r)$ by the spatial Kaplan-Meier method
rawthe uncorrected estimate of $G_{IJ}(r)$, i.e. the empirical distribution of the distances from each point of type $i$ to the nearest point of type $j$
theothe theoretical value of $G_{IJ}(r)$ for a marked Poisson process with the same estimated intensity

Warnings

The function $G_{IJ}$ does not necessarily have a density.

The reduced sample estimator of $G_{IJ}$ is pointwise approximately unbiased, but need not be a valid distribution function; it may not be a nondecreasing function of $r$. Its range is always within $[0,1]$.

The spatial Kaplan-Meier estimator of $G_{IJ}$ is always nondecreasing but its maximum value may be less than $1$.

Details

The function Gmulti generalises Gest (for unmarked point patterns) and Gdot and Gcross (for multitype point patterns) to arbitrary marked point patterns.

Suppose $X_I$, $X_J$ are subsets, possibly overlapping, of a marked point process. This function computes an estimate of the cumulative distribution function $G_{IJ}(r)$ of the distance from a typical point of $X_I$ to the nearest distinct point of $X_J$.

The argument X must be a point pattern (object of class "ppp") or any data that are acceptable to as.ppp.

The arguments I and J specify two subsets of the point pattern. They may be logical vectors of length equal to X$n, or integer vectors with entries in the range 1 to X$n, etc.

This algorithm estimates the distribution function $G_{IJ}(r)$ 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 Gest.

The argument r is the vector of values for the distance $r$ at which $G_{IJ}(r)$ should be evaluated. It is also used to determine the breakpoints (in the sense of hist) for the computation of histograms of distances. The reduced-sample and Kaplan-Meier estimators are computed from histogram counts. In the case of the Kaplan-Meier estimator this introduces a discretisation error which is controlled by the fineness of the breakpoints.

First-time users would be strongly advised not to specify r. However, if it is specified, r must satisfy r[1] = 0, and max(r) must be larger than the radius of the largest disc contained in the window. Furthermore, the successive entries of r must be finely spaced.

The algorithm also returns an estimate of the hazard rate function, $\lambda(r)$, of $G_{IJ}(r)$. This estimate should be used with caution as $G_{IJ}(r)$ is not necessarily differentiable.

The naive empirical distribution of distances from each point of the pattern X to the nearest other point of the pattern, is a biased estimate of $G_{IJ}$. However this is also returned by the algorithm, as it is sometimes useful in other contexts. Care should be taken not to use the uncorrected empirical $G_{IJ}$ as if it were an unbiased estimator of $G_{IJ}$.

References

Cressie, N.A.C. Statistics for spatial data. John Wiley and Sons, 1991.

Diggle, P.J. Statistical analysis of spatial point patterns. Academic Press, 1983.

Diggle, P. J. (1986). Displaced amacrine cells in the retina of a rabbit : analysis of a bivariate spatial point pattern. J. Neurosci. Meth. 18, 115--125. Harkness, R.D and Isham, V. (1983) A bivariate spatial point pattern of ants' nests. Applied Statistics 32, 293--303 Lotwick, H. W. and Silverman, B. W. (1982). Methods for analysing spatial processes of several types of points. J. Royal Statist. Soc. Ser. B 44, 406--413.

Ripley, B.D. Statistical inference for spatial processes. Cambridge University Press, 1988.

Stoyan, D, Kendall, W.S. and Mecke, J. Stochastic geometry and its applications. 2nd edition. Springer Verlag, 1995.

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.

Examples

Run this code

data(longleaf)
     # Longleaf Pine data: marks represent diameter
    <testonly>longleaf <- longleaf[seq(1,longleaf$n, by=50), ]</testonly>
    Gm <- Gmulti(longleaf, longleaf$marks <= 15, longleaf$marks >= 25)
    plot(Gm)

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