Gfox
Foxall's Distance Functions
Given a point pattern X and a spatial object Y,
compute estimates of Foxall's \(G\) and \(J\) functions.
- Keywords
- spatial, nonparametric
Usage
Gfox(X, Y, r = NULL, breaks = NULL, correction = c("km", "rs", "han"), ...)
Jfox(X, Y, r = NULL, breaks = NULL, correction = c("km", "rs", "han"), ...)Arguments
- X
A point pattern (object of class
"ppp") from which distances will be measured.- Y
An object of class
"ppp","psp"or"owin"to which distances will be measured.- r
Optional. Numeric vector. The values of the argument \(r\) at which \(Gfox(r)\) or \(Jfox(r)\) 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
This argument is for internal use only.
- correction
Optional. The edge correction(s) to be used to estimate \(Gfox(r)\) or \(Jfox(r)\). A vector of character strings selected from
"none","rs","km","cs"and"best". Alternativelycorrection="all"selects all options.- …
Extra arguments affecting the discretisation of distances. These arguments are ignored by
Gfox, butJfoxpasses them toHestto determine the discretisation of the spatial domain.
Details
Given a point pattern X and another spatial object Y,
these functions compute two nonparametric measures of association
between X and Y, introduced by Foxall
(Foxall and Baddeley, 2002).
Let the random variable \(R\) be the distance from a typical point
of X to the object Y.
Foxall's \(G\)-function is the cumulative distribution function
of \(R\):
$$G(r) = P(R \le r)$$
Let the random variable \(S\) be the distance from a fixed point
in space to the object Y. The cumulative distribution function
of \(S\) is the (unconditional) spherical contact distribution
function
$$H(r) = P(S \le r)$$
which is computed by Hest.
Foxall's \(J\)-function is the ratio $$ J(r) = \frac{1-G(r)}{1-H(r)} $$ For further interpretation, see Foxall and Baddeley (2002).
Accuracy of Jfox depends on the pixel resolution,
which is controlled by the
arguments eps, dimyx and xy passed to
as.mask. For example, use eps=0.1 to specify
square pixels of side 0.1 units, and dimyx=256 to specify a
256 by 256 grid of pixels.
Value
A function value table (object of class "fv")
which can be printed, plotted, or converted to a data frame of values.
References
Foxall, R. and Baddeley, A. (2002) Nonparametric measures of association between a spatial point process and a random set, with geological applications. Applied Statistics 51, 165--182.
See Also
Examples
# NOT RUN {
data(copper)
X <- copper$SouthPoints
Y <- copper$SouthLines
G <- Gfox(X,Y)
J <- Jfox(X,Y, correction="km")
# }
# NOT RUN {
# }
# NOT RUN {
J <- Jfox(X,Y, correction="km", eps=0.25)
# }