https://github.com/cran/spatstat
Tip revision: 2af0235a57146624ae8d5b148d47d8d0ab351213 authored by Adrian Baddeley on 14 May 2007, 05:36:09 UTC
version 1.11-5
version 1.11-5
Tip revision: 2af0235
nncross.Rd
\name{nncross}
\alias{nncross}
\title{Nearest Neighbour in Another Point Pattern}
\description{
Given two point patterns \code{X} and \code{Y},
finds the nearest neighbour in \code{Y} of each point of \code{X}.
}
\usage{
nncross(X, Y, iX=NULL, iY=NULL)
}
\arguments{
\item{X, Y}{Two point patterns (objects of class \code{"ppp"}).}
\item{iX, iY}{Optional identifiers, used to test whether a point in
\code{X} is identical to a point in \code{Y}. See Details}
}
\details{
Given two point patterns \code{X} and \code{Y} this
function finds, for each point of \code{X},
the nearest point of \code{Y}. The distance between these points
is also computed.
The return value is a data frame, with rows corresponding to
the points of \code{X}. The first column gives the nearest neighbour
distances (i.e. the \code{i}th entry is the distance
from the \code{i}th point of \code{X} to the nearest point of
\code{Y}). The second column gives the indices of the nearest
neighbours (i.e.\ the \code{i}th entry is the index of
the nearest point in \code{Y}.)
Note that this function is not symmetric in \code{X} and \code{Y}.
To find the nearest neighbour in \code{X} of each point in \code{Y},
just use \code{nncross(Y,X)}.
The arguments \code{iX} and \code{iY} are used when
the two point patterns \code{X} and \code{Y} have some points in
common. In this situation \code{nncross(X, Y)} would return some zero
distances. To avoid this, attach a unique integer identifier to
each point, such that two points are identical if their
identifying numbers are equal. Let \code{iX} be the vector of
identifier values for the points in \code{X}, and \code{iY}
the vector of identifiers for points in \code{Y}. Then the code
will only compare two points if they have different values of the
identifier. See the Examples.
}
\value{
A data frame with two columns:
\item{dist}{Nearest neighbour distance}
\item{which}{Nearest neighbour index in \code{Y}}
}
\examples{
# two different point patterns
X <- runifpoint(15)
Y <- runifpoint(20)
N <- nncross(X,Y)$which
# note that length(N) = 15
plot(superimpose(X=X,Y=Y), main="nncross", cols=c("red","blue"))
arrows(X$x, X$y, Y[N]$x, Y[N]$y, length=0.15)
# two patterns with some points in common
Z <- runifpoint(50)
X <- Z[1:30]
Y <- Z[20:50]
iX <- 1:30
iY <- 20:50
N <- nncross(X,Y, iX, iY)$which
plot(superimpose(X=X, Y=Y), main="nncross", cols=c("red","blue"))
arrows(X$x, X$y, Y[N]$x, Y[N]$y, length=0.15)
}
\author{
Adrian Baddeley
\email{adrian@maths.uwa.edu.au}
\url{http://www.maths.uwa.edu.au/~adrian/}
and Rolf Turner
\email{rolf@math.unb.ca}
\url{http://www.math.unb.ca/~rolf}
}
\keyword{spatial}
\keyword{math}