| nnmap {spatstat.geom} | R Documentation |
K-th Nearest Point Map
Description
Given a point pattern, this function constructs pixel images
giving the distance from each pixel to its k-th nearest
neighbour in the point pattern, and the index of the k-th nearest
neighbour.
Usage
nnmap(X, k = 1, what = c("dist", "which"),
..., W = as.owin(X),
is.sorted.X = FALSE, sortby = c("range", "var", "x", "y"))
Arguments
X |
Point pattern (object of class |
k |
Integer, or integer vector. The algorithm will find the
|
what |
Character string specifying what information should be returned.
Either the nearest neighbour distance ( |
... |
Arguments passed to |
W |
Window (object of class |
is.sorted.X |
Logical value attesting whether the point pattern |
sortby |
Determines which coordinate to use to sort the point pattern. See Details. |
Details
Given a point pattern X, this function constructs two pixel images:
a distance map giving, for each pixel, the distance to the nearest point of
X;a nearest neighbour map giving, for each pixel, the identifier of the nearest point of
X.
If the argument k is specified, then the k-th nearest
neighbours will be found.
If what="dist" then only the distance map is returned.
If what="which" then only the nearest neighbour map
is returned.
The argument k may be an integer or an integer vector.
If it is a single integer, then the k-th nearest neighbours
are computed. If it is a vector, then the k[i]-th nearest
neighbours are computed for each entry k[i]. For example, setting
k=1:3 will compute the nearest, second-nearest and
third-nearest neighbours.
Value
A pixel image, or a list of pixel images.
By default (if what=c("dist", "which")), the result is
a list with two components dist and which
containing the distance map and the nearest neighbour map.
If what="dist" then the result is a real-valued pixel image
containing the distance map.
If what="which" then the result is an integer-valued pixel image
containing the nearest neighbour map.
If k is a vector of several integers, then the result is
similar except that each pixel image is replaced by a list of
pixel images, one for each entry of k.
Sorting data and pre-sorted data
Read this section if you care about the speed of computation.
For efficiency, the algorithm sorts the point pattern X
into increasing order of the x coordinate
or increasing order of the the y coordinate.
Sorting is only an intermediate step;
it does not affect the output, which is always given in the same
order as the original data.
By default (if sortby="range"),
the sorting will occur on the coordinate that has the larger range of
values (according to the frame of the enclosing window of X).
If sortby = "var"), sorting will occur on the coordinate that
has the greater variance (in the pattern X).
Setting sortby="x" or sortby = "y" will specify that
sorting should occur on the x or y coordinate, respectively.
If the point pattern X is already
sorted, then the argument is.sorted.X
should be set to TRUE, and sortby should be set
equal to "x" or "y" to indicate which coordinate
is sorted.
Warning About Ties
Ties are possible: there may be two data points
which lie exactly the same distance away from a particular pixel.
This affects the results from nnmap(what="which").
The handling of ties is not well-defined: it is not consistent
between different computers and different installations of R.
If there are ties, then different calls to nnmap(what="which")
may give inconsistent results. For example, you may get a different answer
from nnmap(what="which",k=1)
and nnmap(what="which", k=1:2)[[1]].
Author(s)
Adrian Baddeley Adrian.Baddeley@curtin.edu.au
, Rolf Turner rolfturner@posteo.net , and Jens Oehlschlaegel
See Also
Examples
plot(nnmap(cells, 2, what="which"))