ppp
Create a Point Pattern
Creates an object of class "ppp"
representing
a point pattern dataset in the two-dimensional plane.
Usage
ppp(x,y, ..., window, marks, check=TRUE, drop=TRUE)
Arguments
- x
- Vector of $x$ coordinates of data points
- y
- Vector of $y$ coordinates of data points
- window
- window of observation,
an object of class
"owin"
- ...
- arguments passed to
owin
to create the window, ifwindow
is missing - marks
- (optional) mark values for the points. A vector or data frame.
- check
- Logical flag indicating whether to check
that all the $(x,y)$ points lie inside the specified window.
Do not set this to
FALSE
unless you are sure that this check is unnecessary. - drop
- Logical flag indicating whether to simplify data frames of marks. See Details.
Details
In the "ppp"
. This function
creates such objects.
The vectors x
and y
must be numeric vectors of
equal length. They are interpreted as the cartesian coordinates
of the points in the pattern.
A point pattern dataset is assumed to have been observed within a specific
region of the plane called the observation window.
An object of class "ppp"
representing a point pattern
contains information specifying the observation window.
This window must always be specified when creating a point pattern dataset;
there is intentionally no default action of ``guessing'' the window
dimensions from the data points alone.
You can specify the observation window in several (mutually exclusive) ways:
xrange, yrange
specify a rectangle with these dimensions;poly
specifies a polygonal boundary. If the boundary is a single polygon thenpoly
must be a list with componentsx,y
giving the coordinates of the vertices. If the boundary consists of several disjoint polygons thenpoly
must be a list of such lists so thatpoly[[i]]$x
gives the$x$coordinates of the vertices of the$i$th boundary polygon.mask
specifies a binary pixel image with entries that areTRUE
if the corresponding pixel is inside the window.window
is an object of class"owin"
(seeowin.object
) specifying the window.
xrange, yrange
or poly
or mask
are passed to the window creator function
owin
for interpretation. See
owin
for further details. The argument window
, if given, must be an object of class
"owin"
. It is a full description of the window geometry,
and could have been obtained from owin
or
as.owin
, or by just extracting the observation window
of another point pattern, or by manipulating such windows.
See owin
or the Examples below.
The points with coordinates x
and y
must lie inside the specified window, in order to
define a valid object of this class.
Any points which do not lie inside the window will be
removed from the point pattern, and a warning will be issued.
See the section on Rejected Points.
The name of the unit of length for the x
and y
coordinates
can be specified in the dataset, using the argument unitname
, which is
passed to owin
. See the examples below, or the help file
for owin
.
The optional argument marks
is given if the point pattern
is marked, i.e. if each data point carries additional information.
For example, points which are classified into two or more different
types, or colours, may be regarded as having a mark which identifies
which colour they are. Data recording the locations and heights of
trees in a forest can be regarded as a marked point pattern where the
mark is the tree height.
The argument marks
can be either
- a vector, of
the same length as
x
andy
, which is interpreted so thatmarks[i]
is the mark attached to the point(x[i],y[i])
. If the mark is a real number thenmarks
should be a numeric vector, while if the mark takes only a finite number of possible values (e.g. colours or types) thenmarks
should be afactor
. - a data frame, with the number of rows equal to the number of points
in the point pattern. The
i
th row of the data frame is interpreted as containing the mark values for thei
th point in the point pattern. The columns of the data frame correspond to different mark variables (e.g. tree species and tree diameter).
drop=TRUE
(the default), then
a data frame with only one column will be
converted to a vector, and a data frame with no columns will be
converted to NULL
.
See ppp.object
for a description of the
class "ppp"
. Users would normally invoke ppp
to create a point pattern,
but the functions as.ppp
and
scanpp
may sometimes be convenient.
Value
- An object of class
"ppp"
describing a point pattern in the two-dimensional plane (seeppp.object
).
Rejected points
The points with coordinates x
and y
must lie inside the specified window, in order to
define a valid object of class "ppp"
.
Any points which do not lie inside the window will be
removed from the point pattern, and a warning will be issued.
The rejected points are still accessible: they are stored
as an attribute of the point pattern called "rejects"
(which is an object of class "ppp"
containing the rejected points
in a large window). However, rejected points in a point pattern
will be ignored by all other functions except
plot.ppp
.
To remove the rejected points altogether,
use as.ppp
. To include the rejected points,
you will need to find a larger window that contains them,
and use this larger window in a call to ppp
.
See Also
Examples
# some arbitrary coordinates in [0,1]
x <- runif(20)
y <- runif(20)
# the following are equivalent
X <- ppp(x, y, c(0,1), c(0,1))
X <- ppp(x, y)
X <- ppp(x, y, window=owin(c(0,1),c(0,1)))
# specify that the coordinates are given in metres
X <- ppp(x, y, c(0,1), c(0,1), unitname=c("metre","metres"))
plot(X)
# marks
m <- sample(1:2, 20, replace=TRUE)
m <- factor(m, levels=1:2)
X <- ppp(x, y, c(0,1), c(0,1), marks=m)
plot(X)
# polygonal window
X <- ppp(x, y, poly=list(x=c(0,10,0), y=c(0,0,10)))
plot(X)
# copy the window from another pattern
data(cells)
X <- ppp(x, y, window=cells$window)