Create a Point Pattern

Description

Creates an object of class "ppp" representing a point pattern dataset in the two-dimensional plane.

Usage

  ppp(x,y, ..., window, marks, check=TRUE)

Arguments

Details

In the spatstat library, a point pattern dataset is described by an object of class "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 then poly must be a list with components x,y giving the coordinates of the vertices. If the boundary consists of several disjoint polygons then poly must be a list of such lists so that poly[[i]]$x gives the x coordinates of the vertices of the ith boundary polygon.
• mask specifies a binary pixel image with entries that are TRUE if the corresponding pixel is inside the window.
• window is an object of class "owin" (see owin.object) specifying the window.

The arguments 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. The rejected points are still accessible, as an attribute of the point pattern called "rejects", but will be ignored by all other functions except link{plot.ppp}.

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.

In the current implementation, marks must be a vector, of the same length as x and y, which is interpreted so that marks[i] is the mark attached to the point (x[i],y[i]). If the mark is a real number then marks should be a numeric vector, while if the mark takes only a finite number of possible values (e.g. colours or types) then marks should be a factor.

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 (see ppp.object).

Author(s)

Adrian Baddeley adrian@maths.uwa.edu.au http://www.maths.uwa.edu.au/~adrian/ and Rolf Turner rolf@math.unb.ca http://www.math.unb.ca/~rolf

See Also

ppp.object, as.ppp, owin.object, owin, as.owin

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)))

  ## Not run: 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)
  ## Not run: plot(X)

  # polygonal window
  X <- ppp(x, y, poly=list(x=c(0,10,0), y=c(0,0,10)))
  ## Not run: plot(X)

  # copy the window from another pattern
  data(cells)
  X <- ppp(x, y, window=cells$window)