quad.object

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Class of Quadrature Schemes

A class "quad" to represent a quadrature scheme.

Keywords
spatial
Details

A (finite) quadrature scheme is a list of quadrature points $u_j$ and associated weights $w_j$ which is used to approximate an integral by a finite sum: $$\int f(x) dx \approx \sum_j f(u_j) w_j$$ Given a point pattern dataset, a Berman-Turner quadrature scheme is one which includes all these data points, as well as a nonzero number of other (``dummy'') points.

These quadrature schemes are used to approximate the pseudolikelihood of a point process, in the method of Baddeley and Turner (2000) (see Berman and Turner (1992)). Accuracy and computation time both increase with the number of points in the quadrature scheme.

An object of class "quad" represents a Berman-Turner quadrature scheme. It can be passed as an argument to the model-fitting function ppm, which requires a quadrature scheme.

An object of this class contains at least the following elements: ll{ data: an object of class "ppp" giving the locations (and marks) of the data points. dummy: an object of class "ppp" giving the locations (and marks) of the dummy points. w: vector of nonnegative weights for the quadrature points } Users are strongly advised not to manipulate these entries directly. The domain of quadrature is specified by dummy$window while the observation window (if this needs to be specified separately) is taken to be data$window.

The weights vector w may also have an attribute attr(w, "zeroes") equivalent to the logical vector (w == 0). If this is absent then all points are known to have positive weights. To create an object of class "quad", users would typically call the high level function quadscheme. (They are actually created by the low level function quad.) Entries are extracted from a "quad" object by the functions x.quad, y.quad, w.quad and marks.quad, which extract the $x$ coordinates, $y$ coordinates, weights, and marks, respectively. The function n.quad returns the total number of quadrature points (dummy plus data).

An object of class "quad" can be converted into an ordinary point pattern by the function union.quad which simply takes the union of the data and dummy points.

Quadrature schemes can be plotted using plot.quad (a method for the generic plot).

See Also

quadscheme, ppm

Aliases
  • quad.object
Documentation reproduced from package spatstat, version 1.6-2, License: GPL version 2 or newer

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