pair {sgeostat}

Pair Object

Description

Create a pair object from a point object.

A pair object contains information defining pairs of points contained in a point object. A pair object is a list containing five vectors: from, to, lags, dist, and bins. The length of each of these vectors (except bins) is equal to the number of pairs of points being represented, say k. The vectors from and to contain pointers into the vectors of a point object, pointing to each member of the pair of points (e.g., from[k] points to si and to[k] points to sj). The vector dist contains the distance between the pairs of points. The vector lags contains the lag number to which each pair of points has been assigned. The vector bins contains the spatial midpoint between each lag and is used for plotting.

Usage

pair(point.obj,num.lags=10,type='isotropic', theta=0, dtheta=5, maxdist) 

Arguments

point.obj

a point object generated by point()

num.lags

the number of lags into which to divide the pairs of points in the pair object. The lags are all of equal size.

type

either 'isotropic' or 'anisotropic'. If 'isotropic' then all n \choose 2 possible pairs of points are represented in the pair object. If 'anisotropic', then the arguments theta and dtheta are used to determine which pairs of points to include.

theta

an angle, measured in degrees from the horizontal x axis, that determines pairs of points to be included in the pair object (see Notes below).

dtheta

a tolerance angle, around theta, measured in degrees that determines pairs of points to be included in the pair object (see Notes below).

maxdist

the distance beyond which not to consider pairs of points. A large number of spatial locations can cause the pair function to consume a considerable amount of computation time. In most cases, spatial dependence can be adaquately characterized without examining the entire spaital extent of the data set.

Value

A pair object:

from

vector of indices into the point object for "from" point

to

vector of indices into the point object for "to" point

lags

vector of spatial lags of each pair

dist

vector of distances between each pair

bins

vector of spatial midpoints of each lag (used for plotting)

NOTE

Name of this function changed from pairs to pair to avoid conflicts with R's pairs function!!

Note

When creating an anisotropic pair object, the assumption is that the direction, as well as the distance, between pairs of points is important in describing the variation. Using the theta and dtheta arguments, pairs of points that meet direction requirements can be selected. A pair of points will be included when the angle between the positive x axis and the vector formed by the pair of points falls within the tolerance angle given by (theta-dtheta,theta+dtheta)

References

http://www.gis.iastate.edu/SGeoStat/homepage.html

See Also

point

Examples

maas.pair   <- pair(maas.point,num.lags=10,maxdist=2000)
maas.pair25 <- pair(maas.point,num.lags=10,type='anisotropic',
                      theta=25,maxdist=500)