calc_sde {aspace}R Documentation

Calculate and plot a Standard Deviation Ellipse

Description

This function computes the Standard Deviation Ellipse (SDE) for a set of points. The SDE is a centrographic measure used to characterize the dispersion of point observations along two orthogonal axes. The SDE also captures directional bias in aspatial point pattern and will be oriented in the direction of maximum dispersion. The function provides options for weighting observations, and centring the ellipse on a user defined point, mean centre, or weighted mean centre of the input point locations. Output includes plotting of the SDE and an ASCII (text) file containing elliptical coordinates.

Usage

calc_sde(id = 1, filename = "SDE_Output.txt", calccentre = FALSE, useWMC = FALSE, centre.xy = centre, destmat = activities, titletxt = "Title", verbose = FALSE, plot = TRUE, calcSDxy = TRUE, plotSDEaxes = TRUE, plotdest = TRUE, plotcentroid = TRUE, plotSDxy = TRUE, weightpoints = FALSE, weights = wts, jpeg = FALSE)

Arguments

id An identifier for a given SDE. When running calc.sde in a loop for multiple sets of points, increment id such that it is unique!
filename The name of an ASCII (text) file where SDE coordinates will be written
calccentre Boolean: Set to TRUE if the mean center of the points is to be used as the SDE centroid
useWMC Boolean: Set to TRUE if the weighted mean center is to be used as the SDE centroid
centre.xy A numeric vector of length 2, containing specified x- and y-coordinates to use as the centroid
destmat A numeric matrix or data frame with two columns. The first column represents x-coordinates, the second, y-coordinates. Each row corresponds to a single point location.
titletxt A string to use as the title on the plot
verbose Boolean: Set to TRUE if extensive standard output feedback is desired
plot Boolean: Set to TRUE if the SDE is to be plotted
calcSDxy Boolean: Set to TRUE if the standard deviations in the orthogonal (x and y) directions are to be computed
plotSDEaxes Boolean: Set to TRUE if the orthogonal axes through the centroid are to be plotted
plotdest Boolean: Set to TRUE if input point observations are to be plotted along with the SDE
plotcentroid Boolean: Set to TRUE if the centroid is to be plotted along with the SDE
plotSDxy Boolean: Set to TRUE if the orthogonal standard deviation box should be plotted along with the SDE
weightpoints Boolean: Set to TRUE if the point observations are to be weighted
weights A matrix or data frame of weights for the points
jpeg Boolean: Set to TRUE if the plot should be saved in JPEG format

Details

This function is most powerful when used repetitively within a loop to compute the SDE for subsets of points stored in a large data table.

Value

The returned result is a list:

CALCCENTRE Boolean: Indicates whether the mean centre was computed
WeightPoints Boolean: Indicates whether the points were weighted
UseWMC Boolean: Indicates whether the weighted mean centre is to be used
Orig.x Original x-coordinate of centre
Orig.y Original y-coordinate of centre
CENTRE.x x-coordinate after computation of mean centre
CENTRE.y y-coordinate after computation of mean centre
Sigma.x Half-length of axis along x-axis
Sigma.y Half-length of axis along y-axis
Major String indicating which axis is the major elliptical axis
Minor String indicating which axis is the minor elliptical axis
Theta Rotation angle in degrees
Eccentricity A measure of eccentricity
Area.sde Area of the SDE
TanTheta Trigonometric result
SinTheta Trigonometric result
CosTheta Trigonometric result
SinThetaCosTheta Trigonometric result
Sin2Theta Trigonometric result
Cos2Theta Trigonometric result
ThetaCorr Corrected theta angle for rotation of major axis from north
WMC.x Weighted mean center x-coordinate
WMC.y Weighted mean center y-coordinate

Note

This function can be used on its own (once) or repetitively in a loop to process grouped point data stored in a larger table. When used repetitively, be sure to increment the id parameter to ensure that each SDE has a unique identifier. The output ASCII coordinate file can be further processed using the makeshapes function to generate an ESRI Shapefile for SDE polygons.

Author(s)

Tarmo K. Remmel, Ron Buliung

References

See chapter 4 of the documentation manual for CrimeStat at http://www.icpsr.umich.edu/CRIMESTAT/ and Ebdon, D. 1987. Statistics in geography. 2nd edition. New York, NY Basil Blackwell Ltd. 232 p.

See Also

calc_sdd, calc_mcp, makeshapes

Examples

calc_sde(id = 1, filename = "SDE_Output.txt", calccentre = FALSE, useWMC = FALSE, centre.xy = centre, destmat = activities, titletxt = "Title", verbose = FALSE, plot = TRUE, calcSDxy = TRUE, plotSDEaxes = TRUE, plotdest = TRUE, plotcentroid = TRUE, plotSDxy = TRUE, weightpoints = FALSE, weights = wts, jpeg = FALSE) 

[Package aspace version 1.2 Index]