calc_sde: Calculate the Standard Deviation Ellipse

In aspace: Functions for Estimating Centrographic Statistics

Calculate the Standard Deviation Ellipse

Description

This function computes the Standard Deviation Ellipse (SDE) from 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 a spatial point pattern, the ellipse will be oriented in the direction of maximum dispersion.

Usage

calc_sde(id=1, centre.xy=NULL, calccentre=TRUE,
weighted=FALSE, weights=NULL, points=NULL, verbose=FALSE)

Arguments

id	A unique integer to identify the shape
centre.xy	A vector of length 2, containing the x- and y-coordinates of the SDE centre (Planar Coordinates Only!)
calccentre	Boolean: Set to TRUE if the mean center is to be calculated
weighted	Boolean: Set to TRUE if the weighted mean center is to be computed with weighted coordinates
weights	Weights applied to point observations, number of weights should equal the number of observations
points	A 2-column matrix or data frame containing point coordinates
verbose	Boolean: Set to TRUE if extensive feedback is desired on the standard output

Details

Use the LOCATIONS element in the output list object along with the ATTRIBUTES elements can be used to produce shapefiles or other vector point files for geographic data.

Value

The returned result is a list:

TYPE	The type of calculation results stored in the object: BOX, SDD, SDE, CMD, CF, or CF2PTS, MNC, MDC
DATE	The date and time that the function was run
ID	Identifier for the SDE shape - it should be unique
LOCATIONS	Locations pertinent for the SDE that can be used with ATTRIBUTES if wishing to build a vector point file for geographic data outside of this pacakge.
FORPLOTTING	Coordinates and identifiers used for plotting by plot_sde()
ATTRIBUTES	Attributes for the output SDE that can be used with LOCATIONS coordinates if wishing to build a vector point file for geographic data outside of this package.
id	Identifier for the SDE shape - it should be unique
calccentre	Boolean: TRUE if mean centre is computed
weighted	Boolean: TRUE if the weighted mean centre is to be used instead
CENTRE.x	X-coordinate of the centre
CENTRE.y	Y-coordinate of the 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 (i.e., the flatness of the ellipse)
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

Note

Results specific for plotting are stored in the FORPLOTTING element within the produced list object. Pass the entire object to plot_box() and the function automatically extracts this information. 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 argument to ensure that each SDE has a unique identifier.

Author(s)

Tarmo K. Remmel, Randy Bui, Ron N. 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_box, calc_sde, calc_cmd, calc_cf, calc_cf2pts, calc_mnc, calc_mdc, gridpts

Examples

# SDE EXAMPLE
data(activities)
a <- calc_sde(id=1, centre.xy=NULL, points=activities)
str(a)
print(a)

# IF THE RESULT OF THIS FUNCTION IS STORED TO AN OBJECT, THE plot_sde()
# FUNCTION WILL TAKE THAT OBJECT AS INPUT FOR PLOTTING VIA THE datin ARGUMENT

# SDE TO SHAPEFILE EXAMPLE (REMOVE THE COMMENTS TO RUN)
# shp <- convert.to.shapefile(a$LOCATIONS, a$ATTRIBUTES, "id", 5)
# write.shapefile(shp, "SDE_Shape", arcgis=T)

aspace documentation built on May 29, 2024, 2:04 a.m.