calc_sdd {aspace}R Documentation

Calculate the Standard Distance Deviation (Standard Distance)

Description

This function computes the Standard Distance Deviation (SDD) or Standard Distance from a set of points.

Usage

calc_sdd(id=1, filename="SDD_Output.txt", centre.xy=NULL, calccentre=TRUE, weighted=FALSE, weights=NULL, CMD.npts=10000, points=activities, verbose=FALSE)

Arguments

id A unique integer to identify a SDD estimate
filename A string indicating the ASCII textfile where shape coordinates will be written
centre.xy A vector of length 2, containing the x- and y-coordinates of the SDD centre
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
CMD.npts The approximate number of points to generate for the centre of minimum distance (CMD) calculation
points A 2-column matrix or data frame containing the set of point observations input to the calc_sdd function
verbose Boolean: Set to TRUE if extensive feedback is desired on the standard output

Details

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

Value

The result is a list of terms:

id Identifier for the SDD 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
central.x X-coordinate of the identified central feature
central.y Y-coordinate of the identified central feature
median.x X-coordinate of median centre, median value of the X-coordinate vector
median.y Y-coordinate of median centre, median value of the Y-coordinate vector
CMD.x X-coordinate of estimated centre of minimum distance
CMD.y Y-coordinate of estimated centre of minimum distance
SDD.radius SDD value, radius of the SDD
SDD.area Area of the SDD circle

Note

Results are stored in the r.SDD object (required for graphical visualization using plot_sdd). As the value of 'CMD.npts' increases, the more precise the centre of minimum distance is estimated. 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 SDD has a unique identifier. The output ASCII coordinate file can be further processed using the makeshapes function to generate an ESRI Shapefile for SDD polygons.

Author(s)

Randy Bui, Ron Buliung, Tarmo K. Remmel

See Also

plot_sdd, calc_mcp, calc_sde, calc_box, ellipse3, makeshapes, mcp, gridpts

Examples

calc_sdd(id=1, filename="SDD_Output.txt", centre.xy=NULL, calccentre=TRUE, weighted=TRUE, weights=wts, CMD.npts=10000, points=activities, verbose=TRUE)

[Package aspace version 2.2 Index]