The QGIS Standard Deviational Ellipse Plugin

The QGIS Standard Deviational Ellipse plugin implements the method presented in The Standard Deviational Ellipse; An Updated Tool for Spatial Description Robert S. Yuill. Geografiska Annaler. Series B, Human Geography. Vol. 53, No. 1 (1971), pp. 28-39. URL: https://www.jstor.org/stable/490885

Apparently, there are many definitions of a Standard Deviational Ellipse. CrimeStat implements a method that produces different results from the Yuill method, and this method is offered as an alternative. This method is also implemented in the R aspace package.

Functionality

  • The QGIS Standard Deviational Ellipse plugin can be used to investigate point patterns, showing a summary of the distribution as a standard deviational ellipse.
  • Output is a polygon layer with the standard deviational ellipse as the only feature. The following attributes are available for the layer / ellipse: meanx, meany, majorsd (SD along the major axis), minorsd (SD along the minor axis that is normal to the major axis), majorangle(rad) (major axis angle in radians counter-clockwise relative to x/east), directiond(eg) (“compass” direction - degrees clockwise relative to north) and eccentricity (sqrt(1-b²/a²)).

Options

  • The user can choose between different methods
    • Yuill (see introduction)
    • CrimeStat (as implemented in the R package aspace)
  • Specify if only selected features are to be used (but if no features are selected, all features will be used)
  • Select an attribute for weighting (for Yuill’s method).
  • For Yuill’s method, it is possible to try a DOF (degrees of freedom) correction and a sqrt(2) correction (to make the standard deviational ellipse equal to the standard distance deviation when the distribution of points is random and even in all directions). With both of these corrections applied, the result will be the same as for the CrimeStat method.

Implementation

The calculations of the standard deviational ellipse parameters is performed in a separate thread.

Versions

The current version is 3.0.

  • 3.0: Update to the QGIS 3.0 API. UI rearranged. Close the SDE polygon to make it valid.
  • 1.2.1: Added a help button and removed double quotes from the output layer name.
  • 1.2: Added “compass” direction (for the major axis) and eccentricity. Fixed DOF calculations. User interface and robustness improvements.
  • 1.1: Added the CrimeStat option. Added some more options and modified the user interface.
  • 1.0: First official version.