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, majoranglerad
(major axis angle in radians counter-clockwise relative to x/east),
directiondeg (“compass” direction - degrees clockwise relative
to north), majorsd (SD along the major axis), minorsd (SD along
the minor axis that is normal to the major axis) and eccentricity.
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 1.2.
- 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.