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 in the output (SDE) layer:
    • 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)
    • eccentricity (sqrt(1-b²/a²)).

Options

The plugin GUI offers a number of options:

sde_gui

Choices:

  • For the Yuill method, a Weight field can be used (if the layer has a numeric attribute).
  • Selected features only to be used (if no features are selected, all features will be used)
  • Select method
    • Yuill (see introduction)
    • CrimeStat (as implemented in the R package aspace)
  • For the Yuill method, it is possible to use DoF correction (DoF: Degrees of Freedom) and 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.

The suggested name for the output layer is: SDE_<method>_<input_layer_name>. But that can be modified.

Implementation

The calculations of the standard deviational ellipse parameters is performed in a separate thread that reports its progress through the progressbar in the GUI.

Versions

The current version is 3.0.5.

  • 3.0.5: Eliminated the superfluous “Standard deviational ellipse” submenu under the Vector menu. GUI and documentation fixes.
  • 3.0.4: Fixed issue with selected features in QGIS 3.
  • 3.0.3: Fixed issue with integer fields from Shapefile format datasets not being accepted as weights.
  • 3.0.2: Make icon show in the toolbar and make help show
  • 3.0.1: SDE polygon closed to make it valid.
  • 3.0.0: Update to the QGIS 3.0 API. GUI rearranged.
  • 1.2.1: Help button added. Double quotes removed from the output layer name.
  • 1.2.0: “Compass” direction and eccentricity added (for the major axis). Fixed DOF calculations. GUI and robustness improvements.
  • 1.1: CrimeStat option added. More options added. GUI modifications.
  • 1.0: First official version.

Citation

Would you like to cite / reference this plugin?

Tveite, H. (2016). The QGIS Standard Deviational Ellipse Plugin. http://plugins.qgis.org/plugins/SDEllipse/.

Bibtex:

@misc{tveitesde,
  author =   {Håvard Tveite},
  title =    {The {QGIS} {Standard} {Deviational} {Ellipse} Plugin},
  howpublished = {\url{http://plugins.qgis.org/plugins/SDEllipse/}},
  year = {2016--2020}
}