Standard deviations in the directional distribution tool

Discussion created by JoseeRousseau on Sep 30, 2015
Latest reply on Oct 1, 2015 by JoseeRousseau



I am using the Directional Distribution (Standard Deviational Ellipse) tool under the Spatial Statistics toolbox and Measuring Geographic Distribution toolset for an analysis I wish to publish. The results of running the tool looks great but I want to make sure I understand how they were created.


The ArcGIS help website mentions “When the underlying spatial pattern of features is concentrated in the center with fewer features toward the periphery (a spatial normal distribution) a one standard deviation ellipse polygon will cover approximately 68 percent of the features; two standard deviations will contain approximately 95 percent of the features”. The key word in that sentence is “When […]”. 


  1. How do I know whether each ellipse follows a spatial normal distribution?
  2. If my spatial distribution is not normal, how are the ellipses created?  Does the tool ignores the two standard deviations requested option?
  3. I have less than a handful of ellipses that were created using less than 5 points. Regardless of the low sample size, those ellipse seem to have a buffer between the location of the points and the boundary of the ellipse with the ellipse being larger. How was the size of the ellipse determined? I can provide a figure if needed.


Thank you for all your help,