Directional Distribution (Standard Deviational Ellipse) Tool ArcMap

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Directional Distribution (Standard Deviational Ellipse) Tool, Measuring Geographic Distribution Toolset, Spatial Statistics ArcToolbox
Summary
Creates standard deviational ellipses to summarize the spatial characteristics of geographic features: central tendency, dispersion, and directional trends.

Usage
The Standard Deviational Ellipse tool creates a new Output Feature Class containing elliptical polygons, one for each case (Case Field parameter). The attribute values for these ellipse polygons include X and Y coordinates for the mean center, two standard distances (long and short axes), and the orientation of the ellipse. The fieldnames are CenterX, CenterY, XStdDist, YStdDist, and Rotation. When a Case Field is provided, this field is added to the Output Feature Class, as well.

Calculations based on either Euclidean or Manhattan distance require projected data to accurately measure distances.

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; and three standard deviations will cover approximately 99 percent of the features in the cluster.

The value in the output Rotation field represents the rotation of the long axis measured clockwise from noon.
The Case Field is used to group features prior to analysis. When a Case Field is specified, the input features are first grouped according to case field values, and then a standard deviational ellipse is computed for each group. The case field can be of integer, date, or string type. Records with NULL values for the Case Field will be excluded from analysis.

The standard deviational ellipse calculation may be based on an optional Weight Field (to get the ellipses for traffic accidents weighted by severity, for example). The weight field should be numeric.

For line and polygon features, feature centroids are used in distance computations. For multipoints, polylines, or polygons with multiple parts, the centroid is computed using the weighted mean center of all feature parts. The weighting for point features is 1, for line features is length, and for polygon features is area.

Map layers can be used to define the Input Feature Class. When using a layer with a selection, only the selected features are included in the analysis.

Caution:
When using shapefiles, keep in mind that they cannot store null values. Tools or other procedures that create shapefiles from nonshapefile inputs may store or interpret null values as zero. In some cases, nulls are stored as very large negative values in shapefiles. This can lead to unexpected results. See Geoprocessing considerations for shapefile output for more information.

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