Directional Distribution (Standard Deviational Ellipse) (Spatial Statistics)

Usage tips

• The standard ellipse calculation may be based on an optional weight field.

• Standard Deviational Ellipse creates a new feature class containing an elliptical polygon. The attribute values for these ellipse polygons include two standard distances (long and short axes) and the orientation of the ellipse.

• If the underlying pattern in the data is normally distributed about the mean, the one standard deviation ellipse polygon will cover approximately 68 percent of the features in the cluster. 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 following fields are added to the output ellipse feature class to store ellipse information: CenterX stores the x coordinate of the center of the ellipse, CenterY stores the y coordinate of the center of the ellipse, XStdDist stores the standard distance in the x direction, and YStdDist stores the standard distance in the y direction and Rotation which stores the rotation of the ellipse (if you specify a case field, it is also included in the output feature class).

• The value in the Rotation field represents the rotation of the long axis measured clockwise from noon.

• If a case field is specified, then the input features are grouped according to case field values. A mean center is calculated from the average x and y values for the centroids in each group.

• For line and polygon features, feature centroids are used in the computations.

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

• Current map layers may be used to define the input feature class. When using layers, only the currently selected features are used in the Central Feature operation.

• Learn more about working with layers and table views

• The following environment settings affect this tool: Cluster Tolerance, Extent, M Domain, Configuration Keyword, Coordinate System, Output has M Values, Output Spatial Grid, Output has Z Values, Default Z Value, Output XY Domain, and Output Z Domain.

Command line syntax
An overview of the Command Line window
DirectionalDistribution_stats <Input_Feature_Class> <Output_Ellipse_Feature_Class> <1 Standard Deviation | 2 Standard Deviations | 3 Standard Deviations> {Weight_Field} {Case_Field}

Command line example

workspace e:\project93\data
DirectionalDistribution assault.shp assault_ellipse.shp '1 Standard Deviation' # TYPE

Scripting syntax
About getting started with writing geoprocessing scripts
DirectionalDistribution_stats (Input_Feature_Class, Output_Ellipse_Feature_Class, Ellipse_Size, Weight_Field, Case_Field)

Script example

# Measure the geographic distribution of auto thefts
# Import system modules
import arcgisscripting

# Create the Geoprocessor object
gp = arcgisscripting.create()

# Local variables...
workspace = "C:/chris/data/"
auto_theft_locations = "AutoTheft.shp"
auto_theft_links = "AutoTheft_links.shp"
auto_theft_sd = "auto_theft_SD.shp"
auto_theft_se = "auto_theft_SE.shp"
auto_theft_ldm = "auto_theft_LDM.shp"

try:
    # Set the workspace (to avoid having to type in the full path to the data every time)
    gp.Workspace = workspace

    # Process: Standard Distance of auto theft locations...
    gp.StandardDistance_stats(auto_theft_locations, auto_theft_sd, "1 Standard Deviation", "#", "#")

    # Process: Directional Distribution (Standard Deviational Ellipse) of auto theft locations...
    gp.DirectionalDistribution_stats(auto_theft_locations, auto_theft_se, "1 Standard Deviation", "#", "#")

    # Process: Linear Directional Mean of auto thefts...
    gp.DirectionalMean_stats(auto_theft_links, auto_theft_ldm, "false", "#")

except:
    # If an error occurred while running a tool, print the messages 
    print gp.GetMessages()