Desktop Help 10.0 - Generate Spatial Weights Matrix (Spatial Statistics)

Summary

Constructs a spatial weights matrix (.swm) file to represent the spatial relationships among features in a dataset.

Learn more about how Generate Spatial Weights Matrix works

Illustration

Spatial weights based on Polygon Contiguity.

Spatial relationships based on polygon contiguity, Queen's case: shared edges or nodes.

Usage

Output from this tool is a spatial weights matrix file (.swm). Tools that require you to specify a Conceptualization of Spatial Relationships option will accept a spatial weights matrix file; select GET_SPATIAL_WEIGHTS_FROM_FILE for the Conceptualization of Spatial Relationships parameter and, for the Weights Matrix File parameter, specify the full pathname to the spatial weights file you create using this tool.
This tool also reports characteristics of the resultant spatial weights matrix file: number of features, connectivity, minimum, maximum and average number of neighbors. This summary is accessible from the Results window and may be viewed by right-clicking on the Messages entry in the Results window and selecting View. Using this summary, ensure that all features have at least 1 neighbor (8 neighbors is ideal). In general, especially with large datasets, a low value for feature connectivity is desirable.
Whenever using a distance-based Conceptualization of Spatial Relationships, data should be in a Projected Coordinate System (rather than a Geographic Coordinate System).
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.
The Unique ID field is linked to feature relationships derived from running this tool. Consequently, the Unique ID values must be unique for every feature and typically should be in a permanent field that remains with the feature class. If you don't have a Unique ID field, you can easily create one by adding a new integer field to your feature class table and calculating the field values to be equal to the FID/OID field. You cannot use the FID/OID field directly for the Unique ID parameter.
The polygon contiguity options, CONTIGUITY_EDGES_ONLY and CONTIGUITY_EDGES_CORNERS, for the Conceptualization of Spatial Relationships parameter are only valid for polygon features. These options are available with the ArcInfo license only.
The Number of Neighbors parameter may override the Threshold Distance parameter for Inverse or Fixed Distance Conceptualizations of Spatial Relationships. If you specify a threshold distance of 10 miles and 3 for the number of neighbors, all features will receive a minimum of 3 neighbors even if the threshold has to be increased to find them. The threshold distance is only increased in those cases where the minimum number of neighbors is not met.
The CONVERT_TABLE option for the Conceptualization of Spatial Relationships parameter may be used to convert an ASCII spatial weights matrix file to a .swm formatted spatial weights matrix file. First you will need to put your ASCII weights into a formatted table (using Excel, for example).
Caution:
If your table includes weights for self-potential, they will be omitted from the .swm output file, and the default self-potential value will be used in analyses. The default self-potential value for the Hot_Spot_Analysis tool is one, but this value can be overwritten by specifying a Self-Potential Field value; for all other tools, the default self-potential value is zero.
For polygon features, you will almost always want to choose Row for the Standardization parameter. Row Standardization mitigates bias when the number of neighbors each feature has is a function of the aggregation scheme or sampling process, rather than reflecting the actual spatial distribution of the variable you are analyzing.
The Modeling Spatial Relationships help topic provides additional information about this tool's parameters.

Caution:

When using shapefiles, keep in mind that they cannot store null values. Tools or other procedures that create shapefiles from non-shapefile inputs may store or interpret null values as zero. This can lead to unexpected results. See also Geoprocessing considerations for shapefile output.

Syntax

GenerateSpatialWeightsMatrix_stats (Input_Feature_Class, Unique_ID_Field, Output_Spatial_Weights_Matrix_File, Conceptualization_of_Spatial_Relationships, Distance_Method, {Exponent}, {Threshold_Distance}, {Number_of_Neighbors}, {Row_Standardization}, {Input_Table})

Parameter	Explanation	Data Type
Input_Feature_Class	The feature class for which spatial relationships of features will be assessed.	Feature Class
Unique_ID_Field	An integer field containing a different value for every feature in the Input Feature Class.	Field
Output_Spatial_Weights_Matrix_File	The full pathname for the spatial weights matrix file (.swm) you want to create.	File
Conceptualization_of_Spatial_Relationships	Specifies how spatial relationships among features are conceptualized. INVERSE_DISTANCE —The impact of one feature on another feature decreases with distance. FIXED_DISTANCE —Everything within a specified critical distance of each feature is included in the analysis; everything outside the critical distance is excluded. K_NEAREST_NEIGHBORS —The closest "k" features are included in the analysis; k is a specified numeric parameter. CONTIGUITY_EDGES_ONLY —Polygon features that share a boundary are neighbors. CONTIGUITY_EDGES_CORNERS —Polygon features that share a boundary and/or share a node are neighbors. DELAUNAY_TRIANGULATION —A mesh of nonoverlapping triangles is created from feature centroids; features associated with triangle nodes that share edges are neighbors. CONVERT_TABLE —Spatial relationships are defined in a table. Note: Polygon Contiguity methods are only available with an ArcInfo license.	String
Distance_Method	Specifies how distances are calculated from each feature to neighboring features. EUCLIDEAN_DISTANCE —The straight-line distance between two points (as the crow flies) MANHATTAN_DISTANCE —The distance between two points measured along axes at right angles (city block); calculated by summing the (absolute) difference between the x- and y-coordinates	String
Exponent (Optional)	Parameter for inverse distance calculation. Typical values are 1 or 2.	Double
Threshold_Distance (Optional)	Specifies a cutoff distance for Inverse Distance and Fixed Distance conceptualizations of spatial relationships. Enter this value using the units specified in the environment output coordinate system. A value of zero indicates that no threshold distance is applied. When this parameter is left blank, a default threshold value is computed based on output feature class extent and the number of features.	Double
Number_of_Neighbors (Optional)	An integer reflecting either the minimum or the exact number of neighbors. For K Nearest Neighbors, each feature will have exactly this specified number of neighbors. For Inverse Distance or Fixed Distance each feature will have at least this many neighbors (the threshold distance will be temporarily extended to ensure this many neighbors, if necessary). When there are island polygons and one of the Contiguity Conceptualizations of Spatial Relationships is selected, then this specified number of nearest polygons will be associated with those island polygons.	Integer
Row_Standardization (Optional)	Row standardization is recommended whenever feature distribution is potentially biased due to sampling design or to an imposed aggregation scheme. ROW_STANDARDIZATION —Spatial weights are standardized by row. Each weight is divided by its row sum. NO_STANDARDIZATION —No standardization of spatial weights is applied.	Boolean
Input_Table (Optional)	A table containing numeric weights relating every feature to every other feature in the input feature class. Required fields are the Input Feature Class Unique ID field, NID (neighbor ID), and WEIGHT.	File

Code Sample

GenerateSpatialWeightsMatrix Example (Python Window)

The following Python Window script demonstrates how to use the GenerateSpatialWeightsMatrix tool.

import arcpy
arcpy.env.workspace = "C:/data"
arcpy.GenerateSpatialWeightsMatrix_stats("911Count.shp", "MYID","euclidean6Neighs.swm","K_NEAREST_NEIGHBORS","#", "#", "#", 6,"NO_STANDARDIZATION")

GenerateSpatialWeightsMatrix Example (stand-alone Python script)

The following stand-alone Python script demonstrates how to use the GenerateSpatialWeightsMatrix tool.

# Analyze the spatial distribution of 911 calls in a metropolitan area
# using the Hot-Spot Analysis Tool (Local Gi*)

# Import system modules
import arcpy

# Set geoprocessor object property to overwrite existing output, by default
arcpy.gp.overwriteOutput = True

# Local variables...
workspace = "C:/Data"

try:
    # Set the current workspace (to avoid having to specify the full path to the feature classes each time)
    arcpy.env.workspace = workspace

    # Copy the input feature class and integrate the points to snap
    # together at 500 feet
    # Process: Copy Features and Integrate
    cf = arcpy.CopyFeatures_management("911Calls.shp", "911Copied.shp",
                         "#", 0, 0, 0)

    integrate = arcpy.Integrate_management("911Copied.shp #", "500 Feet")

    # Use Collect Events to count the number of calls at each location
    # Process: Collect Events
    ce = arcpy.CollectEvents_stats("911Copied.shp", "911Count.shp", "Count", "#")

    # Add a unique ID field to the count feature class
    # Process: Add Field and Calculate Field
    af = arcpy.AddField_management("911Count.shp", "MyID", "LONG", "#", "#", "#", "#",
                     "NON_NULLABLE", "NON_REQUIRED", "#",
                     "911Count.shp")
    
    cf = arcpy.CalculateField_management("911Count.shp", "MyID", "[FID]", "VB")

    # Create Spatial Weights Matrix for Calculations
    # Process: Generate Spatial Weights Matrix... 
    swm = arcpy.GenerateSpatialWeightsMatrix_stats("911Count.shp", "MYID",
                        "euclidean6Neighs.swm",
                        "K_NEAREST_NEIGHBORS",
                        "#", "#", "#", 6,
                        "NO_STANDARDIZATION") 

    # Hot Spot Analysis of 911 Calls
    # Process: Hot Spot Analysis (Getis-Ord Gi*)
    hs = arcpy.HotSpots_stats("911Count.shp", "ICOUNT", "911HotSpots.shp", 
                     "GET_SPATIAL_WEIGHTS_FROM_FILE",
                     "EUCLIDEAN_DISTANCE", "NONE",
                     "#", "#", "euclidean6Neighs.swm")

except:
    # If an error occurred when running the tool, print out the error message.
    print arcpy.GetMessages()

Environments

Current Workspace, Scratch Workspace, Output Coordinate System

Output Coordinate System: Feature geometry is projected to the Output Coordinate System prior to analysis.

Licensing Information

ArcView: Yes

ArcEditor: Yes

ArcInfo: Yes

3/7/2012

Generate Spatial Weights Matrix (Spatial Statistics)