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How to: Find closest objects in ArcGIS with Python

As part of my DunesGIS project I had a need to calculate ‘closeness statistics’ for objects in ArcGIS. By ‘closeness statistics’ I mean statistics giving information about how close the objects are to each other. I needed to do this to calculate how dune patterns change over time.

The code below takes a shapefile as input, and then calculates the closest other object to each object. It then returns the mean closeness and standard deviation of all objects.

def CalculateCloseness(Filename):
    # Calculate the distance from each shape (point, line, polygon)
    # to all of the others
    gp.GenerateNearTable(Filename, Filename, "NearTable", \
                         "", "LOCATION", "ANGLE", "ALL")

    # Create a view of the table so that we can run queries on it below
    gp.MakeTableView("NearTable", "tbl", "NEAR_DIST > 0")

    # Search for all of the distances which are > 175 and < 185 degrees
    # that is, basically horizontally
    rows = gp.SearchCursor("tbl", "NEAR_ANGLE >= 175 AND NEAR_ANGLE <= 185", "", "")

    # Get the first row
    row = rows.Next()

    # Create a NumPy array to hold the results
    shortest = numpy.zeros(500)

    # For each row
    while row:
        # Get the previous shortest distance from the array
        prev_value = shortest[row.IN_FID]

        # If the previous value is 0 then this is the first distance
        # we've found so set it to that
        if (prev_value == 0):
            shortest[row.IN_FID] = row.NEAR_DIST
            continue
        # Otherwise if this is a shorter one then use it
        elif (row.NEAR_DIST < prev_value):
            shortest[row.IN_FID] = row.NEAR_DIST

        # Move to the next row
        row = rows.Next()

    # Select all non-zero elements so the zero's don't skew the mean
    non_z_indices = numpy.where(shortest)

    # Calculate mean and stdev
    mean_closeness = numpy.mean(shortest[non_z_indices])
    std_closeness = numpy.std(shortest[non_z_indices])

    # Return results
    return [mean_closeness, std_closeness]

This script is (well, will shortly) be included in RTWTools for ArcGIS, where it is integrated into an ArcGIS Toolbox.

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This post originally appeared on Robin's Blog.

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Categorised as: Academic, GIS, Python