How Point Density works
The Point Density tool calculates the density of point features around each output raster cell. Conceptually, a neighborhood is defined around each raster cell center, and the number of points that fall within the neighborhood is totaled and divided by the area of the neighborhood.
If a Population field setting other than NONE is used, the item's value determines the number of times to count the point. For example, an item with a value of 3 would cause the point to be counted as three points. The values can be integer or floating point. If an area unit is selected, the calculated density for the cell is multiplied by the appropriate factor before it is written to the output raster. For example, if the input ground units are meters, comparing a unit scale factor of meters to kilometers will result in the values being different by a multiplier of 1,000,000 (1,000 x 1,000).
Possible uses include finding density of houses, wildlife observations, or crime reports. The population field can be used to weight some points more heavily than others, depending on their meaning, or to allow one point to represent several observations. For example, one address might actually represent a condominium with six individual units, or some crimes might be weighted more heavily than others in determining overall crime levels.
Increasing the radius will not greatly change the calculated density values. Although more points will fall inside the larger neighborhood, this number will be divided by a larger area when calculating density. The main effect of a larger radius is that density is calculated considering a larger number of points, which can be farther from the raster cell. This results in a more generalized output raster.
The following are some examples of using Area unit scale factor to change the units of the output density:
- Map units are meters, so default units of density are points per square meter in the neighborhood. It is desired to calculate density in points per hectare (10,000 square meters).
- Use a scale factor of 100 (100 by 100 meters is one hectare).
- Map units are feet, and it is desired to calculate density as points per square mile.
- Use a scale factor of 5,280 (the number of feet in a mile).
The table below gives approximate values of unit scale factors to use for some common conversions.
Desired output is density per: | sq foot | sq meter | acre | hectare | 1,000 sq feet | Input -------+---------+----------+--------+---------+---------------| map feet | 1 | 3.208 | 208.7 | 328.083 | 31.623 | units: meters | 0.3048 | 1 | 63.6 | 100 | 9.639 |
Silverman, B. W. Density Estimation for Statistics and Data Analysis. New York: Chapman and Hall, 1986.