How data is calculated for user-created areas
This section describes how data is calculated in the application. The summarization of data delivers accurate estimates in areas that don't match up exactly to common geographic areas such as Census block groups in the United States. For example, frequently a user-created area of interest such as rings, drive times, hand drawn polygons or imported shapefiles cut through part of a Census block group. To account for population that may not be evenly distributed geographically throughout a block group, the application looks at where the population is distributed at the block level (in the United States), which is a more granular level than a block group, and uses this information to summarize the data more accurately.
Data Allocation Method
The Data Allocation method allocates block group data to custom areas by examining where the population is located within the block group and determines how much of the population of a block group overlaps a custom area. This method is used in the United States, and similarly in Canada. The population data reported for census blocks, a more granular level of geography than block groups, is used to determine where the population is distributed within a block group. If the geographic center of a block falls within the custom area, the entire population for the block is used to weight the block group data. The geographic distribution of the population at the census block level determines the proportion of census block group data that is allocated to user specified areas as shown in the example.
Depending on the data, households, housing units or businesses at the block group level are used as weights. Employing block centriods is superior because it accounts for the possibility that the population may not be evenly distributed geographically throughout a block group.
Data Allocation Method for countries other than the United States and Canada
In countries where a geography level analogous to U.S. Census blocks are unavailable, Esri uses a proprietary set of population-weighted points to allocate data available at larger geographies to any custom area. A patent is pending on this Esri-developed data allocation technique.