The GeoEnrichment service employs a sophisticated geographic retrieval methodology to aggregate data for rings and other polygons. A geographic retrieval methodology determines how data is gathered and summarized or aggregated for input features. For standard geographic units, such as states, provinces, counties, or postal codes, the link between a designated area and its attribute data is a simple one-to-one relationship. For example, if an input study trade area contains a selection of ZIP Codes, the data retrieval is a simple process of gathering the data for those areas.
How data is summarized
The geographic retrieval process for ring buffers, drive-time service areas, and other non-standard geography polygons is more complicated, because the input polygon may intersect geographic areas that contain data that needs to be aggregated. The following diagram illustrates this case. The polygon in the center represents an input study area that is being enriched. For example, the GeoEnrichment service can calculate the total population for this area. The polygons labeled represent Census geographies that contain total population values. In the United States, these can be Block Groups with enrichment data; in Canada, they can be Enumeration Areas.
The GeoEnrichment service employs a Weighted Centroid geographic retrieval methodology to aggregate data for rings and other polygons. The Weighted Centroid retrieval approach uses Census Block data to better apportion block groups that are not exclusively contained within a ring. In the United States, Canada, and many other countries, Census blocks are the smallest unit of Census geography. These small areas are used to create all other levels of Census geography. For example, in the United States, one or many blocks are aggregated to create a Block Group.
The GeoEnrichment service uses a dasymetric apportionment technique to aggregate Census based demographic data for smaller areas. If areas are very small and do not intersect any Census block data the service returns zero values. The GeoEnrichment service provides an estimate of population and does not count roof tops. Census data is NOT available at this scale.
In countries where Census Blocks do not exist, postal points and/or small area grids can be used to apportion demographic data. GeoEnrichment services have been configured in all countries to work with small area points that represent how population is distributed throughout the country.
The Weighted Centroid method is illustrated in the following figure:
In the previous figure, Census Blocks are illustrated as black points. Using area P3 as an example, the population weight for this area is determined by summing the block weights within this polygon. The sum of these weights will provide a proportion of area P3 that is within the study area. Summarizing a demography variable such as the Total Population, will use this proportion to aggregate and summarize data. For example, if 90 percent of the P3 Blocks' population are within the study area, and the Total Population of P3 is 100 people, you can determine that 90 people in area P3 are inside the study area.
The weight w1 of the site P1 is calculated as a sum of weights of block points belonging to the intersection of the site P1 and the target polygon T:
Here, ß is a block and W1(ß) is a weight of this block in the site P1.
Summarizing a demography variable such as the Total Population, the weights need to be determined for all intersecting geographies. The GeoEnrichment service calculates the weight W1(ß) as a ratio of the total population associated with the block (ß) belonging to the site P1 to the sum of total population values for all blocks belonging to the site P1: