Subscribers can draw custom regions on a map to highlight a target area, or generate aggregated data for a specific area of interest. Aggregated data for custom regions can be generated by creating a report of a custom region, or loading a custom region in to a table.
When data for a custom region is aggregated in a report or table, data from smaller geographies within the region is used to calculate the values for the custom region. For drawn and radius custom regions, PolicyMap automatically chooses which type of smaller geographies to use in this calculation based on the data indicator and size of the custom region. Large custom regions may have aggregated data comprised of counties, whereas small custom regions may have aggregated data based on census tracts. Not all indicators have data for all geographies, so in some instances, the geography type will be a less exact fit.
Please note: assembled custom regions aggregate data based on the geographies used to assemble the region, regardless of the region’s size.
The 50% Rule – Custom regions that are drawn or are radii around a point don't line up with the existing geographies that data is aggregated from. Some geographies may be mostly inside the custom region, but partially outside. Some geographies may be mostly outside the custom region, but poke into it.
For a geography to be included in the custom region data aggregation in a report or table, at least 50% of the geography must be contained within the custom region.
In the first example on the right, only the green shaded census tracts (38.02, 38.03, 37.02, 35.01, and 34.01) would be used when aggregating data. Tracts 37.01, 35.02, and 34.02 do not have at least 50% of their area contained within the radius custom region.
In the second example below, we have drawn a custom region following streets but these streets do not correspond to census tract boundaries. Because of the 50% Rule, our custom region will use the green highlighted census tracts (37.02, 34.01, 33.01, 34.02, and 33.02) when aggregating data, while 36.01, 38.03, and 35.01 will not be included since less than 50% of their area are contained within the custom region.
To read a full guide on all features on PolicyMap, please look at our Primer available here. If you want to know more about a feature or topic, send your request to firstname.lastname@example.org subject "Tutorial Topics."