How BlockAtlas computes population within an area

This page explains how the radius and travel time tools turn a point on the map into population and demographic estimates.

Population: 2020 census blocks

Population counts come from the 2020 United States decennial census at the block level. Census blocks are the smallest geography the Census Bureau publishes. There are roughly eight million of them, and in cities a block often matches a literal city block.

A block counts toward an area when its center falls inside the area.

Circles and travel time areas

Distance radii are true circles measured on the surface of the earth, not flat map circles, so a 10 mile radius covers the same ground area anywhere in the country.

Drive, walk, and bike areas are isochrones: the set of points reachable within a time budget. BlockAtlas computes them with the Valhalla routing engine on the OpenStreetMap road network, using typical speeds for each mode of travel.

Demographics: ACS tract allocation

Indicators such as income, rent, age, and housing come from the American Community Survey, which is published for census tracts rather than blocks. To estimate a tract level value for an arbitrary area, BlockAtlas allocates each tract in proportion to the share of the tract's 2020 population living on blocks whose centers fall inside the area.

Counts are summed across the allocated tracts. Rates are computed by summing the numerator and denominator separately, then dividing. Medians are recomputed from the underlying bracket counts rather than averaged.

When the tracts with data cover less than the whole area's population, the value is flagged as approximate. A count is never rescaled to compensate for missing coverage.

Small areas

A very small area may capture too few census blocks for centroid counting to mean anything. In that case BlockAtlas reports the area as not measurable rather than showing a misleading zero or a shaky estimate.

Points of interest

Counts of grocery stores, restaurants, schools, and other places come from OpenStreetMap (ODbL). These are exact counts of mapped points inside the area, not estimates, and their completeness depends on OpenStreetMap coverage.

Sources

  • 2020 United States decennial census, block level population and land area
  • American Community Survey 5 year estimates
  • Census Bureau TIGER/Line boundary geometries
  • OpenStreetMap road network and points of interest (ODbL)

Try it yourself: population by drive time or population by radius. Questions about the method are welcome at [email protected].