This statement should raise a few eyebrows. Anecdotally, LA is a sprawl of concrete, palm trees, and small homes. It’s a place where people drive several blocks to get a cup of coffee. How could the population density there be higher than where most people would suppose has the highest population density, New York City, with its packed sidewalks and towering skyscrapers?
In a March, 2012 press release Census made the following claim: “The nation’s most densely populated urbanized area is Los Angeles-Long Beach-Anaheim, Calif., with nearly 7,000 people per square mile. The San Francisco-Oakland, Calif., area is the second most densely populated at 6,266 people per square mile, followed by San Jose, Calif. (5,820 people per square mile) and Delano, Calif. (5,483 people per square mile). The New York-Newark, N.J., area is fifth, with an overall density of 5,319 people per square mile.”
Since then, this statistic has lingered around, occasionally popping up in an article somewhere, perturbing unsuspecting readers. How could this be the case?
To understand how Census came to this counterintuitive conclusion requires clarification of what Census is referring to when they say “Los Angeles.” Are they referring to the city limits, or to the broader metropolitan area? Depending on the definition the results may vary. What exactly is Census referring to when they say “urbanized area?”
It turns out that Census has three definitions of metro areas: Combined Satistical Areas (CSAs), Core-Based Statistical Areas (CBSAs), and Urbanized Areas (UAs). The final of these is confusing when used to refer to specific boundaries, given that it could have a generic interpretation as well. CBSAs with over 50,000 people are Metropolitan Statistical Areas, while those with less are Micropolitan Statistical Areas.
And for the city proper? The city of Los Angeles is one of Census’s Census-Designated Places (CDPs), which according to Census “are the statistical counterparts of incorporated places, and are delineated to provide data for settled concentrations of population that are identifiable by name but are not legally incorporated under the laws of the state in which they are located.” When referring to the city of LA in this analysis, it is the Los Angeles CDP (Census also has a Consolidated City definition, but it doesn’t apply here).
CSAs and CBSAs, like CDPs, are also clearly delineated geographically. They are both defined using county boundaries to set their limits. According to Census: “CBSAs consist of the county or counties or equivalent entities associated with at least one core (urbanized area or urban cluster) of at least 10,000 population, plus adjacent counties having a high degree of social and economic integration with the core as measured through commuting ties with the counties associated with the core. The general concept of a CBSA is that of a core area containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core… CSAs consist of two or more adjacent CBSAs that have substantial employment interchange. The CBSAs that combine to create a CSA retain separate identities within the larger CSA.”
UAs, meanwhile, have a less clear deliniation: “UAn urbanized area consists of densely developed territory that contains 50,000 or more people.” There is no clear definition of how UAs are bounded or how their limits are defined.
The simplest way to calculate population density is population divided by area. Thus one can see that the definition of area matters immensely. The map below shows LA using four definitions: CBSA, CSA, UA, and CDP. By zooming and scrolling, one can see how the definition impacts the results. The shading reflects the population density by quintile at the Census tract level.
Here we can see the four definitions of LA, including the largest geographically, the CSA. Looking at both the CSA and CBSA it is clear that they contain a lot of rural area. Because the definitions are based on county lines, even if only a faction is urban (like in San Bernadino County), the whole county gets included in the metro area definition. This is why Census created the “Urbanized Area”" definition. The shading shows population density by quintile for all the Census tracts within the bounds.
Zooming in one can see that the UA neatly circumscribes the parts of Los Angeles with the highest population density. What is unclear, however, is why the boundaries end when they do. There are pockets of density–and hence urban population–immediately adjacent to where the boundary stops to the south and east.