if the counties were part of the planning or jurisdictional region of an existing census statistical areas committee.
For the 1990 decennial census, the Census Bureau assigned block numbers to all parts of the United States and the Outlying Areas. It opened the census tract program to include all counties (and statistical equivalents) with sufficient local interest to form a census statistical areas committee. All other counties (or equivalents) were part of the BNA program. (For details, see the section in this chapter entitled “Census Tracts and BNAs for the 1990 Census.”)
Even though local participation in the census tract program evolved over time in response to user demand and the growth in MAs, the underlying rationale for delineating census tracts has remained constant. They define a set of small geographic areas for the enumeration, tabulation, and publication of census data.
For the 1990 census, the Census Bureau changed the concept of the BNA dramatically. By redefining the BNA from a geographic area delineated solely as the framework for assigning census block numbers (1940 through 1980), to an entity sharing the same basic attributes as the census tract (1990), the Census Bureau has established a nationwide set of comparable small geographic areas.
The census tract and BNA criteria recognized by the Census Bureau identify boundary, size, and demographic requirements, and establish conventions for numeric identification and stability.
The need for appropriate boundaries is a longstanding concern of census geography. Census tract and BNA boundaries generally follow permanent, visible features, such as streets, roads, highways, rivers, canals, railroads, and high-tension power lines. Pipelines and ridge lines may be acceptable when no other choice is available.
