The most important attribute of census tract/BNA boundaries is that they be visible, that is, readily identifiable in the field.
The Census Bureau often is urged to accept the use of governmental unit boundaries as census tract and BNA boundaries. Census tract/BNA boundaries always follow the boundaries of States and counties (or their statistical equivalents). Census tract and BNA boundaries follow other governmental unit boundaries only in selected instances. Many early census tract plans covered only large cities; as a result, the city limits were census tract boundaries. This posed no problem at a time when governmental unit boundaries remained unchanged for long periods of time, and thus, their location was well known. Later, as annexations became more frequent and incorporated places expanded into surrounding areas, the governmental unit boundaries in many States became more irregular and subject to change, and fewer people knew their precise location. This lack of stability meant that the governmental unit boundaries became less suitable as census tract boundaries.
For the 1970 census, the Census Bureau began providing data for that portion of each census tract inside a governmental unit, and for the census tract as a whole. Until then, the only way to obtain census tract data within a governmental unit was by recognizing the governmental unit boundaries as census tract boundaries. Currently, the Census Bureau makes the full range of census data available for all governmental units and for census tracts/BNAs, thereby negating the need for census tract/BNA boundaries to follow governmental unit boundaries.
When first delineating census tracts, the Census Bureau requests that the average population of all census tracts in a county (or statistically equivalent entity) be about 4,000 people (approximately 1,500 housing units), with individual census tracts ranging from 2,500 to 8,000 inhabitants (1,000 to 3,000 housing units). For the 1990 census, the Census Bureau requested that the average number of housing units in each BNA be around 1,500 (approximately 4,000 people), ranging from 600 to 3,000 housing units (1,500 to 8,000 inhabitants).
