Within urban areas, they can be as small as a city block. At the other extreme, blocks can be thousands of acres in areas that are largely unpopulated public lands. For example, the figure at right shows the 4,146 census blocks for Mendocino County. The blocks become quite small in cities like Ukiah, but are very large in the portions of the county that are largely rural and dominated by public landholdings. Housing units in the census data include traditional forms of housing and also teepees, caves, and other less conventional dwellings.
Census blocks can extend offshore or include lakes to account for houseboats.
It is important not to confuse the notion of housing units as used by the Census Bureau with structures, a term that does not always connote human residence. Because the census data represent conditions in 1990, they do not portray post-1990 population changes. Nonetheless, the data provide a reliable source of spatially referenced population statistics, and are in common use by numerous public and private entities for a variety of purposes. Teale Data Center maintains digital spatial data for SRA and DPA on a continuous basis for the Department. The remaining 1,000 people are assumed to live within the remaining non-SRA private lands.