Land and water area measurements, which were reported in thousandths of square kilometers in the original data, were changed to square kilometers in the coverages. The state coverages were appended into one coverage for the conterminous United States. The individual codes could have been combined in any way. In the data tabulations, a geographic block group may be split. Block groups do not cross census tract or block numbering area boundaries.
Census tracts are small, relatively permanent statistical subdivisions of a county.
Census tract numbers range from 0001 to 9499.9 and are unique within a county. The suffix .99 identifies a census tract that was populated entirely by persons aboard one or more ships. BNA numbers range from 9501 to 9980.99 and are unique within a county. The suffix .99 identifies a census tract that was populated entirely by persons aboard one or more ships. Suffixes in the range of .80 through .98 usually identify census tracts or BNAs that were revised or created during the 1990 census.