Page:Geographic Areas Reference Manual (GARM).pdf/258

 To identify all census tracts/BNAs affected by a county boundary update, the Census Bureau added a special suffix in the range of .70 to .98 (starting with .98 and assigned in descending sequence) to the basic census tract/BNA number of each census tract/BNA that lost territory. The Census Bureau also assigned a new census tract/BNA number to the portion of the census tract/BNA in the county that gained territory. These new census tract/BNA numbers fit within the numbering scheme of each county, but were identifiable by the special suffix. The addition of these special suffixes fulfilled data user requests for a flag to identify any areas changed after the Census Bureau produced the products used in the early 1990 census operations. Because many census tracts/BNAs with this special suffix have very small areas with little or no population or housing, some users have chosen to aggregate one or more such census tracts/BNAs with an adjacent census tract/BNA for data analysis.

One of the changes brought about by the TIGER System was the need to include all area (land and water) within a census tract/BNA. Rather than extending the census tract/BNA boundaries into the Great Lakes or out to the three-mile limit in coastal waters, the Census Bureau decided to close off the census tract/BNA boundaries along the shoreline or just offshore. The Census Bureau then assigned a default census tract/BNA number 0000 to the coastal and Great Lakes waters not assigned to any other census tract/BNA.

In the decennial census geographic hierarchy, census tracts/BNAs are subdivisions of, and nest within, counties (and their statistical equivalents). The block groups, the next lower level in the decennial census geographic hierarchy, are subdivisions of census tracts/BNAs and always nest within a specific census tract/BNA. The Census Bureau assigns census block numbers within block groups to identify the smallest geographic areas for which it collects and tabulates census data. It does this by using the block Census Tracts and Block Numbering Areas10-15