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 and EDs for all other areas. This provided complete geographic coverage for all 1980 census geography, and served as a basis for structuring the 1990 geographic entities.

All features in the TIGER data base were classified according to feature type and characteristic. For example, single- and double-line drainage as shown on USGS topographic maps were differentiated, and roads were classified by type. Census block number information from the 1980 census was preserved only for GBF/DIME-File areas.

Within counties with census tracts, the Census Bureau invited local census statistical areas committees to participate in delineating the 1990 BGs at the same time they delineated their 1990 census tracts. Agencies were permitted to delineate as many as nine BGs within each BNA or census tract. The guidelines specified an ideal size for a BG of 400 housing units, with a minimum of 250, and a maximum of 550 housing units. The guidelines further required that BG boundaries follow clearly visible features, such as roads, rivers, and railroads.

In the summer of 1985, the Census Bureau offered State governments, as well as the governments of Puerto Rico and the Virgin Islands of the United States, the opportunity to delineate, for use in the 1990 census, BNAs and BGs in counties or statistically equivalent entities that did not have census tracts. Where local and State agencies chose not to participate, the Census Bureau completed the delineation of BNAs and BGs. The program resulted in the delineation of 224,691 collection BGs in the United States, and a total of 228,202 BGs in all areas under U.S. jurisdiction. The average number of BGs per census tract was 3.7 for counties with census tracts, and 3.9 per BNA for counties with BNAs.

Although most people intuitively think of census blocks as being rectangular or square, of about the same size, and occurring at regular intervals, as in many cities of the United States, census block configurations actually Census Blocks and Block Groups11-9