Generalized Boundaries

U.S. Department of Commerce

The boundary files for the mapping of states, counties, block groups, minor civil divisions (MCDs), metropolitan areas (MAs), and congressional districts (CDs) are generalized, which means that the number of lines making up the perimeter of those polygons are reduced, and the polygon shapes are simplified. The purpose of generalizing the boundaries is to reduce the size of the map files, and to speed their display. The "Alaska Native Lands", "Indian_Lands", "Places", and "Census Tracts" layer are not generalized. The "Census Block Grps-generalized" layer is generalized, while the "Census Block Groups" layer is not. The latter resides on the individual county maps.

Original boundary coordinates and geocodes were extracted from the Census Bureau's TIGER database -- a system of internal files equivalent to the publicly available TIGER/Line files. Coordinates and geocodes were imported into an ArcInfo® environment where generalization processes were applied to reduce boundary complexity and the size of the files. The generalization process included reduction of coordinate pairs, elimination of small polygons for discontiguous entities, and elimination of small Census tract/BNAs and Block Groups with zero population.

Different degrees of coordinate reduction or thinning were used for different boundary types. State, county, MCD and MA boundaries were aggressively thinned, although lines coincident amongst these separate boundary networks retain a precise nesting quality. The Census Block Group boundaries were moderately thinned, while CD boundaries were only minimally thinned. Geographic relationships amongst boundary types can not be inferred from boundary location, because boundary networks were thinned independently of one another (except for the previously mentioned State, county, MCD and MSA boundaries). After generalization, all boundary networks were clipped against a national coastline. This coastline was also derived from the TIGER database and then generalized within the ArcInfo environment.