Generalization is the process of taking complex lines and making them simpler. They will draw faster, take up less file space, and if done well, will not be distinguishable at the the selected scale from the higher detail original data.
These examples show Anne Arundel County, MD, part of whose boundary is the Chesapeake Bay.
|Get Geographic areas, Counties and equivalents
69 MB zip download, entire US
Boundaries in the center of the Chesapeake Bay
|Tiger cartographic boundary files,
|Generalized 500K (8.8 MB zip, entire US)
This is probably the best bang for the buck, since it is reasonably small, and does not include a lot of extra water along the edges.
|Tiger cartographic boundary files, http://www.census.gov/geo/maps-data/data/cbf/cbf_counties.html||Generalized 5M|
|Tiger cartographic boundary files, http://www.census.gov/geo/maps-data/data/cbf/cbf_counties.html||Generalized 20M (1.3 MB zip, entire US)|
|SW corner of Anne Arundel County, with the 4 boundaries.
The generalized 500K and the detailed TIGER map are very similar at this scale. The big difference comes on the Chesapeake Bay, where there detailed map goes down the center of the bay, and the 500K follows the coastline. You would have to carefully consider which worked better for your purposes.
|Coastline of Anne Arundel County.
Note the detailed TIGER data in the center of the bay, and and three generalized data sets indistinguishable generally following the coast. Note also that one island in the NE corner of the map is bisected by the generalized coastline.
|Added coastlines from the National Map. This is a 4.5 MB download (which requires Unix Tar/Gzip decompression). Whether this improves on the Tiger data is up to the user. It is more complex, but maybe not more accurate.|
|NED along the Arizona-Mexico border. The terrain
in Mexico is from the 250K DEM, which grossly simplifies topography.
National Atlas. Note that this does not match the topography from NED.
Census Bureau Note the much better match with topography.
Both files are essentially the same size (about 13 MB for the three files making up the shape file).
The program will look first for the Census Bureau shapefile, but if that cannot be found, will try to find the National Atlas file. They should be placed in c:\mapdata\database\us\counties.
The USGS National Boundary Dataset (NBD) Downloadable Data is organized by state.
last revision 9/27/2015