These are statewide shapefiles representing the 2000 US Census Bureau Urban Area boundaries as derived from 2000 TIGER. Two of the files are in LAT/LONG coordinate system and two are in Michigan Georef projection. The 'byco' in the name represents the shapefile polygons that are split by county line. The shapefiles without the 'byco' in the name have been 'dissolved' across county lines base on the UA Code.