The following are approximate tallies of current listings by county. These counts are based on entries in the National Register Information Database as of March 13, 2009 and new weekly listings posted since then on the National Register of Historic Places web site. There are frequent additions to the listings and occasional delistings and the counts here are approximate and not official. New entries are added to the official Register on a weekly basis. Also, the counts in this table exclude boundary increase and decrease listings which modify the area covered by an existing property or district and which carry a separate National Register reference number.
^The latitude and longitude information provided in this table was derived originally from the National Register Information System, which has been found to be fairly accurate for about 99% of listings. For about 1% of NRIS original coordinates, experience has shown that one or both coordinates are typos or otherwise extremely far off; some corrections may have been made. A more subtle problem causes many locations to be off by up to 150 yards, depending on location in the country: most NRIS coordinates were derived from tracing out latitude and longitudes off of USGS topographical quadrant maps created under the North American Datum of 1927, which differs from the current, highly accurate WGS84 GPS system used by most on-line maps. Chicago is about right, but NRIS longitudes in Washington are higher by about 4.5 seconds, and are lower by about 2.0 seconds in Maine. Latitudes differ by about 1.0 second in Florida. Some locations in this table may have been corrected to current GPS standards.