During much of the twentieth century, as the United States continued to grow, surveyors who were determined to plot the location of the center of the nation had to keep altering their decision, accommodating the addition of new states whenever necessary. When Hawaii became the 50th and last state to join the Union in 1959, the National Geodetic Survey at the National Oceanic and Atmospheric Administration took up the task once again and determined the location of the geographic center of the United States to be a point in the middle of the South Dakota prairie in the county of Butte along old Highway 85.
The decision was reached using a technique known as "the center of gravity method'' which is akin to making a cardboard map of the country, all one thickness, and attempting to balance it atop some sort of narrow post. The geographic center would be the point at which the map would balance. (Before Alaska and Hawaii were added, the center of the U.S. was determined to be Lebanon, Kansas. With the addition of Alaska, the center moved to Castle Rock, Butte County, not far from the current location.)
Practically inaccessible now that Highway 85 has been moved, the center of the U.S. is designated by a small marker, a flag, and a hand-written sign declaring the site's importance. Some visitors make the trek off the main highway to this - the "real'' center - determined to take photos near the exact location. Others, however, now flock to the nearest town, Belle Fourche, SD, located about 20 miles from the official site, where guests are content to note that they were close to the geographic center of the U.S.
The town of Belle Fourche, a small agricultural enclave, has always been the closest town to the official geographic center and has long hoped to benefit from the designation, which local officials believed would attract tourists to the town. Even though that failed to happen, especially after the highway was moved, Belle Fourche decided in 2007 to erect a new and larger monument touting the designation - one that has succeeded in attracting scores more visitors who now arrive to take pictures of their loved ones near the marker.
The Belle Fourche Chamber of Commerce chose a 21-foot diameter structure fashioned from about 27 tons of South Dakota granite for the monument. On the marker in the center is a large map of the United States enclosed in a compass rose with all lines radiating from Belle Fourche. Visitors often stand in their home state while snapping photographs, marking their visit to someplace not so far from the Geographic Center of the United States.