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While any measurement of the exact center of the United States will always be imprecise for a number of reasons, such factors as the exact placement of thousands of miles of changing shoreline, the U.S. National Geodetic Survey (NGS) has produced designated measurements for the center of the United States to the best of its ability to do so.
From the time of the 1912 additions of New Mexico and Arizona, until the 1959 additions of Alaska and Hawaii, the NGS placed the geographic center of the entire United States at a point near the town of Lebanon, Kansas. Since 1959 this location still stands as the Geographic Center of the Contiguous United States, while the geographic center of the entire U.S. moved to a point northeast of Belle Fourche, South Dakota.
The location for contiguous U.S. center is pinpointed by a historical marker that is located within a small park at the intersection of AA Road and K-191, accessible by a turn-off from U.S. Route 281 approximately a mile north of the town of Lebanon.
The marker supposedly indicates the actual point measured by a survey performed in 1918. The measured center is about a half to three-quarters of a mile away on a private farm. The farm's owner did not want tourists trudging through his field, so the marker was placed as close as possible. There is usually an American flag flying on the pole placed on top of the monument. Just to the south of the monument, yards away, is a covered picnic area and small four-pew chapel where a couple can take wedding vows.
In 1918, the Coast and Geodetic Survey found this location by balancing on a point a cardboard cutout shaped like the U.S. Although this method was only accurate to within twenty miles, and the Geodetic Survey no longer endorses any location as the center of the U.S., the identification of Lebanon has stuck. 
The GEOGRAPHIC CENTER of the UNITED STATES
LAT. 39°50' LONG. 98°35'
NE 1/4 - SE 1/4 - S32 - T2S - R11W
Located by L.T. Hagadorn of Paulette & Wilson - Engineers and L.A. Beardslee - County Engineer. From data furnished by U.S. Coast and Geodetic Survey.
Sponsored by Lebanon Hub Club. Lebanon, Kansas. April 25, 1940.