Geographic Center of North America - Rugby, North Dakota - American Guide Series on Waymarking.com

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From the North Dakota - A Guide to the Northern Prairie State  Tour 6 section:

At 163.5 m. is the junction with ND 3, a graveled highway.  At this crossroads is a stone cairn marking the GEOGRAPHIC CENTER OF NORTH AMERICA.  In 1931 the U. S. Geological Survey determined that the geographic center of the continent is in Pierce County, N. Dak., and the marker at the highway junction is in the approximate center of the country  The survey states, "The geographic center of an area may be defined as that point on which the surface of the area would balance if it were a plane of uniform thickness, or in other words, the center of gravity of the surface... It would not be feasible, therefore, to specify for such a large irregular area as that of North America the exact section, township and range in which the geographic center lies."  Some years before 1931 Pierre, S. Dak., claimed to be the approximate center of the continent, based on the fact that two lines drawn from corner to corner of a map of North America intersected near Pierre.

The cairn was relocated in 1971 across the Hwy 2 when the highway was expanded to four lanes.  The old location is now a frontage road northwest of the current location.  Rugby has embraced its status as the geographic center and includes that fact in its city seal and slogan.

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