The Meades Ranch Triangulation Station is a survey marker in Osborne County in the state of Kansas in the Midwestern United States. The marker was initially placed in 1891. From 1901, it was the reference location for establishing a system of horizontal measurement in the United States, known as geodetic datum. In 1913, the datum was adopted across all of North America, and the system revised and formalized as the North American Datum of 1927 (NAD27). A similar reference for vertical measurement was established in 1929 as the National Geodetic Vertical Datum of 1929. The NAD27 was later supplanted by the North American Datum of 1983 (NAD83), which was formally adopted by the United States in 1989 and Canada in 1990; the new system moved the reference point to a point in the earth's core, and the Meades Ranch marker lost its special significance to the geodetic datum system. In 1973, the site was listed in the National Register of Historic Places as Geodetic Center of the United States. History In surveying, a geodetic datum is a point or set of points used to establish a coordinate system. By designating the location of one point, and the direction from that point to a second point, one can establish a system relative to which other points can be located and mapped. As one surveys a larger area over which the curvature of the Earth becomes significant, it also becomes necessary to define an ellipsoid: a curved surface that approximates the shape of the Earth in the area of interest. The first nationwide datum in the United States was established in 1879. At that time, a region of contiguous triangulation existed from Maine to Georgia. The New England Datum was chosen with a benchmark in Maryland, near the middle of this region, as the horizontal datum, and a second Maryland mark used to define the azimuth (horizontal) direction. To account for the Earth's curvature, the datum used a model called the Clarke spheroid, developed in 1866. Meades Ranch station The U.S. Coast and Geodetic Survey established a triangulation station, officially known as the Meades Ranch Triangulation Station, in 1891. The location lay near the intersection of two transcontinental arcs of triangulation: one running from the Atlantic to the Pacific Ocean, roughly following the 39th parallel; the other running from Mexico to Canada, along the 98th meridian. In 1901, the Meades Ranch station