According to Wikipedia, the North American Datum (NAD) is the horizontal datum used to define latitude and longitude (horizontal coordinates) in North America. A datum is a formal description of the shape of the Earth along with survey markers serving as “anchor” points for the coordinate system. In surveying, cartography, and land-use planning, two North American Datums are in use: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems based on slightly different assumptions and measurements.