NAD27, or the North American Datum of 1927, is a geodetic datum used for mapping and surveying in North America. It provides a framework for measuring locations on the Earth's surface, helping to create accurate maps. NAD27 is based on the Clarke 1866 ellipsoid, which approximates the shape of the Earth. This datum was widely used until the introduction of NAD83, which offers improved accuracy and is based on a more modern ellipsoid. While NAD27 is still referenced in some older maps and surveys, most current applications rely on the more precise NAD83 or WGS84 systems for geographic data.