NAD 27, or the North American Datum of 1927, is a geodetic datum used for mapping and surveying in North America. It was established to provide a consistent framework for geographic coordinates based on the Clarke 1866 ellipsoid. This datum is primarily used in the United States, Canada, and parts of Mexico. NAD 27 is important for various applications, including land surveying, navigation, and geographic information systems (GIS). However, it has largely been replaced by NAD 83, which offers improved accuracy and is based on a more modern reference system.