Survey township, sometimes called Congressional township, as used by the United States Public Land Survey System, refers to a square unit of land, that is nominally six (U.S. Survey) miles (~9.7 km) on a side. Each 36-square-mile (~93 km²) township is divided into 36 one-square-mile (~2.6 km²) sections, that can be further subdivided for sale, and each section covers exactly . The townships are referenced by a numbering system that locates the township in relation to a meridian (north-south) and a base line (east-west). Townships were originally surveyed and platted by the US General Land Office using contracted private survey crews and are marked on the U.S. Geological Survey topographic maps.