Survey township

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 640 acre. To be more precise, the subdivisions of a section are frequently the quarter-section (160 acres) and the quarter-quarter section (40 acres). In the Homestead Act of 1862, one quarter-section of land was the amount allocated to each settler. Stemming from this are the idiomatic expressions, "the lower 40", which is the 40 acre on a settler's land that is lowest in elevation, in the direction towards which water drains toward a stream, and the "back forty", the portion farthest from the settler's dwelling. 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 of the United States of America.

Prior to standardization, some of the Ohio Lands were surveyed into townships of five miles (8 km) on each side. These are often known as Congressional Townships.

Survey townships are distinct from civil townships. A survey township is used to establish boundaries for land ownership. A civil township is a form of local government. In states that use both forms, civil townships generally use the boundaries established by survey townships. County lines, especially in western States, usually follow township lines, leading to the large number of rectangular counties in the West, which are agglomerations of townships. Other counties may have county lines that form a Jack-O-Lantern tooth pattern of townships.

In western Canada, the Dominion Land Survey adopted a similar format for survey townships, which do not form administrative units. These townships also have the area of 36 square miles (six miles by six miles).