Precise or first‐order leveling, as it is now called, was first undertaken by the United States Coast and Geodetic Survey in 1878 when field‐work was begun on the line of levels of high precision which was to follow the general trend of the Transcontinental Arc, an arc of triangulation extending across the United States from Chesapeake Bay to the Golden Gate and following approximately the 39th parallel. From this single‐line beginning, the control‐leveling of the Survey has been extended until the fundamental leveling net now includes more than 260,000 miles of first‐ and second‐order leveling. About two‐fifths of this is first‐order leveling, the remaining three‐fifths being second‐order or s