Control leveling

Control leveling is used to determine precise elevations of bench marks (monumented points) above or below a reference surface called a datum. The National Geodetic Survey (NGS) of the National Ocean Survey (NOS), formerly the U. S. Coast and Geodetic Survey (USC&GS), now a component of the National Oceanic and Atmospheric Administration, determines elevations in meters above or below the National Geodetic Vertical Datum of 1929. Bench marks are located at distances of about 1.6 km along railroads and highways in the United States to form level lines. These level lines form a net of closed loops. The present program for development of the fundamental vertical control net calls for first-order lines at approximately 160-km intervals with a subdivision of second-order lines spaced at 40 to 80 km. In certain areas of intense development, the net is again subdivided by second-order lines spaced at 8 to 16 km. The current net consists of 370,000 km of first- and second-order leveling.