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A calibration base line (CBL) is a precisely measured, straight-line course of approximately 1,400 m used to calibrate Electronic Distance Measuring Instruments (EDMI). A CBL usually consists of four segments ranging from 150 to 1,400 m whose lengths are measured with two high precision EDMI. Since the program began in 1974, NGS has established more than 300 base lines throughout the United States in cooperation with government agencies, universities and professional groups. Participating organizations must select a suitable site that is accessible for public use and install the base line markers 2 to 3 months before measurements are taken. Published CBL distances are available from the National Geodetic Survey, 301-713-3242 or ngs.infocenter@noaa.gov. For more information, please refer to: [1] NOAA Technical Memorandum NOS NGS-8 - Establishment of Calibration Base Lines (1994). [2] NOAA Technical Memorandum NOS NGS-10 - Use of Calibration Base Lines (1977). Both publications are available in pdf from http://www.ngs.noaa.gov/PUBS_LIB/pub_index.html.