Calibration Base Lines for Electronic Distance Measuring Instruments (EDMI)

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.

CBL's provide surveyors with a means to detect and correct errors in Electronic Distance Measuring Instruments (EDMI). These highly accurate base lines provide a locally accessible standard for length measurement.In addition, NGS makes available the PC program CALIBRAT. This program is used to determine the scale and constant corrections for electronic distance measuring instruments by utilizing measurements taken over previously determined base lines. The formulas used in the program are found in NOAA Technical Memorandum NOS NGS-10, "Use of Calibration Base Lines."

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Both mark-to-mark and horizontal distances are published. When the suggested grade specifications are met, it is a simple matter to place the instruments or reflectors at about the same height above the monuments. A direct comparison with the published mark-to-mark data can then be made. Unless steps are taken to set the instruments or reflectors such that the measured distances are in the horizontal plane between monuments, computations are required to compare the observed distances with the published horizontal values.

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The accuracy of each distance segment is determined through a least squares adjustment procedure on the assumption the data points were established in a straight line. Accuracies are on the order of 0.1 to 0.3 mm per line segment. Actual accuracies are stated on each individual CBL document.

The coordinates for each CBL are generally scaled from a topo map or positioned using a GPS unit.

The published elevations are for distance reduction purposes only. They are not tied to the the National Vertical Datum. The relative differences between elevations are better than +/-12 mm.

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