Two-color terrameter

Abstract

The Terrameter is a two-wavelength EDM instrument that has been tested under changing meteorological conditions. This system, based on the dispersive character of the air, can determine the measured distance to better than 1 part in 10 7, without the usual meteorological observations and corrections. The Terrameter simultaneously measures the optical path lengths at two wavelengths (red, 638.8 nm and blue, 441.6 nm). With an internal micro-computer, the correct baseline distance is computed, eliminating the firstorder effects of temperature, pressure and relative humidity fluctuations along the measured line. The instrument is designed to be easily positioned over a horizontal control point to a precision of better than 0.1 mm. Present strainmeters lack the portability and range for high accuracy measurements, and other available distance-measuring instruments do not have the necessary accuracy to measure the strain rates in a reasonably short period of time. The speed, high accuracy, portability and range of the Terrameter make it possible to determine the rate of tectonic deformation in a period of months rather than years. Other applications, such as the determination of the stability of rock faces and salt domes, and the measurement of structural deformation of dams, reservoirs and construction sites, are also possible with this instrument.