Corrections for Refractive Index as Applied to Electro-Optical Distance Measurements

摘要

During the past year (1968) the United States Coast and Geodetic Survey has made use of a small aircraft, equipped with thermistors, for obtaining temperatures along lines measured with electro-optical instruments. This procedure is being used in mountainous areas where it is not possible to obtain temperatures with the use of balloons. In June 1968 when the transcontinental traverse was extended across the northwestern part of the U.S., in an area of northern Idaho, distances measured on 3 days over each of two lines failed to agree by 8 to 10 cm. Because results of the distance measurements in this area did not meet the specified requirements; that is, 1.7 cm plus 1 p.p.m. of the instance, the field party was instructed to observe simultaneous zenith distance angles before and after a series of geodimeter measurements and to repeat this procedure for 6 to 8 hours. The ground profile along the test line is shown. Results of these and other tests are discussed. In order to evaluate results of the zenith distance angles, as applied to the refractive index corrections, plans were made to carry out additional test measurements making use of an aircraft to obtain temperatures along the lines. Results of these tests are also discussed.