AbstractThe design and manufacture of distance measuring instruments for geodesy has advanced to the point where the problems of defining refractive index along a measured line provide the largest source of error in the measurement of distance. A series of studies has been initiated to determine the possible advantages of measuring lines in pairs. The results of studies made thus far in two locations show that use of line pairs provides useful information about atmospheric refraction during the time of measurement and may improve the accuracy of trilateration and traverse.