The most of geodetic measurements are carried out in the lowest non-stable air levels over the terrain, which are very difficult and change in time with the optical environment. These air levels are approximately parallel with the earth surface, reaching the elevation of several metres, and about 99 % of all geodetic measurements are realized here. In the last years there was a great development in the area of constructional solution of measuring instruments. Modern electronic theodolites of the highest accuracy class reach the accuracy of direction (angle) measurement of 0,1 – 0,2 mgon and electronic rangefinders more than 0,5 mm for 1 – 2 km distances. That’s why the situation rise up where the important source of systematic errors in the measuring process are not instrumental errors but errors of the environment, in which is the measuring process realized. Because of this high accuracy of geodetic instruments, the refraction influence is a base limiting factor of increasing the measurement accuracy. In the past, the refraction influence was mostly neglected in the engineering geodesy but, in a present, requirements are given on a higher accuracy. That’s why various theoretical refraction models and practical inspecting processes are used, which enable us to decrease the refraction influence on the measured values.