When determining height differences by using trigonometric levelling method, some affecting values are present. Besides measuring imperfection due to instrumental imperfection and surveyor's errors, quality of the trigonometric levelling results also depends on following external affecting values: height difference between observed points, distance between observed points and temperature gradient (change of temperature with height) between observed points. Therefore, if the height difference is bigger, the influence of vertical refraction on the sightline is bigger. Also, temperature gradient in layers of air directly affects the sightline, that is, on the measuring of vertical angles. When distance between observed points is longer, the effect of Earth's curvature is bigger, so, to eliminate this effect, trigonometric levelling was conducted between relatively close points, approximately 200-meter distant. Besides that, levelling was conducted by means of two different methods to obtain partially independent height differences between close points. In that way, it was possible to calculate the refraction influence from difference of the results obtained by two methods.