The causes of the violating horizontal positioning of the vision axis as the main geometric leveling condition were studied. The importance of this condition at leveling forward performed as the main method of altitude measurements for topographic and geodetic engineering works was particularly emphasized. When analyzing the ways of calibrating the main position, the authors highlighted the general regulations for different methods of determining the calibration base and mirror rotation of the tested level to the differences in visualization the visual scale readings depending on the length of shoulders of the upcoming leveling process. The obligatory required return of testing to initial altitude of calibration base creates a closed loop of heights that enables to define the correction of horizontal sight axis and the difference between the inequality sums of rear and front shoulders. For performing field works, a new scheme of calibration the main position of level with its offset from alignment calibration base is offered.