The paper presents discusses a formula for the mean error m, of the refraction factor, determined basing on synchronie and bi-directional observations of vertical angles, performed for a section of trigonometric levelling. It turns out from analysis of the formula that the mean error of this factor is mostly influenced by mean errors of vertical angles and heights of distance meters and prisms over survey points. The error of a distance may be neglected; this leads to the simple working formula.

In the paper the formula for equation of the observation correction is introduced, which also includes observation terms, which are not the subjects of adjustment. The system of such type of correction equations is the basis for calculation of intermediary unknowns, which are not only the function of observations being the subjects of adjustment, but also of observations, which are not deformed by corrections. The paper presents proofs of formulae for calculation of mean errors of intermediary unknowns and functions of those unknowns for a considered case. An important general conclusion results from those proofs: both, the mean en-or of the ith unknown, and the mean error of the function of unknown, obtained from the discussed system of equations can not be smaller than the corresponding error obtained form a system, which does not include those observations. Presented formulae may be used in the case of adjustment of a connected network to higher order points, which co-ordinates are considered as observations, which are not the subjects of adjustment. Therefore we assume them as constant in a narrower range, i.e. we assume constancy of their values after adjustment; however their mean errors are considered in accuracy analysis. Derived formulae may be also used for calculation of mean errors of explicitly determinable geodetic constructions connected to known points, if the influence of mean errors of co-ordinates of those points on the mean error of a given function of unknowns is to be considered.