The study presents results of the internal reliability analysis of structural modules used for the determination of horizontal displacement in incomplete trigonometric network. The influence of such elements as: number of control points, sight line length and arrangement of control points around the instrument station on reliability was analysed. Furthermore the analysis of the influence of diversification of reliability indices calculated for individual observations on the detection efficiency of non-dislocated control points was performed. The presented numerical example illustrates the possibility of incorrect valuation of control point stability because of a large diversification of reliability indices. The summary contains recommendations from the point of view of internal reliability for optimal designing of structural modules in incomplete trigonometric networks.