Statement of the problem. During geodetic monitoring of the foundations of buildings and structures, quantitative characteristics of displacements of deformation marks are obtained. However, these data may contain errors due to the lack of a fixed initial basis. Thus it is proposed to use a conditional (fictitious) basis. Results. Using the example of a reactor ring, a conditional basis passing through the center of the reactor is considered, the projection of the conditional axis of the reactor onto a horizontal plane is taken as the starting point, and the conditional horizontal axis passing through the center of the reactor is taken as the direction. The calculation of the assessment of the stability of the signs of the basic planned altitude network and the assessment of the accuracy of determining the conditional axis of the reactor using these signs were performed. Conclusions. By changes in the value of the radius of the circumscribed circle, the deviation of the position of signs relative to the central point — the point of the corresponding projection of the conditional axis of the reactor — can be estimated.