Nowadays, the method of displacement measurements on constructions is well known. A variety of equipment of different accuracy and capacity is available. In this paper authorspresent a combined method of tacheometric and photogrammetric measurements and displacements measured by a pressure length transducer. In our laboratory, the loading of the pre-stressed concrete plate occurred in steps of 3 kN. The Mathematica 5.0 program was used to calculate the interpolation polynomials. All results were compared to the calculated values (using in the Eurocode 2) with the help of interpolation polynomials. The results show that, in future, we will not need to use so many load phases, because the intermediate displacements can be calculated from the obtained equations and the displacements under different loads predicted.