The research described in this article include an analysis of the accuracy of the assignation of temporary water level, based on bathymetric measurements. Data came from measuring with echo sounder Hi-Target HD380 integrated with satellite receiver Hi-Target V30. The area of research was Chechło Reservoir in the Trzebinia commune. Plane of the water level was determined using an approximation method of least squares. The impact of interval of the registration to determine the correctness of the plane was analyzed. The planes were compared with a control plane which was obtained on the basis of independent measurement of the circumference of the water reservoir using a satellite receiver. The results allowed to state that the control plane allows correct the inclination of the plane of the water, and its use can be facultative and not obligatory.