The authors carried out an analysis of the accuracy of data obtained from a hydroacoustic survey performed with the ultrasonic sonar LMS-527C DF and GPS Company Lowrance Electronics, Inc. The location of probing points was carried out in two ways: using an 12-channel GPS built-in echo sounder and, independently, a surveying equipment. In the second case, the measuring technique GPS-RTK (Real Time Kinematic)was used enabling accurate position determination in real time. The field measurement was conducted in July 2009 at Pilchowice Reservoir, which is located in the upper part of the Bóbr River near Jelenia Góra. Fieldwork was carried out by the staff of the Institute of Geodesy and Geoinformatics, University of Life Sciences in Wroclaw. Assessment of accuracy was based on analysis of the determined coordinates in XY plane, measured depth D and obtained spatial coordinates XYH. As a result of the analysis, the accuracy of the measurements was determined. It was found that using a built-in GPS receiver the position of the sonar transducer XY could be determined with an average error of 4.2 m (accuracy is heterogeneous and changes with time). Hydroacoustic survey with an ultrasonic sonar and a GPS receiver embedded in it (assuming that the level of water table is known) can provide bathymetric data of an accuracy of 0.20 m. However, if for sonar transducer positioning the measuring technique GPS-RTK was used, (with known or unknown water level), the depth measurement accuracy was at a level of 0.13 m. The calculations were performed in the Scilab environment.