This thesis presents the results of field measurement of GNSS at a part of lake Cerknica, more specifically in the surroundings of the village Dolenje Jezero, in January and March 2014. We took measures before and after the sleet storm and later when the amount of water was increased above the average water level of the lake. We describe the results and findings of determining the medium-altitude surface of the lake and the quality of the DTM. The first part of the thesis concerns the theoretical description of the methods GNSS surveying, later we focused on the description of GNSS surveying on the field. In the practical part of the thesis the problems encountered in the processing of observations static GNSS surveying methods are highlightened. We checked the accuracy of RTKmeasurement method with reference to the VRS in a given area. The main part of the thesis includes graphic displays of a medium altitude surface of the lake set by using the measured discrete points with RTK-method of surveying. The aim of the task in this part is to compare the data of the RTK-method measurement of water levels to the DTM based simulation of water level during the flood. Fluctuation of water levels was introduced with the comparison of the results on the graphic basis of orthophoto. The final part of the thesis points out the suggestions for a possible upgrade of the task in the future.