In the master's thesis we present the results of five relative gravimetric measurements, conducted between March 2016 and September 2017 during the various watercourses of Cerknica Lake. We describe the results and findings of the influence of the lake's water level variations on the changes of gravity and, consequently, the effect of the changes in gravity to the normal height of the points on which we carried out a relative gravimetric measurement. The influence of the water level variations is related to the water mass in individual gravimetric measurements. In field work, RTK-measurement method (Real Time Kinematic) of GNSS (Global Navigation Satellite System) was used to determine the approximate point positioning along the lake and later for setting-out them. The first part of the master's thesis relates to a theoretical description of the gravimetric survey and the RTK-measurement method of GNSS, followed by a description of the practical part. Here we highlight the problems that arise from the unusual behavior of the gravimeter The aim of the master's thesis is to evaluate the influence of the quantity of water on the change of gravity between individual measurements.In the final part we present proposals for upgrading the task in the future.