The goal of the article is to compare two methods used for suppression of an effect of terrain unevenness on accuracy of height measurement in UAV with integrated ultrasound altimeter during landing. Secure landing is one of the main requirements of current UAV operation. When the UAV is controlled by an operator on direct visibility, the landing process is possible to control manually or alternatively to use auxiliary system that informs about current height. However recently, the development is focused on regimes without direct visibility between UAV and its operator or the autonomous and semiautonomous modes of UAV flight. The course of the whole flight itself is possible to divide into three phases, take off, cruising and landing. Just autonomous landing is usually the most safety critical of these phases. For this reason it is crucial to continually measure the UAV height as accurately as possible and to monitor the landing area for potential obstacles. The obstacle detection can be performed in various ways, today most often with on-board camera. For height determination it is beneficial to use ultrasound. In this case it is necessary to compare requested measurement accuracy with actually achievable. Measurement accuracy is dependent on several factors caused by the altimeter itself and by outside conditions. Here it is possible to consider two essential effects. These are transmission environment properties and the character of the terrain from which the ultrasound signal is reflected during the height measurement. This article is focused on the ways to suppress the impact of terrain unevenness on measured height. Two of the basic methods are compared here. The first method is a continuous averaging of the measured values. The second method is the Kalman filter. The use of the both methods presumes that any obstacles have been eliminated from the landing side detection process in another way, so the area of the application of these methods is obstacle free only with different degree and character of terrain unevenness. In the article, several model cases of the terrain unevenness are used with their simulated ultrasonic height measurements. These values are then applied to the both different processing methods and their results are mutually compared.