In the last decade, the development of laser scanning allowed the widespread use of this technology in Poland. Currently, Airborne Laser Scanning (ALS) become the most reliable source of elevation data for large areas. It was selected as a main source of data in the project IT System of the Country’s Protection against extreme hazards (ISOK), which provides LiDAR data for almost whole country. Despite progress in availability of accurate elevation data, there are specific areas were quality of the data collected in ISOK is not sufficient, e.g. Kampinoski National Park (KPN). The lower data quality can be expected in marsh ecosystems covered by dense vegetation in the floodplain and former river valleys. The aim of this work is to compare ALS Digital Elevation Models (DEM) collected with different scanning parameters, evaluate and determine causes of their quality degradation.