Filtering methods based on morphological operations have been developed in some previous studies. The biggest challenge for these methods is how to keep the terrain features unchanged while using large window sizes for the morphological opening. Zhang et al. (2003) tried to achieve this goal, but their method required the assumption that the slope is constant. This paper presents a new method to achieve this goal without such restrictions, and methods for filling missing data and removing outliers are proposed. The experimental test results using the ISPRS Commission III/WG3 dataset show that this method performs well for most sites, except those with missing data due to the lack of overlap between swaths. This method also shows encouraging results for laser data with low pulse density.