A semi-automated method to create a lidar-based hydro-flattened DEM

Abstract

ABSTRACTThe response of water surfaces to light detection and ranging (lidar) pulses is unpredictable, which results in sparse lidar point density with varying intensity values. Due to the sparseness of the point cloud and lack of natural breaklines, lidar-derived digital elevation model (DEM) can produce unnatural surface over waterbodies. Such surfaces are not cartographically pleasing and can cause issues in the hydrologic and hydraulic modelling of a river. Hydro-flattening is the process of creating a lidar-derived DEM in which water surfaces appear and behave as they would in traditional topographic DEMs generated from photogrammetric digital terrain models. Hydro-flattened DEMs, created using lidar data, exclude the lidar points over waterbodies and include three-dimensional (3D) bank shorelines. In this article, a semi-automated method is presented for extracting bank shorelines for the purpose of creating lidar-derived hydro-flatten DEMs. Lidar point cloud and an approximate stream centreline are the primary data for this process. In the first step, a continuous bare ground surface (CBGS) is created by eliminating non-ground lidar points and by adding artificial underwater points. In the second step, the lowest elevation from the lidar point cloud within a radius distance from the river centreline is used to create a virtual water surface (VWS). This VWS is revised to consider water surface undulations such as ripples or waves, protruding underwater objects, etc. The revised VWS is then intersected with the CBGS to locate the two-dimensional (2D) bank shorelines. The 2D shorelines are assigned the elevations of the VWS and are used to produce a hydro-flattened DEM. The planimetric absolute mean separation of 0.94, 0.69, and 0.63 m for the three water surfaces is observed between the bank shoreline extracted using raw lidar points and a GPS (global positioning system) survey. The mean separation using vendor classified lidar points is 0.74, 0.67, and 0.64 m which are very similar to those using raw lidar.