A semi-automatic method to Hydro-flatten LiDAR data

Triangulation of LiDAR data can produce unnatural water surface. Therefore, it is necessary to hydro-flatten LiDAR data. Hydro-flattening is driven primarily by cartographic mapping needs, and can also help hydrologic analysis, because hydro-flattened surfaces behave naturally; allowing water flow from upstream to downstream. Detailed hydro-flattening requirements have been stated by USGS, which are to be used by LiDAR contractors. In this paper, a new method is presented which extracts bank shoreline of inland water bodies to hydro-flatten LiDAR data. Unclassified LiDAR data acquired over Big Rapids, MI and a river centerline from the NHD were used as inputs to this method. LiDAR data within a certain distance of the centerline was processed to create a continuous bare ground surface by removing LiDAR points over trees and other non-ground features and by adding synthetic bathymetric points in the water area with sparse LiDAR points. Then cross-sections were placed at regular intervals perpendicular to the centerline, and the lowest elevation points around every intersection were searched. These elevations were assumed to be at, or near the actual water surface and assigned to their respective cross-section. These elevations were checked and revised to have a natural drop in elevation from upstream to downstream, then raised arbitrarily by 0.5 m so that they were between the minimum and the bank full elevation of the river at any cross-section. A virtual water surface was created using the raised elevations, which was then intersected with the conditioned bare earth surface to obtain bank shoreline polygons. Any voids greater than 200 m2 were filled and the polygons were smoothed using tools in ArcGIS software for better cartographic appearance. These polygons were then converted to a 3D feature by extracting elevation from the virtual water surface and were used to hydro-flatten the LiDAR data. The proposed method is also modified to be used for hydro-flatten ponds.