Hydro-flattening is a common requirement when it comes to delivering surface models to the U.S. Geological Survey (USGS) National Elevation Dataset (NED). According to the USGS LIDAR Base Specification Version 1.2, a hydro-flattened surface refers to a specific kind of digital elevation model (DEM) that is required for inclusion in the NED. A hydrologically flattened waterbody, stream, rivers and other water surfaces should be flat and level from bank-to-bank. Rivers and streams should follow a downhill water surface, which would reflect the more natural behavior of the environment.
“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 created from photogrammetric digital terrain models (DTMs).”
There are different methods a user can employ to create hydro-flattened surfaces. The method outlined within this article is only one of those options. In this suggested method breaklines will be created to represent river flattening, and then used to enforce the surface, resulting in a flattened surface that flows in the downstream direction.