With the USGS 3D-Elevation Program (3DEP) goal of elevation-mapping the contiguous US to the QL2 specification and Alaska to the QL5 specification, and the 3D Nation initiative focusing on mapping from the tops of mountains to the depths of the ocean, additional attention has been directed toward upgrading the National Hydrography Dataset (NHD) to be more closely matched to the high precision elevation data. Toward this goal, the USGS Lidar Base Specifications 1.3, released in Feb 2018, introduced an EleHydro data dictionary as a guide for how breaklines would be compiled. Though not a requirement, the concept was offered as a guide for hydrological breaklines and a roadmap to deriving flowlines from elevation data. Since its initial release, the NHD has been revised and upgraded to reflect higher resolution imagery and topography. Likewise, the EleHydro guidelines are evolving into a Elevation-Derived Hydrography (EDH) specification to incorporate the higher precision lidar/IfSAR elevation data into the flowlines and waterbodies of the nation. NHD data is currently being developed with the new EDH specifications under USGS pilot project programs using either QL2 lidar-derived and QL5 IfSAR-derived digital elevation models. This presentation discusses the significant differences between lidar-derived and IfSAR-derived digital elevation models (DEMs) relative to flowline extraction, compares the currently available techniques for preparing DEM surfaces and feature (flowline) extraction, and explores some of the challenges with using IfSAR-derived DEMs for NHD flowline extraction in Alaska.
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