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Elevation-derived Hydrography and the National Hydrography Datasets

By NGP Standards and Specifications

Summary

Background

Hydrography and elevation data historically have been acquired and managed separately because of differences in data formats and relative accuracies. The success of the 3D Elevation Program (3DEP) and resulting rapid increase in highly detailed and accurate lidar (IfSAR in Alaska), coupled with increased computing power and a proliferation of available tools, make it possible to derive hydrography data from elevation data at a fine resolution and on a broad scale. Deriving National Hydrography Dataset (NHD) features from 3DEP elevation data greatly enhances the vertical and horizontal spatial integration between terrain and the stream network, providing the level of accuracy and detail required for local scale applications while also providing the terrain basis for modeling the flow of water across landscapes. 

Respondents of the Hydrography Requirements and Benefits Study (2016) identified integrated hydrography and elevation data as necessary to their most critical activities. The most highly required data integration type for Federal agency mission critical activities was the alignment of hydrography data with elevation data. Deriving NHD features from elevation data at an accuracy of +/- 7 feet, which equates to deriving hydrography from Quality Level 2 3DEP elevation data, would meet 65% of the overall reported user requirements (76% Federal, 73% state, 44% other). 

 

Discussion

While some elevation-derived hydrography has been ingested into the NHD, other data, particularly from smaller and more research-focused science projects, are not. These independent projects are a missed opportunity to leverage expertise and resources across sectors to improve the quality of the NHD and better meet the needs of users. To address this, the USGS is responding to the need for integrated hydrography and elevation data in multiple ways, including: 

 

The production of NHDPlus High Resolution datasets nationwide 

As a first step toward integrating elevation and hydrography information, the USGS is building the NHDPlus High Resolution (NHDPlus HR) nationally using 3DEP elevation data, the NHD, and the Watershed Boundary Dataset (WBD). This product will enable high resolution hydrologic flow modeling across landscapes and into and through the stream network anywhere in the country. Currently, the NHDPlus HR production process uses a workaround to deal with disagreement between the elevation and hydrography data. The product will be greatly enhanced by elevation-derived hydrography. 

The NHDPlus HR is currently available as a Beta product for 80% of the U.S. with completion of CONUS, Hawaii, and the U.S. territories planned for FY20. Regional NHDPlus HR Beta production has begun in Alaska and will continue throughout the 2020s. Once the Beta product is completed, an updated version of the data will begin production. 

 

Pilot projects to better understand capabilities to produce elevation-derived hydrography 

In 2017, the USGS funded two pilot projects with contractors to derive NHD features from elevation data in five distinct landscapes across the contiguous U.S. The project focused on better understanding the costs and utility of deriving hydrographic features from lidar data. It documented both the need to provide training on the NHD data model, and the need for clearer data capture guidelines. In 2019, the USGS began a second pilot project with three contractors to derive new NHD data from 3DEP IfSAR elevation data for a watershed in Alaska. As a part of the pilot, the contractors are using recently developed draft USGS elevation-derived hydrography specifications and capture condition documents to guide their work, and in turn, are providing valuable reviews and feedback about the documents. 

 

Development of USGS elevation-derived hydrography specifications 

The USGS is currently developing two documents that are crucial for deriving elevation-derived hydrography data that is NHD-ready: Elevation-derived Hydrography Acquisition Specifications and Elevation-derived Hydrography— Representation, Extraction, Attribution, and Delineation Rules. These documents, currently in final review with an expected publication in mid-FY20, lay the foundation for creating the next generation of NHD data. 

 

Research into elevation-derived hydrography methods 

Federal agencies, state and local governments, academic institutions, non-profits, and consultancies are actively researching elevation-derived hydrography production methods, and many of these techniques are already being used to update hydrography data across the country. The USGS has a long history of research and collaboration in this area, and continues to investigate how to provide better integrated, current elevation and hydrography products to increase the return on geospatial data infrastructure investments for the Nation. The USGS Center of Excellence for Geospatial Information Science is currently exploring how to derive hydrography data directly from elevation data, with a focus on updating the NHD. 

 

Creation of a USGS programmatic infrastructure for elevation-derived hydrography data 

The USGS does not plan to hold contractors to using a specific elevation-derived hydrography production method because the effectiveness of any given technique may differ based on the specific geography, climate, or other computing variables. Rather, the USGS is actively developing a process to inspect data against specifications, much like 3DEP’s inspection process for lidar data. The USGS is in the second year of research into making NHD data ingestion easier and less costly and plans to work with contractors in FY20 to test these solutions. Lastly, NGP has requested a new hydrography position within Topographic Data Services (TDS) to focus on and oversee NHD elevation-derived hydrography acquisition, specifications, and partnerships. 

 

Outreach and strategic planning for NHD elevation-derived hydrography acquisition 

In FY20, NGP will outreach to partners and stakeholders, including contractors, to ensure they have the information necessary to ingest elevation-derived hydrograph into the NHD. To support this, USGS is leading a special session at the AWRA Geospatial Technology Conference in March 2020 to introduce the specifications and capture condition documents to the community. The session includes lightning talks from several contractors and subcontractors involved in pilot work. The USGS is also adding information about elevation-derived hydrography to the USGS National Hydrography website to make information broadly available. Lastly, NGP is developing an internal program plan for a national approach to acquiring elevation-derived hydrography for the NHD.