Visit the NHD Web site for more complete background and production status information.
The NHD provides a national framework for assigning reach addresses to water-related entities, such as industrial dischargers, drinking water supplies, fish habitat areas, wild and scenic rivers. Reach addresses establish the locations of these entities relative to one another within the NHD surface water drainage network in a manner similar to street addresses. Once linked to the NHD by their reach addresses, the upstream/downstream relationships of these water-related entities and any associated information about them can be analyzed using software tools ranging from spreadsheets to Geographic Information Systems (GIS). GIS can also be used to combine NHD-based network analysis with other data layers, such as soils, land use and population, to help better understand and display their respective affects upon one another. Furthermore, since the NHD provides a nationally consistent framework for addressing and analysis, water-related information linked to reach addresses by one organization (national, state, local) can be shared with other organizations and easily integrated into many different types of applications to the benefit of all. California's experience with RF3 illustrates these benefits.
The establishment and maintenance of permanent feature identifiers is one of the fundamental goals of the NSDI Framework. Reach codes will play this role within the National Hydrography Dataset. Like the street name in a mailing address, it is the uniqueness and stability of the reach code in a reach address that makes it so valuable for integrating water quality information. The National Hydrography Dataset is designed to provide comprehensive coverage of hydrologic data for the US. While initially based on 1:100,000-scale data, the NHD is designed to incorporate -- and encourage the development of -- higher-resolution data required by many users and, thus, it supports another basic tenant of the Framework. The NHD will facilitate the improved integration of water-related data in support of the application requirements of a growing national user community and will enable shared maintenance and enhancement.
Data preparation and integration is an automated (blind) process, known as the "Blind Pass", during which the content and organization of the DLG-3 hydrography file and the alpha release of RF3 (RF3-Alpha), are converted into more feature-oriented interim datasets, known as DLG-F and RF3", respectively. They are then conflated, i.e., overlaid to facilitate the automated transfer of reach attributes onto the DLG linework. As part of Blind Pass processing, extensive content verification is performed including the validation of RF3-Alpha names against the latest USGS Geographic Names Information System (GNIS II). The other significant NHD ingredient datasets are centerlines for wide rivers and lakes, GNIS II point feature names, and an improved version of the USGS hydrologic Cataloging Unit (CU) boundaries. The centerlines were generated using a raster-based, least-cost-path algorithm to compute the cheapest path through the wide river and lake polygons defined within the DLG. In addition to the GNIS-validated RF3-Alpha names mentioned above, approximately 100,000 new GNIS II names were assigned to DLG wide river and lake polygons using a point-in-polygon technique. These ingredient datasets are combined and distributed for visual review in the form of quad-based GIS workspaces.
"Visual Pass" processing entails the distributed visual review and correction of the output from the Blind Pass. Initially, quad-based quality-assurance/quality-control (QA/QC) is performed to resolve any reach conflation or centerline integration errors. CU-based navigation QA/QC is then performed to ensure the integrity of the linear drainage network that is comprised of all reaches within each Cataloging Unit. This CU-level review requires that the set of quads covering each CU be combined.
As Visual Pass processing is completed, the data processed by different sites will be combined and central QA/QC performed prior to final database loading by the USGS.
The NHD design is intended to support the nesting of more detailed hydrography data into the 1:100,000-scale base with minimal modifications to existing reach codes, which will serve as permanent surface water feature identifiers across scales. Currently, the state of Florida is conducting a 1:24,000-scale integration pilot project in cooperation with the USGS, USEPA and other state and local cooperators to evaluate and report on issues related to this process. The Florida pilot project, which was initiated under a Federal Geographic Data Committee grant, has also facilitated the prototyping and initial development of a suite of update procedures and tools. Given the increasingly widespread interest in higher resolution hydrography, we are pursuing similar projects with potential maintenance partners located in the different parts of the Country. These projects will allow us to refine and enhance NHD update procedures and tools while further exploring longer term maintenance roles.
Keven S. Roth
U.S. Geological Survey - National Mapping Division
511 National Center
Phone: 703-648-5471
Fax: 703-648-4722
Internet ID: KROTH@USGS.GOV