"The National Hydrography Dataset (NHD) is a feature-based database that interconnects and uniquely identifies the stream segments or reaches that comprise the nations surface water drainage system. It is based initially on the content of the U.S. Geological Survey 1:100,000-scale Digital Line Graph (DLG) hydrography data, integrated with reach-related information from the U.S. Environmental Protection Agency Reach File Version 3.0 (RF3). More specifically, it contains reach codes for networked features and isolated lakes, flow direction, names, stream level, and centerline representations for aerial water bodies. The NHD also incorporates the National Spatial Data Infrastructure framework criteria set out by the Federal Geographic Data Committee."
"The National Hydrography Dataset combines elements of the DLG and RF3: spatial accuracy and comprehensiveness from the DLG and network relationships, names, stream level, and a unique identifier (reach code) for surface water features from RF3. The NHD supersedes DLG and RF3 by incorporating them, not by replacing them. Users of DLG and RF3 will find the National Hydrography Dataset both familiar and greatly expanded and refined. 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 effects upon one another. Furthermore, because 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. The National Hydrography Dataset is designed to provide comprehensive coverage of hydrologic data for the U.S. 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. It 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."
"The accuracy of the attributes of the Digital Line Graph data is estimated to be 98.5 percent. One or more of the following methods were used to test attribute accuracy: - manual comparison of the source with hardcopy plots. - symbolized display of the digital line graph on an interactive computer graphic system. - Selected attributes that could not be visually verified on plots or on screen were interactively queried and verified on screen. In addition, software validated feature types and characteristics against a master set of types and characteristics, checked that combinations of types and characteristics were valid, and that types and characteristics were valid for the delineation of the feature. Feature types, characteristics, and other attributes conform to the Standards for National Hydrography Dataset (USGS, 1999) as of the date they were loaded into the database. All names on reaches were validated against a March 1999 extract from the Geographic Names Information System. The entry and identifier for the names match those in the Geographic Names Information System. The association of each name to reaches has not been methodically checked, and so a name may be applied to the wrong reaches. Anecdotal reviews indicate that 80 percent or more of the named reaches have the correct name. Reaches were delineated with a batch procedure and were checked extensively during the visual pass steps of processing. Based on automated quality assurance/quality control checks performed at various intervals during the processing, approximately 99 percent of the reaches are delineated according to standards."
"Areas conform to topological rules. Gaps and overlaps among areas do not exist. All areas close."
"The completeness of the data reflects the content of the sources, which, in the initial release of the National Hydrography Dataset, most often are U.S. Geological Survey topographic maps. Features found on the ground may have been eliminated or generalized on the source graphic because of scale and legibility constraints. Most attention in applying geographic names was given to transport reaches that follow stream/rivers and waterbody reaches. Detailed capture conditions are provided for every feature type in the Standards for National Hydrography Dataset (USGS, 1999), available online through <http://mapping.usgs.gov/standards/.>"
"Statements of horizontal positional accuracy are based on accuracy statements made for U.S. Geological Survey topographic quadrangle maps. These maps were compiled to meet National Map Accuracy Standards. For horizontal accuracy, this standard is met if at least 90 percent of points tested are within 0.02 inch (at map scale) of their true position. Additional offsets to positions may have been introduced where there are many features to improve the legibility of map symbols. In addition, the digitizing of maps is estimated to contain a horizontal positional error of less than or equal to 0.003 inch standard error (at map scale) in the two component directions relative to the source maps. Visual comparison between the map graphic (including digital scans of the graphic), and plots or digital displays of points, lines, and areas, is used to assess the positional accuracy of digital data. Linear features of the same type along the adjoining edges of data sets are aligned if they are within a 0.02 inch tolerance (at map scale). To align the features, the midpoint between the end of the corresponding features is computed, and the ends of features are moved to this point. Features outside the tolerance are not moved; instead, a feature of type connector was added to join the features."
"Flow validation performed after initial release of data: 1.Convert RF3 to RF3 double prime. This batch operation processed Reach File version 3 to delete duplicate reaches, reassign reaches to the correct cataloging unit, validate geographic names assigned to reaches against data from the Geographic Names Information System (December 1996 extract), apply updates supplied by the States of California and Arizona, redelineate reaches based on the standards used for the National Hydrography Dataset, and identify inflow/outflow points where transport reaches entered and exited waterbodies. 2. Create artificial paths. Using waterbodies from the features data and inflow/outflow points extracted from RF3 double prime, this process automatically generated the lines used to delineate artificial paths within waterbodies using the ESRI ARC/INFO GRID routine. 3. Blind pass. This batch step conflated RF3 double prime reaches, and transferred reach information (reach code, reach date, name, stream level, and flow relationships), to the features. It also integrated the artificial paths generated in the previous step with the other features, built reaches on the artificial paths, and assigned geographic names to waterbodies (February 1995 extract). 4. Quadrangle-based visual pass. During this interactive step, analysts ensured that the data developed in the previous batch processes conformed to reach delineation rules and that reaches were assigned to the appropriate cataloging unit. Batch procedures identified and developed a list of possible errors. Using the list, software presented each case to analysts to make appropriate edits to the data. Analysts recorded notes where repairs could not be made; these notes are encoded in the cataloging unit digital update units under a separate process description. 5. Build superquads. After the quadrangle-based visual pass was complete, all quadrangles that cover all or part of each cataloging unit using were paneled into a superquad. In this batch process, reaches that cross quad boundaries were corrected to conform to reach delineation rules. 6. Cataloging unit-based visual pass. Like the quadrangle-based visual pass, analysts ensured that reaches conformed to reach delineation rules. Batch procedures identified and developed a list of possible errors. (Errors not detected by the software may continue in the data.) Analysts examined each error, and corrected the data. Analysts recorded notes about repairs that could not be made, and other errors in the data; these notes are encoded in the cataloging unit digital update units under a separate process description. 7. Central quality assurance/quality control. This step (1) confirmed that integrity checks were performed successfully during the visual pass activity, and (2) assessed statistics gathered during the earlier processes to determine if additional review was needed. A check of data from the cataloging unit-based visual pass was run in batch; any data that did not pass the procedure were reviewed interactively. If substantive changes were required, the data were reprocessed using procedures (as required) described in previous steps. The edited data then were rechecked using the central quality assurance/quality control process. 8. Data preparation and database load. This batch procedure performed final processing to the data emerging from the quality assurance/quality control step. Some of the activities include assigning the final reach codes, building waterbody reaches, adding final artificial paths in waterbodies, and implementing any recent changes in standards for the National Hydrography Dataset. The spelling of geographic names were replaced from the March 1999, data extract from the Geographic Names Information System. Following this procedure, reaches, features, characteristics, geographic names, and relations were loaded into the database that holds the National Hydrography Dataset. 9. Flow relation correction and validation. The flow relations were checked for consistency through a batch procedure, which generated a list of possible errors. Software presented possible errors to analysts, who corrected flow relations and, occasionally, the delineation of reaches. Changes were posted to the database. 10. Extract distribution copies of data. Data for a cataloging unit were extracted from the database, and converted into an ESRI ARC/INFO workspace containing coverages and other files related to the National Hydrography Dataset. Data available in the Spatial Data Transfer Standard format were developed from the workspaces. The workspaces, and the Spatial Data Transfer Standard-formatted files, were made available to the public. This process converted Digital Line Graph (DLG-3) data to the features and converted the coordinate system to geographic (longitude-latitude) coordinates in the North American Datum of 1983, in five steps: 1. The U.S. Geological Surveys Batch DLG-3 to DLG-F Conversion System converted DLG-3 nodes, lines, areas, and associated attribute codes to temporary features and associated characteristics. Known conversion problems or uncertain conversions (for example, the DLG-3 attribute code for reservoir sometimes should convert to the feature type Reservoir and other times to feature type Lake/Pond) were flagged for later inspection. Only known conversion problems were flagged and no additional steps to detect or repair discrepancies in the original DLG-3 or the converted NHD were taken. 2. A default value of a characteristic was added in cases where the description was incomplete. 3. All instances in which problems and uncertainties were flagged were inspected and resolved interactively. 4. Feature delineation rules were applied to the temporary features in a batch process to create the final version of features. 5. Coordinate values were converted to geographic coordinates, the North American Datum of 1983, using version 2.1 of NADCON software (National Geodetic Survey, n.d.).This process generated the features data."