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The shapefile is converted to SDC (Smart Data Compression) format using either ESRI SDC Data Development Kit Professional 2 (DDKP2) or tools in ArcGIS. The SDC data set is then loaded into ArcSDEŽ to verify and validate the geometry.

After processing, the data set is checked for drawing display and number of records and file sizes compared with source materials. Some of the cartographic disparities with this data, inherent in the original 1:100,000 source data, remain in that the density of features can change abruptly. [From National Hydrography Dataset (NHD) documentation - 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. In general, streams longer than one mile (approximately 1.6 kilometers) were collected. Most streams that flow from a lake were collected regardless of length. Only definite channels were collected so not all swamp/marsh features have stream/rivers delineated through them. Lake/ponds having an area greater than 6 acres (approximately 2.4 hectares) were collected. Note, however, that these general rules were applied unevenly among maps during compilation. Some map quadrangles have a much sparser pattern of hydrography than do adjoining maps and these differences continue in the digital rendition of these features. Transport reaches are defined on nearly all features of type stream/river, canal/ditch, artificial path, pipeline, and connector. Waterbody reaches are defined on the subset of lake/pond features that were identified as waterbodies during the development of Reach File Version 3. Most attention in applying geographic names was given to transport reaches that follow stream/rivers and waterbody reaches. Near the international boundaries with Canada and Mexico, only the parts of features within the United States are delineated. Detailed capture conditions are provided for every feature type in the Standards for National Hydrography Dataset (USGS, 1999), available online through http://geography.usgs.gov/standards/.]

The data set originally comes from several sources. Most of the data is from U.S. Geological Survey topographic quadrangle maps or sources that exceed its horizontal accuracy. 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. At 1:100,000 scale, 0.02 inch is approximately 167 feet (50.8 meters).

Process_Description:

Convert DLG [Digital Line Graph] data to features:
This process converted DLG data to features and associated characteristics and converted the coordinate system to geographic (longitude-latitude) coordinates in NAD83 in five steps:
1. The USGS's [U.S. Geological Survey] "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 conditions for which conversions could not be reliably made were flagged for later inspection. Only known conversion problems were flagged, and no additional steps were taken to detect or repair discrepancies in the original DLG-3 or the converted NHD [National Hydrography Dataset].
2. A default value of a characteristic was added in cases where the description was incomplete.
3. All instances in which data 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 and to the NAD83 using the NADCON software version 2.1 (National Geodetic Survey, n.d.).

This process generated the "features" data.
Build reaches:
The basic steps for building reaches are as follows:
1. Convert RF3 [Reach File Version 3] to RF3" (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 on the basis of standards used for the NHD, and identify inflow/outflow points where transport reaches entered and exited waterbodies.
2. Create artificial paths. Using waterbodies from the feature data and inflow/outflow points extracted from RF3", this process automatically generated the centerlines used to delineate artificial paths within waterbodies by using subroutines within the ARC/INFOŽ GRID module.
3. Blind pass. This batch step conflated features and RF3" 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 (February 1995 extract) to waterbodies.
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. (Errors not detected by the software may continue in the data.) Using the list, software presented each case to analysts to make appropriate edits to the data. Analysts recorded notes about repairs that could not be made and about other errors in the data. (These notes are encoded in the cataloging unit digital update units.)
5. Build superquads. After the quadrangle-based visual pass was complete, all quadrangles that cover all or part of each cataloging unit 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. As they did with 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 about other errors in the data. (These notes are encoded in the cataloging unit digital update units.)
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 of data 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 included assigning the final reach codes, building waterbody reaches, adding final artificial paths in waterbodies, and implementing any recent changes in standards for the NHD. The spelling of geographic names was replaced using the March 1999 data extract from the Geographic Names Information System. After this, reaches, features, characteristics, geographic names, and relations were loaded into the database that holds the NHD.
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 ARC/INFOŽ workspace containing coverages and other files. 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.

Modified flow relationships for a subbasin.

Updated stream level attributes based on flow relationships produced by the flow validation process. Stream level updates were based upon names for named reaches and the arbolate sum for unnamed reaches. Arbolate sum for any reach on the network is the sum of the lengths of all upstream reaches connected to that reach through flow relationships.

Modified Names: Added names to NHD reaches by comparing them to the GNIS [Geographic Names Information System] coordinates using ArcView NHD Tools.

Modified Flow Relationships using NHD ArcView Tools: Performed flow quality assurance/control and repaired flow relationships, as needed.

Modified Names using NHD ArcView Tools: Added names to NHD reaches and waterbodies by comparing them to the GNIS coordinates.

Replaced branched and disjoint (disconnected) reaches with linear reaches in NHDFlowline. This change occurred during the conversion to the geodatabase model. Each NHDFlowline that was part of a branched or disjoint reach was assigned to a new reach. The ComID of the NHDFlowline was unchanged. The change information from the old reach to the new reach was captured in the NHDReachCrossReference table. Each new reach has M values assigned from 0 to 100.

Merged and/or split area features (NHDWaterbody, NHDArea, region.wb, region.lm) and updated associated artificial path WBAreaComID (wb_com_id) values. This change occurred during the conversion to the geodatabase model. If adjacent or overlapping area features had identical classifications, including ReachCode where applicable, the features were merged. If an area feature exceeded 100 sq. km. (25 sq. km. for canal/ditch, stream/river, or submerged stream) and crossed a subbasin boundary, the feature was split at the subbasin boundary. If any sliver polygons of a size of less than 5 sq. km. adjoined the split features, the slivers were merged back into the larger area feature. Merged or split area features were assigned a new ComID.

Deleted name and GNIS Id values from NHD features where GNIS feature type is valley. This update, at the request of the GNIS staff, is to remove the possible application of a valley name (hypsographic feature) to a stream/river (hydrographic feature). In most instances, the valley name cannot be presumed to apply to the stream/river. Where the name does apply to the stream/river, the GNIS name must be updated from valley to a hydrographic feature type in order for the name to be valid in the NHD.

The following steps were performed by ESRI: Downloaded the over 200 compressed files from ftp://nhdftp.usgs.gov/SubRegions/Medium/ and extracted them. Used all features from the NHDWaterbody source file of each NHD source tile. Used Canal/Ditch, Inundation Area, and Stream/River features from the NHDArea source file of each NHD source tile. Merged all NHD source tiles into a single data set. Replaced the numeric values with text strings for the attribute FTYPE. Merged and dissolved each of the Great Lakes and Great Salt Lake into single lake features and named them correctly. Added attributes SQKM and SQMI and calculated their values. Created ArcGISŽ layer file (.lyr), projection file (.prj), and spatial indices.

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The SDC file contains the geospatial and attribute data. The SDI file contains the spatial index. The PRJ file contains the coordinate system information (optional). The XML file (*.sdc.xml) contains the metadata describing the data set (optional).

U.S. Water Bodies

Metadata: