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NHD Frequently Asked Questions


02/01/11

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What is the National Hydrography Dataset?

The NHD is a digital vector dataset used by Geographic Information Systems (GIS). The NHD contains features such as lakes, ponds, streams, rivers, canals, dams and stream gages. These data are designed to be used in general mapping and in the analysis of surface water systems. NHD Flowlines are important features in the NHD because they contain flow direction and form a network. In mapping, the NHD is used with other data themes such as elevation, boundaries, and transportation to produce general reference maps. The NHD is often used by scientists using GIS technology. GIS takes advantage of a rich set of attributes that can be processed to generate specialized information. These analyses are possible because the NHD contains a flow direction network that traces the water downstream or upstream. The NHD also uses an addressing system to link specific information about the water such as discharge rates, water quality, and fish population. Using the basic NHD attributes, flow network, linked information, and other characteristics, it is possible to study cause and affect relationships such as how a source of poor water quality upstream might affect a fish population downstream. The features in the NHD are organized into polygons, lines and points. The polygons most commonly portray waterbodies such as lakes while lines commonly portray streams. The stream lines are broken into shorter segments stretching from confluence-to-confluence. The segments are then linked together to trace the flow of water across the landscape. Flowlines attributed as artificial paths are added inside water bodies to maintain the flow network.

The primary features making up the nation's surface water are labeled with nationally unique and permanent identifiers known as reach codes. These unique identifiers give features an identity for inventory and analysis. Water chemistry, for example, can be linked to a stream or a lake using reach codes. Many features also are labeled with the name of the feature, such as the Ohio River. The feature names must be approved by the Board of Geographic Names (BGN) in order to qualify for inclusion in the NHD.

The network of lines contains linear measurements, making it possible to locate the position of a stream gage, dam, or other event attached to a flow line. Measures, known as M-Values in the NHD, are used for linear referencing and are similar to the address of a house on a street. By recording the measurements upstream on a reach code it is possible to uniquely identify any position along a waterway. Linear referencing makes it easier to perform calculations in a GIS such as identifying dams upstream from a stream gage, and then determining the distance to those stream gages. The system of linear referencing also makes it easy for any agency to link data to the NHD without having to customize the NHD.

For more information about NHD features, see the Feature Catalog at http://nhd.usgs.gov/FeatureDirectory.swf .

What is the Watershed Boundary Dataset (WBD)?

The Watershed Boundary Dataset (WBD) defines the aerial extent of surface water drainage to a point, accounting for all land and surface areas, determined solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects nor particular program or agency. The intent of defining Hydrologic Units (HU) for the Watershed Boundary Dataset is to establish a base-line drainage boundary framework, accounting for all land and surface areas. The selection and delineation of hydrologic boundaries are determined solely upon science-based hydrologic principles, not favoring any administration, special project, program, or agency. At a minimum, the WBD is being delineated and georeferenced to the USGS 1:24,000 scale topographic base map meeting National Map Accuracy Standards (NMAS). Hydrologic units are given a Hydrologic Unit Code (HUC). For example, a hydrologic region has a 2-digit HUC. A HUC describes where the unit is in the country and the level of the unit. For more information go to http://nhd.usgs.gov/wbd.html.

What is the history of the NHD?

1988 - USGS completes national coverage of 100K (Medium Resolution) Digital Line Graphs (DLG's)
1992 - Environmental Protection Agency completes national RF3 coverage
1997 - USGS / EPA cooperative effort to combine elements of DLG and Reach File version 3.0 (RF3)
2001 - National coverage of 100K (Medium Resolution) NHD completed
2002 - USGS, USFS, State Agencies, and Others begin work to produce 24K (High Resolution) NHD
2003 - Feature Operational Database (FOD) is retired and Geodatabase comes online
2007 - National coverage of High Resolution NHD completed

What are the advantages of the NHD?

GIS technologies take advantage of a rich set of attributes imbedded in the NHD to generate specialized information. These analyses are possible because the NHD contains a flow network that allows for tracing water downstream or upstream. It also uses an addressing system based on reach codes and linear referencing to link specific information about the water such as water discharge rates, water quality, and fish population. Using basic NHD features like flow network, linked information, and other characteristics, it is possible to study cause and effect relationships, such as how a source of poor water quality upstream might affect a fish population downstream.

What is the difference between attributes and features?

A feature is a geometric representation of a water feature, like a lake, river, or swamp. Attributes are information stored in a database format that are linked to feature. For instance, a river feature could have attributes indicating its flow direction, length, and name.

What is a reach?

A reach is a continuous piece of surface water with similar hydrologic characteristics. Some unconnected (isolated) features are also reaches, for example, isolated lakes and single, unconnected streams. In the NHD, each reach is assigned a reach code. A reach may be composed of a single feature, like a lake or isolated stream, but reaches may also be composed of a number of contiguous features due to the preservation of reach codes that came when the 24K NHD was created from the 100K NHD. A reach code is a 14 digit code comprised of two parts. The first eight digits are the HUC for the subbasin in which the reach exists, and the last six digits are a sequential number that is assigned when reach codes are allocated in the subbasin. Each reach code occurs only once throughout the nation, but a reach code may be on a number of contiguous features, if they all belong to the same reach. Once assigned, a reach code is permanently associated with its reach. If the reach is deleted, its reach code is retired.

What is the difference between connectors and Artificial Paths?

A connector establishes a known, but non-specific connection between two non-adjacent network segments that have flow.

An artificial path is a feature that represents flow through a 2-dimensional feature, such as a lake or a double-banked stream.

What is the positional accuracy of the NHD?

Original hydrographic data were compiled to meet National Map Accuracy standards. These standards have been maintained in the process of creating the NHD. At 1:100,000 scale, ninety percent of well-defined features are within 167 feet (0.02 inches at map scale) of their true geographic position. USGS Map Accuracy standards for 1:24,000 scale require ninety percent of well-defined features to lie within 40 feet of their true geographic position.

What sources were used to produce the data?

The NHD is a combination of USGS hydrologic digital line graph files (DLG) and EPA reach files, version 3.0 (RF3). The USGS files were used for spatial accuracy and the EPA files were used for attribute information.

Why was this dataset created?

The NHD was created to assist scientists in modeling hydrologic features and is also useful for mapping purposes. The geometric features that came from the USGS DLG files, combined with the flow direction, reach codes, and other attributes taken from the EPA RF3 file, make the NHD a powerful tool for modeling.

In what scale are the data?

For the conterminous United States, Hawaii, and the Virgin Islands, medium and high resolution NHD data are available at 1:100,000 and 1:24,000 scale, respectively. NHD data are available for Puerto Rico at 1:20,000 scale, and at 1:63,360 scale in Alaska. Users are encouraged to work with their state's principal steward to produce and submit data at higher resolutions.

In what format are the data?

Data are offered as a file or personal geodatabase, but can also be downloaded as a shapefile. NHD is available as a State Extract, in pre-staged subregions, or in a specified extent from the The National Map Viewer.

Who can I contact for NHD information?

Visit http://nhd.usgs.gov, contact the Natural Science Information network at http://ask.usgs.gov/sils_index.html, call 1-888-ASK-USGS, or email nhd@usgs.gov.

In what coordinate system are the data?

NHD data is distributed in the Geographic coordinate system with units in decimal degrees.

Upon what datum is the NHD cast?

North American Datum of 1983.

How current are the data?

How current the data are depends upon the production date of the initial line work and whether this line work was updated when the DLG files were created by USGS. In addition, many states and federal agencies have been working to update portions of the NHD, therefore the vintage of the line work can vary from the 1950s to the present.

How can the NHD be used to support modeling?

The NHD takes advantage of a rich set of attributes that can be processed to generate specialized information. These analyses are possible because the NHD contains a flow direction network that traces the water downstream or upstream. It also uses an addressing system to link specific information about the water such as water discharge, water quality, and fish population. Using the basic water features, flow network, linked information, and other characteristics, it is possible to study cause and affect relations, such as how a source of poor water quality upstream might affect a fish population downstream.

The Hydrography Event Management (HEM) Tool provides full functionality for adding and editing events in the NHD. Events are informational data linked to the NHD using a linear referencing system on NHDFlowlines. The use of events is a key advantage of the NHD and allows vast amounts of scientific information to be linked to the NHD while keeping the design simple and by making advanced analysis techniques possible. The HEM tool handles linear referencing mechanics to make working with events easy. The HEM tool works on point, line, and area events and allows events to be located interactively, imported, or calculated. An ID value is created that provides the link between the event location and the informational data tied to the location. In addition, metadata creation and network measures are supported by the HEM tool.

Can the NHD be reprojected?

Yes, the NHD data can be reprojected using GIS tools. NHD data currently exists in geographic coordinates (unprojected lat/long), decimal degrees, NAD83.

A reach has both a reach code, common identifier (ComID), and a permanent identifier (PermID). What is the difference?

A reach code uniquely identifies each reach. This 14-digit code has 2 parts: the first 8 digits are the hydrologic unit code for the subbasin (formerly, known as cataloging unit) in which the reach exists; the last 6 digits are a sequence number assigned in arbitrary order to the reaches within that subbasin. Each reach code occurs only once throughout the Nation. Once assigned, a reach code is associated with its reach permanently. If a reach is deleted, its reach code is retired.

Reach codes facilitate geocoding or linking of observations, such as a water quality sampling sites, to reaches. Reach codes form the basis of a national linear referencing system which supports linking such observations to a point along a reach, an entire reach, or groups of reaches.

Reach codes are stored in the data element named "ReachCode". In addition to the reach code, the date on which the code was assigned in the NHD is also stored in the data element named "RCH_DATE". The only link between NHD at different resolutions is the reach code. Once a reach is defined and assigned a reach code, only mapping errors and changes to the hydrography will cause the reach code to change. As reach codes change, they are tracked in a special NHD cross-reference table.

The common identifier (ComID) is a 10-digit integer value that uniquely identifies the occurrence of each NHD feature (including reaches). Each value occurs only once throughout the Nation. Once assigned, the value is associated permanently with its feature. When features are deleted or split or merged, their ComIDs are retired. The common identifier is stored in a data element named "COM_ID". ComIDs are different between medium resolution and high resolution. Changes to common identifiers are not tracked.

Permanent Identifiers (PermIDs) store registry style strings consisting of 36 characters enclosed in curly brackets. These strings uniquely identify a feature or table row within a geodatabase and across geodatabases. PermIDs allow features to be  tracked in one-way and two-way geodatabase replication.

Can I link my own data to the NHD?

Yes, the best way to link data to the NHD is using the reach code. Reach codes are permanent, tracked, multi-scale, and form the basis of a controlled linear referencing system. The Hydrography Event Management (HEM) Tool provides full functionality for adding and editing events in the NHD. Events are informational data linked to the NHD using a linear referencing system on NHDFlowlines. The use of events is a key advantage of the NHD and allows vast amounts of scientific information to be linked to the NHD while keeping the design simple and by making advanced analysis techniques possible. The HEM tool handles linear referencing mechanics to make working with events easy. The HEM tool works on point, line, and area events and allows events to be located interactively, imported, or calculated. An ID value is created that provides the link between the event location and the informational data tied to the location. In addition, metadata creation and network measures are supported by the HEM tool.

Where can I find information about NHD attributes?

The NHD Data Dictionary provides details on all feature classes and attribute tables found in the NHD, and can be accessed at http://nhd.usgs.gov/NHDDataDictionary_model2.0.pdf. The NHD Model (v2.0) is available in poster form from  http://nhd.usgs.gov/NHDv2.0_poster_6_2_2010.pdf . The poster provides a diagram of the tables, the table items, the item definitions, and the relationships between the tables in the NHDinGEO data model.

Does the NHD contain data for Mexico and Canada?

The USGS along with their Mexico counterpart agency, Instituto Nacional de Estadística Geografía e Informática (INEGI), is currently working on a cross-border hydrography harmonization project for the U.S.-Mexican border. The harmonized datasets (28 HU8s) will eventually be available for public download along the international border. The US data is from the 1:24,000-scale National Hydrography Dataset (NHD), while the Mexico data is from their 1:50,000-scale hydrography dataset. USGS and USEPA are working with Natural Resources Canada (NRCan), Environmental Canada (EC), and Agriculture and Agri-Food Canada to harmonize trans-boundary hydrography data. The final product will be complete and hydrographically correct drainage areas and network of the U.S.-Canadian border.

I see an error in the data. What do I do?

If you find an error in the data, you may inform us by using the Report Data Errors Button on the NHD home page at http://nhd.usgs.gov/. You can also go to the form directly at http://webhosts.cr.usgs.gov/steward/st2maintenance.html

How do I download NHD Data?

NHD Data can be downloaded by using The National Map Viewer, the ftp site that contains Pre-staged Subregions, or an ftp site that contains NHD State Extracts. Fore more information on how to download NHD, see the NHD User Guide.

The National Hydrography Dataset (NHD) is stored in a geodatabase implementation of the NHD model. This provides great flexibility and efficiency to allow the data to work well in analysis using a geographic information system (GIS). To maximize the capability of the NHD, users should download the data in a file-based or personal geodatabase known as NHDinGEO. For those using the NHD to create simple maps, a shapefile version known as NHDinSHAPE also is available.

How do I use NHD data?

For directions on how to use NHD Data, go to http://nhd.usgs.gov/help/index.html?overview.htm . Open the folder called "Using NHD Data".

How are partners using the NHD?

There are many different applications of the NHD, including StreamStats, NHDPlus, WATERS, NRIS, Basins, and New England SPARROW. For more information on these NHD applications go to http://nhd.usgs.gov/applications.html

The USGS provides users with access to an assortment of analytical tools that are useful for water-resources planning and management, and for engineering design applications, such as the design of bridges. StreamStats allows users to easily obtain streamflow statistics, drainage-basin characteristics, and other information for user-selected sites on streams.

Between 1996 and 2000, the Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS), and other federal, state and local agencies collaborated to produce the National Hydrography Dataset (NHD), a comprehensive set of digital geospatial data about surface water features such as streams, rivers and lakes. These data can be used by water quality managers to make maps, perform upstream/downstream queries, and link other water-related information to the NHD network.

In 2006, this interagency collaboration produced NHDPlus, a suite of application-ready geospatial products that build upon, and extend, the capabilities of the medium-resolution NHD. NHDPlus integrates the NHD with the National Elevation Dataset (NED) and the Watershed Boundary Dataset (WBD). It includes an enhanced NHD stream network with improved names, value-added attributes (such as stream order), incremental drainage areas with landscape characteristics, and flow volume and velocity estimates for pollutant dilution modeling. EPA and USGS have linked many water quality databases to NHDPlus, including stream gaging stations, water quality monitoring sites, and impaired waters, enabling these databases to be queried and analyzed in upstream/downstream order. NHDPlus greatly enhances the ability of researchers and water quality managers to analyze and model water quality data. For more information on NHDPlus, visit www.epa.gov/waters. Click on the NHDPlus Quick Link on the right side of the web page.

Watershed Assessment, Tracking & Environmental ResultS (WATERS) is an integrated information system for the nation's surface waters. The EPA Office of Water manages numerous programs in support of the Agency's water quality efforts. Many of these programs collect and store water quality related data in databases. These databases are managed by the individual Water Programs and this separation often inhibits the integrated application of the data they contain. Under WATERS, the Water Program databases are connected to a larger framework. This framework is a digital network of surface water features, known as the National Hydrography Dataset (NHD) . By linking to the NHD, one Water Program database can reach another, and information can be shared across programs.

Originally introduced in 1996 with subsequent releases in 1998, 2001, and 2004, BASINS, or Better Assessment Science Integrating Point & Non-point Sources, is a multipurpose environmental analysis system designed for use by regional, state, and local agencies in performing watershed and water quality-based studies. This system makes it possible to quickly assess large amounts of point source and non-point source data in a format that is easy to use and understand. Installed on a personal computer, BASINS allows the user to assess water quality at selected stream sites or throughout an entire watershed. This invaluable tool integrates environmental data, analytical tools, and modeling programs to support development of cost-effective approaches to watershed management and environmental protection, including TMDLs.

The NRIS Water application is being modified to incorporate the NHD spatial model as the basis for geo-referencing data to streams and lakes. The 1:100,000 scale NHD is complete for the lower 48 states and will be flow validated by May of 2001. This represents a national wall to wall coverage of streams and their underlying spatial model (flow direction, level, etc.). NRIS Water V1.2 will be available for use with the 1:100,000 NHD data at this time.

This NHD-based version of the USGS SPARROW water-quality model aids the development of regional nutrient water-quality criteria and total maximum daily loads (TMDL) for New England streams.

Distribution


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What is the cost of the data?

NHD Data are available for free download. The data is public domain so it can be used for any purpose. USGS has no restrictions on use.

How can I determine which subbasins I need?

Use the EPA's "Surf Your Watershed" site or the NHD Viewer.

What data are currently available?

Medium and high resolution NHD data are available at 1:100,000 or 1:24,000 scale for the conterminous US, and Hawaii, and the Virgin Islands. NHD data are available for Puerto Rico at 1:20,000 scale. In addition, data for Alaska are being produced at 1:63,360 scale. Users are encouraged to produce and submit data at higher resolution.

Where can I download the data?

NHD Viewer, NHD Pre-Staged Subregions, State Extracts. These need to be hyperlinked.

NHD Tools


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What is the Hydrography Event Management Tool?

The Hydrography Event Management (HEM) Tool provides full functionality for adding and editing events in the NHD. Events are informational data that are linked to the NHD using a linear referencing system on NHDFlowlines. The use of events is a key characteristic of the NHD by allowing vast amounts of scientific information to be linked to the NHD while keeping the design simple and by making advanced analysis techniques possible. The HEM tool handles all the linear referencing mechanics to make working with events easy. It works on point, line, and area events and allows events to be located interactively, imported, or calculated. An I.D. value is created that provides the link between the event location and the informational data tied to the location. It also creates metadata linked to the event. The tool also provides network measuring to determine distances through the flowline network. The HEM tool was developed by the Department of the Interior - Bureau of Land Management

What is the NHD GeoConflation Tool

The GeoConflation Tool is one of several methods available to update the National Hydrography Dataset (NHD). The tool automates the NHD Create Process used to generate the high resolution (24K) NHD data from the medium resolution (100K) data. NHD conflation is the integration of two datasets into one. The basic goal of conflation is to conserve ReachCodes and ComID's by duplicating them from the original dataset to the modified or target dataset whenever possible. This allows the history of the ReachCodes, and by association any information tied to the ReachCode, to be preserved.

What is the USFS Reprojection Tool Version 2.0?

The USFS Reprojection Toolbar changes geographic projection and can transform datums using all available ESRI ArcObjects algorithms. It was developed by the U.S.D.A.-Forest Service for the National Hydrography Dataset (NHD) NHDinGEO format, but can function on any personal or file geodatabase. This is an add-on toolbar written for ESRI's ArcGIS 9.2 and 9.3. The advantage is that it reprojects whole personal and file geodatabases at once, and updates the metadata at both the Geodatabase, Dataset, and Feature class levels in both the Data Quality and Spatial Reference sections of standard FGDC metadata--or creates its own metadata if there is none. Version 2.0 works on ArcGIS 9.2 while version 2.1 works on ArcGIS 9.3.

What is the NHD GeoEdit tool?

The NHD GeoEdit tool is a specific partner, USFS and/or USGS, created toolset designed for NHD maintenance editing operations.

What is NHD Network Builder?

After reprojecting NHDinGEO data, it is necessary to re-build the network(s) to restore flow and directionality to your network data. Network_PGDB_S0_FL.exe is a tool that rebuilds a network and sets the flow direction within the context of the geometric network based on the flowdir attribute. The tool does not rebuild or update the NHDFlow table.

What is Geographic Names Information System?

The Geographic Names Information System (GNIS), developed by the U.S. Geological Survey in cooperation with the U.S. Board on Geographic Names, contains information about physical and cultural geographic features in the United States and associated areas, both current and historical (not including roads and highways). The database holds the Federally recognized name of each feature and defines the location of the feature by state, county, USGS topographic map, and geographic coordinates.

Other feature attributes include names or spellings other than the official name, feature designations, feature class, historical and descriptive information. The database assigns a unique feature identifier, a random number, that is a key for accessing, integrating, or reconciling GNIS data with other datasets. The GNIS is our Nation's official repository of domestic geographic feature names information.

Maintenance


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How is the NHD maintained and/or updated?

The NHD is a large and comprehensive database. Building it required the cooperation of many government agencies at the Federal, state, and local level. These agencies banded together into a partnership to share data and resources, making it possible to complete nationwide coverage. The U.S. Geological Survey played a key role in coordinating the partnership. Agencies such as the U.S. Environmental Protection Agency, the U.S. Department of Agriculture Forest Service, the U.S. Department of Interior's National Park Service and Bureau of Land Management, along with state agencies played key roles in developing and building the NHD. These partnerships will remain intact to continue to maintain the NHD.

The NHD has an in-house maintenance program that runs the data through a variety of quality control checks including:

  • Resolve line (1-D) topology errors
  • Resolve Gapped or Branched Reach Codes
  • Resolve Gapped GNIS Names
  • Verify all major GNIS Names exist in NHD
  • Verify Geometric Network connectivity
  • Resolve polygon (2-D) topology errors
  • Check all reaches to ensure that they have MValues
  • Check all WBAreaComID's and correct or add as necessary
  • Check all Coastlines (Coincident with 2-D features) and correct as necessary
  • Split 2-D stream/rivers at sub basin boundaries
  • Insure that offshore islands have coastline
How does the NHD account for changes in landscape?

Hydro-Image Integration is a process by which high resolution NHD data is visually compared to the most current imagery from the National Agriculture Imagery Program (NAIP). Hydro-Image Integration corrects major errors in the NHD data, such that 2-D streams are corrected if they are over 100 feet wide and deviate from the imagery by at least 1,000 feet. Lakes are corrected if they are over a square kilometer, and the shoreline deviates from the imagery by 500 feet or more. New lakes are added to the NHD if they are over 500,000 square meters in size.

What is NHD Stewardship?

Just as building the NHD required a large partnership across the nation, maintaining the NHD also requires an extensive partnership, and can best be accomplished by those closest to the hydrography. Users within the states and federal lands understand the hydrography around them and are motivated to ensure the accuracy of the NHD to meet their business needs; therefore, they are ideally suited to become the stewards of the data; an agency in each state will manage the maintenance activities within the state. The maintenance will be performed by that agency or other agencies in the state. The United States Geological Survey (USGS) will facilitate the overall process, providing national management, coordination, tools, standards, documentation, training, quality assurance, data archival, and data distribution. Updates to the NHD will be made by the stewards, transmitted to the USGS, processed, and made available in the national dataset distribution. For more information, go to the Stewardship website at http://webhosts.cr.usgs.gov/steward/ .

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Last modified: Thursday, 14-Feb-2013 18:11:08 EST