It's a combination of USGS hydrologic digital line graph files and EPA reach files, version 3.0 (RF3). The USGS files are used for spatial accuracy and the EPA files are used for attribute information.

The idea is to combine spatial accuracy with detailed features, attributes, and values. Information such as flow paths, permanent reach IDs, and hydrologic ordering can now be used in modeling. The NHD is not designed to replace or supercede existing datasets, but offer users another tool with more flexibility.

Medium resolution NHD data are available at 1:100,000 scale for the conterminous US and Hawaii. High resolution NHD data are available for Puerto Rico at 1:20,000 scale and the Virgin Islands at 1:24,000 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. Whether the data is medium resolution or high resolution can be determined from the NHD.met file. Medium resolution data has a title 'National Hydrography Dataset' while high resolution NHD has a title 'National Hydrography Dataset - High resolution'. The scale of the source material used to create the NHD can be found in the source citation abbreviation listing in the subbasin level metadata.

Data are offered in both compressed ARC workspaces and compressed SDTS transfer format.

Decimal degrees.

North American Datum of 1983.

Compressed ARC coverages average 0.80 MB; uncompressed ARC coverages average 3.34 MB. Compressed SDTS files average 0.19 MB; uncompressed SDTS files average 1.86 MB.

It will depend upon the production date of the initial linework and whether this linework was updated when the DLG files were created by USGS. Therefore the vintage of the linework can vary from the 1950s to the mid-1980s.

In the process of building the NHD from RF3 and DLG3 (Digital Line Graph Version 3), the rules for delineating reaches were changed in some significant ways. For example, RF3 has reaches on the shorelines of wide rivers and lakes, while NHD has reaches on artificial paths that run through wide rivers and lakes. Another example is, RF3 did not have waterbody reaches and NHD does. Consequently the spatial representation of an RF3 reach will not equal the spatial representation of an NHD reach in some cases. However, a large number of RF3 reaches are 1-1 with NHD reaches. For example, almost all of the single line stream confluence-to-confluence reaches in RF3 have the same spatial extent in NHD. (The coordinates in NHD are more accurate/precise than those in RF3 and you will generally see a slight shift in the line work between RF3 and NHD.)

It is important to know that even the reaches that are 1-1 between RF3 and NHD will have new reach codes. The reach code in RF3 was CU+Segment+MarkerIndex (17 characters) and in NHD it's CU+Segment (14 characters). Every reach received a new code when NHD was built, however an RF3-to-NHD crosswalk (also known as the RF3-to-NHD Cross Reference Archive) was also created. Although the cross walk is not perfect, it works very well for the 1-1 reaches and for small to medium-sized lakes, and less well on wide rivers and large lakes.

We say that NHD is a 'framework' for modeling. This framework consists of two basic components. First, there are permanent features (reaches) with permanent, public identifiers (reach code) that support the linear referencing (reach route system) of attributes to the stream network. Second, the reaches have flow relationships (RFlow table) that enable you to programmatically walk up and down the network encountering the information that you have linked to the network through linear referencing.

To perform meaningful modeling, you'll have to link additional information on the NHD network. For example, if you wanted to perform impact analysis, you might link pollution events to the network (through linear referencing) and then navigate downstream (with the flow table) to see what streams would be affected by the events. If you want to do dilution modeling, you'll need lots of additional information such as flow volume, velocity, travel time, etc. for NHD reaches. So the NHD provides the framework, but the modeler needs to acquire and link to the NHD any attributes needed for a particular type of modeling.

The NHD ArcView Toolkit provides some of the basic functions that you would need to get into the modeling business. For example, the NHD Reach Indexing Tool provides tools to perform linear referencing of attributes to the stream network and NHD Navigate provides tools to walk upstream and downstream on the network.

On the NHD ftp site, along with all the NHD workspaces, a status file is provided to assist users in tracking changes in the NHD. The file is in text format and called 0000_CU_STATUS_LIST_FILE.txt. A sample of the file content is shown in the following image.

When an NHD workspace is updated, the status file entry for that workspace is updated. Each entry in the file provides the following information:

CU Number: The eight digit HUC number for the NHD subbasin (formerly known as cataloging unit) workspace.

Distribution Status: Generally, this column will say 'distributable'. Occasionally, when an update process experiences problems, this column will say 'not distributable' which means that updates are pending and a new workspace is not yet available.

Date Updated: The date that the updates were processed and stored in the NHD central database.

Date Distributed: The date that the workspace was distributed to the NHD ftp site.

Process Description: The first 60 characters of the most recent metadata process description. This describes what updates were made to the workspace.

By comparing the 'Date Distributed' with the date of the workspace held by the user, it is possible for a user to determine when an updated workspace is available. If the user uses an ftp client to download NHD workspaces and if that ftp client preserves the workspace file date, the user can use the workspace file date for comparison. If the user downloads NHD workspaces with a method that does not preserve the file date, then the user should use the file date of the OPENME.TXT file inside the compressed NHD workspace for the comparison.

No, the NHD does not contain stream order. It does contain the attribute 'stream level', which also categorizes streams but using a different approach. An explanation of stream level can be found on the NHD website at:

Yes.

The relates that are distributed with the NHDinARC data were developed for application in ArcInfo Workstation. Unfortunately these relates are not applicable in ArcMap. ArcMap, however, does provide capabilities to link to related tables. For the labeling application that you mention below, probably the best bet in ArcMap would be to use a table join. Load the route.drain subclass into your ArcMap session and right click on the layer. Select Join... In the form that appears, enter RCH_COM_ID in #1, route.rch in #2, and COM_ID in #3. This join will provide access to the route.rch attributes from the route.drain layer. If you choose to label the drains, choose the route.rch:NAME attribute for the label. Although this labeling application is just one example, the NHD data contains multiple identifier cross references to link associated tables.

The NHD data are not projected. It is in geographic coordinates (unprojected lat/long), decimal degrees, NAD83. You should be able to project the data just fine if you specify lat/long or geographics as the input projection.

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 the geocoding or linking of observations, such as a water quality sampling sites, to reaches. They form the basis for 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 "RCH_CODE". 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 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, the values for their identifiers are retired. The common identifier is stored in a data element named "COM_ID". Com_ids are different between medium resolution and high resolution. Changes to common identifiers are not tracked.

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 NHD Toolkit, specifically the NHD Reach Indexing Tool (RIT) component, provides an interactive environment for creating point, line, and area events linked to NHD reaches.

The United States is divided into 2,267 subbasins which vary in size according to the drainage area. There are 2,120 subbasins identified for the conterminous US and Hawaii. Of these, 10 are "empty" subbasins (mainly water) and there is no NHD data. There are 136 subbasins in Alaska, 6 subbasins in Puerto Rico, 2 subbasins in the Virgin Islands. This data will be available as high-resolution NHD data. There are 3 subbasins in the Caribbean Outlying Area that will not be included in the NHD.

Data are available for free download.