GEOG 160
Mapping Our Changing World

16. Theme: Hydrography

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The NSDI Framework Introduction and Reference (FGDC, 1997) envisions the hydrography theme in this way:

Framework hydrography data include surface water features such as lakes and ponds, streams and rivers, canals, oceans, and shorelines. Each of these features has the attributes of a name and feature identification code. Centerlines and polygons encode the positions of these features. For feature identification codes, many federal and state agencies use the Reach schedule developed by the U.S. Environmental Protection Agency (EPA).

 

Many hydrography data users need complete information about connectivity of the hydrography network and the direction in which the water flows encoded in the data. To meet these needs, additional elements representing flows of water and connections between features may be included in framework data (p. 20).

Identification

FGDC had the National Hydrography Dataset (NHD) in mind when they wrote this description. NHD combines the vector features of Digital Line Graph (DLG) hydrography with the EPA’s Reach files. Reaches are segments of surface water that share similar hydrologic characteristics. Reaches are of three types: transport, coastline, and waterbody. DLG lines features represent the transport and coastline types; polygon features are used to represent waterbodies. Every reach segment in the NHD is assigned a unique reach code, along with a host of other hydrological attributes including stream flow direction (which is encoded in the digitizing order of nodes that make up each segment), network connectivity, and feature names, among others. Because the order of reach codes are sequential from reach to reach, point-source data (such as a pollutant spill) can be geocoded to the affected reach. Used in this way, reaches comprise a linear referencing system comparable to postal addresses along streets  (USGS, 2002).

Diagram showing how water flow is attributed to reaches

 How flow attributes are associated with reaches in the National Hydrographic Dataset (USGS, 2000).

 NHD parses the U.S. surface drainage network into four hierarchical categories of units: 21 Regions, 222 Subregions, 352 Accounting units, and 2150 Cataloging units (also called Watersheds). Features can exist at multiple levels of the hierarchy, though they might not be represented in the same way. For example, while it might make the most sense to represent a given stream as a polygon features at the Watershed level, it may be more aptly represented as a line feature at the Region or Subregion level. NHD supports this by allowing multiple features to share the same reach codes. Another distinctive feature of NHD is artificial flowlines--centerline features that represent paths of water flow through polygon features such as standing water bodies. NHD is complex because it is designed to support sophisticated hydrologic modeling tasks, including point-source pollution modeling, flood potential, bridge construction, among others (Ralston, 2004).

Diagram illustrating how hydrographic features are represented with points, lines, and polygons

How vector features are used to represent various types of reaches in the National Hydrographic Dataset (USGS, 2000).

 NHD are available at three levels of detail (scale): medium (1:100,000, which is available for the entire U.S.), high (1:24,000, production of which is underway, “according to the availability of matching resources from NHD partners” (USGS, 2002, p. 2), and local, which "is being developed where partners and data exist" (USGS, 2006c).

Spatial Reference Information

NHD coordinates are decimal degrees referenced to the NAD 83 horizontal datum.

Distribution

Try This!

Download and view an extract from the National Hydrographic Dataset

  1. From the NHD home page (http://nhd.usgs.gov/) click the Get NHD Data link and then follow the link to the NHD Viewer. (There is a Help button in the upper right.)
  2. Use the GIS Tools (Standard tab) to zoom in on an area of interest. Then, click the Download Data button. Choose a reference area from the pick list, then click on the map. A link to the available data sets will appear in the left hand pane under the Selection tab. (The All Results button will list the multiple areas you click on.) Follow the Download link of the area you are interested in.
  3. In the USGS Available Data window that opens, select Hydrography from the Theme column, and choose a file format from the pick list in the Format column. I chose Shapefile format for my extract, because I know it is compatible with Global Mapper / dlgv32 Pro. If you were working in ArcGIS you could choose the File Geodatabase option. Ralston (2004, p. 187) observes that NHD "is precisely the type of information that could benefit from an integrated data model in an object relational database."
    Click the Next button.
  4. From the list of Hydrography Products check the box for what you wish to download. To be assured of getting the data in Shapefile format select a product referred to as a Dynamic Extract.
    Click the Next button.
    Go the the Cart pane on the right will open. (It may open automatically.)
    Go to Checkout, supply your e-mail address, and submit your order.
    You will quickly receive an e-mail telling you that your order has been placed. About an hour and a half after I submitted my request I received a second e-mail containing a download link.
  5. Extract the contents of the .zip file and view the Shapefile data set(s) in Global Mapper.
  6. Use the Identify pointer tool to reveal attributes of the reaches. In the example below I have highlighted a flowline associated with Cedar Creek in western Michigan.