Transportation network data are valuable for all sorts of uses, including two we considered in Chapter 4: geocoding and routing. The Federal Geographic Data Committee (1997, p.19) specified the following vector features and attributes for the transportation framework theme:
|Roads||Centerlines, feature identification code (using linear referencing systems where available), functional class, name (including route numbers), and street address ranges|
|Trails||Centerlines, feature identification code (using linear referencing systems where available), name, and type|
|Railroads||Centerlines, feature identification code (using linear referencing systems where available), and type|
|Waterways||Centerlines, feature identification code (using linear referencing systems where available), and name|
|Airports and ports||Feature identification code and name|
|Bridges and tunnels||Feature identification code and name|
As part of the National Map initiative, USGS and partners are developing a comprehensive national database of vector transportation data. The transportation theme "includes best available data from Federal partners such as the Census Bureau and the Department of Transportation, State and local agencies" (USGS, 2007).
As envisioned by FGDC, centerlines are used to represent transportation routes. Like the lines painted down the middle of two-way streets, centerlines are 1-dimensional vector features that approximate the locations of roads, railroads, and navigable waterways. In this sense, road centerlines are analogous to the flowpaths encoded in the National Hydrologic Dataset (see previous page). Also like the NHD (and TIGER), road topology must be encoded to facilitate analysis of transportation networks.
To get a sense of the complexity of the features and attributes that comprise the transportation theme, see the Transportation Data Model (This is a 36" x 48" poster in a 5.2 Mb PDF file.) [The link to the Transportation Data Model poster recently became disconnected. Instead look at the model diagrams in the Part 7: Transportation Base of the FGDC Geographic Framework Data Content Standard.]
In the U.S. at least, the best road centerline data is that produced by NAVTEQ and Tele Atlas, which license transportation data to routing sites like Google Maps and MapQuest, and to manufacturers of in-car GPS navigation systems. Because these data are proprietary, however, USGS must look elsewhere for data that can be made available for public use. TIGER/Line data produced by the Census Bureau will likely play an important role after the TIGER/MAF Modernization project is complete (see Chapter 4).
View and download National Map transportation data
- Access the Viewer here.
- Expand the pane containing the layer options by clicking on Overlays at the upper-left.
- Under Base Data Layers, check the box for the Transportation layer. You can expand the Transportation list and sub-select different layers.
- As you zoom in to larger map scales (using the slider bar at the upper-left of the map), additional transportation layers will become visible.
- If you wish to download an extract from the transportation database, visit the National Map Download site.