Finding geospatial data for your area becomes (almost) seamless with help from the USGS.
Need digital map data for some region of the United States, but don't know your FIPS codes from your NADs? Help is at hand in the USGS Seamless Data Distribution System, or SDDS. The SDDS is a web-based, DHTML interface that allows you to zoom in on a map of the U.S., select an area visually, and download the data layers of your choice, corresponding to your visual selection. This sure beats the traditional method of fetching GIS data from random FTP servers by USGS topo quadrangle name or, worse yet, some obscurely coded filename!
Start by firing up http://seamless.usgs.gov/ in your web browser. At the time of writing, the service is still in beta, and new data sources are being added regularly. Follow the links to the SDDS service.
At a minimum, you should be able to get elevation models from the Shuttle Radar Topography Mission (SRTM) data sets, at 30 m or 90 m resolution, as well as feature data including road and hydrography (i.e., water) features. A full list of what's available now is available on the right of the display, under the Download tab. Presently, it's possible to view a lot more on the display than is actually available for download.
When the SDDS browser loads, you're presented with a map of the whole U.S., zoomed all the way out. The navigation options on the top left let you zoom in and out, and also let you select an area to zoom in on: click and hold, then drag the mouse to show the area you've outlined expanded in red or green.
Figure 6-25 shows a sample region selection in the SDDS, outlined in green. Whether the outline is red or green depends on whether it's possible to download data for that area directly from the SDDS, or whether you'll have to order hardcopy media. This in turn depends on how much data you want. If you select the 30 m SRTM data over a large area, which has high data density, you won't be able to download the whole set through Seamless. The SDDS also lets you zoom in directly to a state or a major city. Some local areas may have extra data available; for example, Sacramento, California, has a separate set of local orthophotos in addition to the U.S.-wide imagery.
Figure 6-25. Selecting a region to download via the SDDS
Once you've zoomed in to the extent you want to download data for, click the "Select Extent" icon on the lower left. Select a region as described earlier, and a Request Summary page will open, showing you the data sets available for that region, including data format and file size. Each data set is delivered as a ZIP file with an eight-digit filename, which contains all the relevant data and metadata. Download and unzip this file, and you should be able to load up the data set in a GIS browser like QGIS or GRASS right away.
The SDDS toolbar has a host of other nifty features. You can enter a lat/long pair in decimal format to zoom directly to that location. Or you can click on the map to get a lat/long coordinate for that point; unfortunately, this comes in degrees, minutes, and seconds! You can also measure distances between points and along a line that you describe by adding points. You can see elevation data for a point by clicking on the map, or you can create an elevation profile, a 2-D plot of the ground topography between the points, useful for line-of-sight evaluation for radio transmission.
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