Geographic Information Systems (GIS) Services Frequently Asked Questions:
How do I download data from the USGS Seamless Data Distribution Center?

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The USGS Seamless Data Distribution Website is an excellent source for spatial data.
A few highlights of the data available from the site:

• National Elevation Dataset (NED): seamless, national digital elevation data with 30m spatial resolution
• 1/3 and 1/9 NED available for some areas (~10m and ~3.3m)
• Elevation data updated bi-monthly
• High-resolution Orthoimagery for many cities, including 2002 imagery for Chapel Hill, Raleigh, & Durham
• 1992 and 2001 National Land Cover Data
• DRGs (Digital Raster Graphics), Scanned USGS topo maps- listed under "Orthoimagery"

Step-by-step Instructions
To download the data:
Go to:
http://seamless.usgs.gov/
If your browser has a pop-up blocker, make sure to set it to allow pop-ups for this site.
Click on "View and Download United States Data", in the upper right-hand corner of the page.
When the viewer opens, use the zoom/pan tools on the left side of the viewer to zoom in to the area of interest in the map display. This is important because different data products are available for different areas.
Select the layers you want to download. Click on the "Download" tab on the table of contents to the right of the map viewer.
Check the layers you are interested in downloading. Make sure there are no other layers checked. The National Elevation Dataset (NED) is set as the default download layer.
After selecting the desired data, define the download area...
There are two ways to define the area for which data will be downloaded. One is to use the define download area tool, the other is to define the area by coordinates. Both of these methods are explained below.
Using the Define Download Area tool
On the left side of the page, use the zoom in tool to zoom to the area of interest. Then find the Define Download Area tool. Use this tool to draw a box around the area of interest. As long as the box stays green, the data can be downloaded for free. If the rectangle turns red, greater than 100MB of data have been selected. The data will have to be paid for or downloaded in pieces and mosaiced together.
After the download area has been selected, a new window will appear. An interim "extracting data" message may show. Once the data has been extracted, a download button will appear. Click that button to download the data, and save it to your local machine.
Unzip the file. Use the extract function in WinZip to get all of the necessary files.
Using the Define Area By Coordinates Option
If the exact coordinates of the area of interest are known, the "Define Area By Coordinates" option can be used to download data. The link for this option is under the Downloads section on the left side of the page. To use this option, the bounding coordinates of the area of interest must be determined beforehand. This can be done using ArcMap. Simply look up the bounding coordinates for a shapefile of the study area under the shapefile's properties. The shapefile must be in a Geographic Coordinate System- ArcMap will provide the coordinates in decimal degrees.
Coordinates can be entered into the Seamless interface in either decimal degrees or degrees, minutes, and seconds. To switch between the two, just use the link at the bottom of the page that says "Switch to.."
Enter the bounding coordinates.
Click "select area"
At the Request Summary Page, Click on "Download"
Save the file to disc.
Unzip the data. Use the extract function in WinZip to get all of the necessary files.
Up to 100MB of data can be downloaded for free.

If you need help, please ask at the Davis Library Reference Desk.

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For more GIS assistance,
Please contact Amanda C. Henley, GIS Librarian
amanda.henley@unc.edu
(919) 962-1151

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