ESRI Schools and Libraries

Using BASINS

The U.S. Environmental Protection Agency (EPA) has collected digital data for every watershed in the conterminous United States, about geographic features that influence our everyday lives: elevation, streams, main roads, dams, land use patterns, toxic release sites, and so on. (See more about "hydrologic units" at this U.S. Geological Survey [USGS] Web site: http://water.usgs.gov/GIS/huc.html.) For every watershed in the lower 48 states, there are many such layers of data available for free download at the EPA BASINS Web site. (EPA continually upgrades this site, so links may change.)

BASINS stands for Better Assessment Science Integrating Point and Nonpoint Sources. The whole Web site provides a large volume of data organized by watershed and brought together with a set of computer modeling tools, allowing people to investigate potential pollution sources and their areas of impact. Using the modeling tools effectively requires a firm grasp of some powerful science concepts and principles. However, some of the digital data collected and made freely available is appropriate for many different activities, even for young students.

The instructions on this page can help you access and use the data. (You might find it handy to print these instructions.) There are three major processes to go through:

Accessing the BASINS Data
Preparing the BASINS Data
Using the BASINS Data

I. Accessing the BASINS Data

Enter the BASINS site.
Go to the EPA BASINS front page: www.epa.gov/ostwater/basins. You'll see a front page with lots of information and important links to explore. When you're ready to download data, click the Web Download button.

Choose BASINS 3 Data.
The second page has good information and links to explore. When you're ready to download data, go to the BASINS 3 Data from the Web section. Click the link in line 3: http://www.epa.gov/ost/ftp/basins/gis_data/huc.

Click into your Watershed.
It helps to know in advance the name or eight-digit hydrologic unit code (HUC) number for your watershed. You can check this out in advance using the watersheds data from the ESRI GIS for Schools and Libraries CD (version 5 or higher) or using the EPA Surf Your Watershed Web site: www.epa.gov/surf. Compare that information with the information atop the BASINS page. If you need to have a slightly different watershed, you can change the watershed number manually in the address box of your Web browser.

Download the desired elements.
One by one, click on the file(s) you want to download:
- DEMG = elevation data in grid format, for use with ArcView for PC plus either Spatial Analyst or 3D Analyst extensions
- core = local features: roads, dams, toxic release sites, gauging stations, and so forth
- dem = elevation data in shapefile format
- pc3 = permit compliance system sites with water quality parameters
The DEMG data is in grid format, so make sure your GIS software can display it before downloading. The core data is quite large (typically 5-25 MB) and takes a long time to download. The pc3 data requires deep knowledge of water monitoring. But the dem data sets are very useful even for GIS novices and are pretty small files (typically 0.5-3 MB). Your browser will ask you what you want to do with each file. Choose to save it in a place where you can find it.

Set the target storage site and name.
Your browser will likely ask you what you want to name the file as you are saving it and where you want to store it. Note carefully where you want to store it such as C:\BASINS\DOWNLOAD. Navigate to the desired drive and directory, and provide a sensible name with an .exe file extension. You might want to name it according to the watershed and data such as 07010001_dem.exe or 14121001_dem.exe. (HINT: Either save the files into a directory called DEM or CORE or rename the files you are saving, or both. Be careful with this; if you download or unzip two files with the same name into the same place, you will replace the first with the second.) Begin the saving process, and wait for the download to be completed.

Choose more data or exit.
Once the file is downloaded, you can use your browser to go back to previous screens, even to choose a new watershed, and repeat the process. Once all your data is downloaded, you have completed the stage for which the browser is necessary.

II. Preparing the BASINS Data

Downloading the data files is just the first step in using BASINS data. Next, you need to decompress the files, putting the results in a place where you can find them.

Self-check: decompression software?
The BASINS data is prepared as self-extracting archive files.
- On PCs running Windows, no special software is needed; the files will decompress themselves.
- On Macintoshes, you will need a decompression utility that handles standard PC ZIP files. A common choice is Expander by Aladdin Systems: http://www.aladdinsys.com.
Navigate to the downloaded EXE file.
Using your Windows Explorer or Finder, navigate through your drives and directories to where you stored the EXE file.

Double-click the downloaded EXE file.
- On PCs running Windows, the file will begin the self-decompression process.
- On Macintoshes, if your computer asks what you want to do with the file, choose to open the file using the decompression software.
Extract to the desired folder.
Indicate that you want to extract the files to a particular location. Keep track of where the files are being deposited. You may want to take this time to create and name a special new directory for storage such as C:\BASINS\EXTRACTED\01010002\DEM and C:\BASINS\EXTRACTED\01010003\CORE. Again, you need to be careful here. Some files for a single watershed can be named the same, so you must either store them in separate directories (easiest) or rename each set of extracted files.

Look at the extracted files.
Navigate to the site holding your newly extracted files. For each watershed, you will find (watershednumber).shp, (watershednumber).dbf, and (watershednumber).shx.

III. Using the BASINS Data

Engage ArcView software.
Begin ArcView or ArcVoyager. Since ArcVoyager is an ArcView application, the descriptions below will apply to both. To use this data in ArcVoyager, you may choose any of the Designing Global Adventures: Point Me projects or Creating New Worlds: Turn Me Loose project; you cannot access the BASINS data within Viewing World Snapshots: Show Me. (You may also use BASINS data within ArcExplorer, which requires a slightly different set of procedures that will not be covered here.)

Open the desired view.
In ArcView or ArcVoyager, either create a new view or work with a preexisting view. If you are using a preexisting view, notice if the view is in decimal degrees or is projected. This will help you anticipate what your new data should look like.

Add the desired theme.
Add themes in the standard fashion, navigating to where you have stored the data and making sure that you are seeing Feature Data Source files (rather than Image Data Source files).

Zoom to the new theme and turn it on.
Make your new theme(s) active and click the Zoom to Active Theme button. This will bring you to the scale of the watershed(s) added. Click the check box to the left of the theme name to turn it on.

Speed up draw times for elevation.
Because there are more than 1,000 records in the DEM layers, it helps to speed their draw times by creating a Spatial Index. (NOTE: This option is not available using ArcVoyager Special Edition.)
1. Make the desired DEM theme active.
2. Open the theme table.
3. Click the SHAPE field at the far left, darkening the header.
4. From the FIELD menu, choose Create Index. If this choice is greyed out and Remove Index is listed, the index already exists so just exit this menu.

Classify and symbolize the elevation data.
Use standard procedures for choosing the symbolization style for each feature. If you are using more than one watershed, be sure to make the legends consistent, spanning the entire range from the lowest elevation in the set of watersheds to the highest.
1. Find the full range of elevations represented.
2. Open the Legend Editor for one DEM theme.
3. Choose the Graduated Color map.
4. Choose the Classification Field to be elev_ft.
5. Click the Classify button and choose Equal Interval, 64 classes, rounded to just d. Click OK.
6. If the numbers are not quite right, key in new numbers for both Values and Legends. (As a default, Legend will use the same numbers engaged in Values, so you can use the TAB key to speed through the keystroking sequence.)
7. Set a deep green through bright yellow to brick red color scheme.
  1. First, set the lowest elevation as deep green.
  2. Next, set the highest elevation as brick red.
  3. Then, set the middle color as bright yellow, and leave this elevation zone selected (with the black bar showing).
  4. Finally, click the ramp colors button (farthest right button, just above the preconstructed color ramp choices in the Legend Editor window). The colors will ramp from one end to the selected color, and from the selected color to the other end.
8. Last, set the borders for each color symbol as none.
  1. Double-click the top color box to bring up the Symbol Window. Move it off to the side so you can see both windows.
  2. Click once on the symbol for the lowest elevation, selecting that row.
  3. Scroll all the way down to the highest elevation. Hold down the SHIFT key. Click once on the color symbol for the highest elevation. Let go of the SHIFT key. If you have done this properly, all rows will be selected (black).
  4. Set the Symbol Window to Fill Palette. At the bottom of the window, set Outline to None. Close the Symbol Window.
9. Apply your changes.
10. Save your new legend. At the top right of the Legend Editor window, click Save. Navigate to where you want to store the legend and save the file.
11. For other elevation themes, simply LOAD and apply the newly saved legend.

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