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A choropleth map is a map which colors regions or areas according to the value of data defined for that area. The choropleth maps presented here color states, or counties, according to aggregations of TRI data reported by facilities in that area. Coloring an area with one color is not meant to imply that the TRI releases are uniformly distributed throughout that area, it should be interpreted as a color representing, for example, the total releases reported by facilities in that area. Choropleth maps are one of the most commonly used means of displaying areal data.

This software was designed as a dynamic data visualization tool that allows the user to dynamically interact with the choropleth map by using the slider control, changing data views, and zooming map scales. The map is updated in real-time in response to user interaction.

Interacting with the Map

Interacting with the Map

System Requirements

You will need at least 800x600 resolution to see the full map and slider control. Go to control panel to set resolution if needed. The applet is compliant with Java virtual machine 1.1 or later. If your browser does not have the Java virtual machine you can download it from Get Java Virtual MachineExit EPA Disclaimer. If java is not working check that Java is enabled in your browser:

  1. Open the Preferences window using Edit -> Preferences.
  2. Click on Advanced.
  3. Check that a check appears next to "Enable java".

Changing Map View

The U.S. map can be viewed with State or County detail. In state detail view the data are aggregated to the state level by summing the county data. The menu tabs are used to select map view. Click the county or state detail tabs to view the U.S. map at county or state data aggregation detail.

Interacting with the map using the slider control

The counties are grouped by color according to the slider control settings. Initially the cyan color shows the lower 33% of counties in terms of county data values, the yellow shows the middle 34%, and the red show the top 33%. These groupings can be changed by moving or resizing the slider. The map will be updated in real-time.

The slider can be moved as a scroll bar by dragging it or it can be resized. To resize the slider place the mouse over one of the ends and drag it to the new position. The mouse pointer will change to a splitter when the mouse is placed over a slider end.

The slider is used to define the percentile ranges for map coloring. Move the slider or the slider end-points to alter ranges. The slider end-points set the percentages of the distribution, and the percentiles are automatically calculated.

The slider has four percentile values above it and three percentages below it. Initially, the percentiles are (from left to right): The minimum county value (the 0th percentile); the 33rd percentile (the data value below which are 33% of counties), the 67th percentile (the data value below which are 67% of counties), and the maximum county value (the 100th percentile).

The slider can be moved like a scroll bar by clicking on the yellow section, holding down the mouse button, and dragging left or right. This will keep the yellow section the same size and resize the blue and red sections. In addition, you can resize the yellow section by placing the mouse over one end, holding down the mouse button, and dragging the dividing point left or right.

Move the mouse over the leftmost or rightmost number to see the associated map area, which will turn white.

The extreme values of data can be shown by moving the lower end-point of the red band to the right, or by moving the upper end-point of the cyan band to the left. For example, to isolate the 10% of counties with the highest data value move the lower end-point of the red band to the right until the percentage is 10%.

Drilling down in the map

Move the mouse over a county (or state) to identify it, then right-click the mouse to bring up a popup menu of web tools into which you can drill down for more information specific to that area. Currently reports are available from TRI Explorer, Enviromapper, Census, and FedStats.


From the state or county view of the U.S., click a state to zoom in to that state. Zoom out by clicking tha state map, or by selecting a detail tab.

For help with these pages, contact the EPA TRI Explorer.

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