Custom AirNow Maps - Defining Your Map Area

If you want to change from the default display of the lower 48 to a specific location, you can specify that in the URL using values for the west, south, east, and north edges of the map you want. The AirNow map (provided by ArcGIS) uses Web Mercator values (xmin, ymin, xmax, ymax). These values can be obtained using a tool from ArcGIS available at: http://help.arcgis.com/en/webapps/flexviewer/extenthelper/flexviewer_extenthelper.html.

(Please note that this tool requires Flash. If you do not wish to use this particular tool, there are other tools on the web to help you determine Web Mercator values.)

Once you open this tool, zoom in to the area you wish your map to cover.

In the upper left of the window, you will see something like the following:
Fig: ARCGis FlexViewer for Deriving Web Mercator Values for Liberty Island

(Your map may look slightly different depending on many factors, like screen width, resolution, browser, etc.)

While this tool does not explicitly define which value is which, they are always in the order shown below:

  • The first value, -8243520, is the xmin value (the West/Left edge of your map)
  • The second value, 4966309, is the ymin value (the South/Bottom edge of your map)
  • The third value, -8241890, is the xmax value (the East/Right edge of your map)
  • The fourth value, 4967064, is the ymax value (the North/Top edge of your map)

At this point, you can place these values into the following link:

https://gispub.epa.gov/airnow/index.html?xmin=-8243520&ymin=4966278&xmax=-8241890&ymax=4967020

Custom AirNow Default Map
Fig: AirNow Map Image Showing Liberty Island Centered After Using Web Mercator Values

(Please note that the minus signs (-) MUST be preserved.)