Measuring Light Pollution

To get a night-time image of an observatory test site, go to the web page of the Earth Observation Group of the Defense Meteorological Satellite Program at the National Geophysical Data Center.

There are a number of useful and interesting data sets available on this site that can be accessed using the menu bar on the left.

To get a map suitable for measuring the light intensity at a test site, select “Online Maps and Web Services.” In the list of maps that appears, select “Interactive Map” on the top line: “DMSP OLS Global Composites Version 2.” The map that appears is a composite global map of all fixed man-made lights like street and building lights. (If you want to see all the lights, including aurora, fishing boats, oil flares, etc., select the Interactive Map by “Nighttime Lights of the World.”)

Use the Hand and Magnifying Glass icons at lower left to center your selected test site and zoom in five or six levels. You may need to move the map using the hand icon to keep you site centered. It is also useful to keep some recognizable geographic feature - like a national or state boundary – in the image for scaling (see below).

Take a screen capture of the map and open it in ImageJ (You may use any image analysis program with similar functionality, but the directions below are given for ImageJ).

Now:

  1. Convert the image to 8-bit gray scale. Click on the image and then select Image/Type/8-bit. You can make useful measurements in the resulting gray scale image.
  2. Mark the location of the selected site using the pencil tool. Save the image with an appropriate name. Determine the X, Y coordinates of the site using the cross-hair of the pencil tool. You may need to use the zoom tool.

  3. Set the scale of the image to kilometers. Using a good map of your area, find the distance in kilometers between any two points in your image. Use the Straight Lines Selection tool and draw a line between the two points. Then select Analyze/Set Scale. A dialog box will appear with the length in pixels of the line you drew on the image in the top box. Fill in the Known Distance in kilometers, change the Unit Length to km, and click OK. Now any distances or areas you measure in the figure will be in km or square km.

  4. Enter the elevation of the selected site in the Horizon/Distance figure (previous page) and determine the distance to the horizon from the site. For example, the elevation of Kitt Peak is 2120 meters, which corresponds to a Horizon Distance of about 165 km.

  5. Select the Elliptical Selection tool and draw a circle with a diameter of twice the Horizon Distance. The values by the “w” and “h” in the tool bar give, respectively, the width and height in kilometers of the ellipse you are drawing. Adjusting the “w” and “h” until they are equal gives a circle. Click inside the circle and drag it until it is centered on the observatory site.

  6. Select Analyze/Set Measurements, and select Area and Integrated Density (and anything else you might want to measure) by clicking in the box next to them. Click OK.

  7. Select Analyze/Measure and pop-up box will appear with your measurements. The Area is the area of your circle in square kilometers and Integrated Density is the sum of all the pixel values in the circle. Since dark areas have value zero, the sum is a measure of all the light in the circle.

  8. Save these measurements to compare with the other observatory sites. Make sure you have set the scale in each image to kilometers so that your measurements will be in the same units for each site.

ImageJ is a versatile image processing software program that can be downloaded free from:

http://rsb.info.nih.gov/ij/

Click on Download and select the version appropriate for your computer.