Vegetation Measurements
One way to identify different surface covers has been to compare the intensity of the near infrared to the visible light being reflected from the Earth’s surface. An early technique was to subtract the visible red from the infrared intensity. Since leaves of plants reflect more IR than visible light, the difference between IR and red is an indication of vegetation cover—a sort of “Difference Vegetation Index.”
The Difference Vegetation Index works well for ground without steep slopes, but for steep surfaces, there is a shadow effect: some areas reflect different intensities of light just because of their slope. But the percent of light reflected is the same, regardless of the intensity of light, so one way to eliminate the shadow/slope problem is express the Difference Vegetation Index as percent by dividing the difference between IR and red intensities by the total light being reflected. This technique is called normalization, and the scheme, which is commonly used to identify the amount of vegetation cover, is called the “Normalized Difference Vegetation Index” or NDVI. You will need to decide what to do when the intensity of infrared and red light are both 0 since the sum of the intensities is zero and cannot be used to divide the difference of intensities.
For more information about vegetation indices using satellite imagery, visit NASA's Earth Observatory.
Materials
Download the SurfaceType software see a brief tutorial.
Use the three satellite images on the right and below.
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Download the three satellite images below by right-clicking and saving to your computer.
This picture was taken of Mt. St. Helens in southwestern Washington in 1973.
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