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Vegetation Measurements

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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.

Download the three satellite images below by right-clicking and saving to your computer.

Picture of Mt. St. Helens in southwestern Washington in 1973

This picture was taken of Mt. St. Helens in southwestern Washington in 1973.

Materials

    Download the DEW software bundle. You can use either the DigitalImageBasics "False Color" window or theAnalyzingDigitalImages program for this investigation.

    Use the three satellite images above and below.


Mt. St. Helens shortly after the violent eruption in 1983.
Mt. St. Helens shortly after the violent eruption in 1983.
Mt. St. Helens in 1996.
Mt. St. Helens in 1996.

What To Do...

Using the 1973, 1983, and 1996 images of Mt. St. Helens in the SurfaceType program, fill in the following table.
Location in Satellite Image
Satellite Sensor Data
Vegetation Indices
Surface Feature
X
Y
Year
IR
Red
Green
(IR-Red)
(IR-Red)
(IR+Red)
319 180
1973
           
1983
           
1996
           
321 167
1973
           
1983
           
1996
           
278 257
1973
           
1983
           
1996
           
167 77
1973
           
1983
           
1996
           
186 89
1973
           
1983
           
1996
           
276 206
1973
           
1983
           
1996
           

Question: What are the maximum and minimum values possible for NDVI, provided you cannot divide the sum of the intensities when equal to zero. Based on the colors observed on the Landsat imagery of Mt. St. Helens, an NDVI value near the maximum value corresponds to what type of land surface cover? What about for NDVI values near 0? And what appears to produce NDVI values near the minimum value? Does this 
technique appear to work for the satellite images of Mt. St. Helens? Does it have problems?

Now that you have studied these three satellite images of Mt. St. Helens, read the accompanying article provided by the United States Geological Survey for a discussion of the observed changes around Mt. St. Helens. Read the article on the colors observed on Landsat images. Do your findings agree?

If possible, read the article on remote sensing provided by the USGS since the many figures and illustrations are available in color and are linked to the text.