ABCs of DEW Software Investigation 7-1:

Changes in Mt. St. Helens

Mount St. Helens seen from space in 1973  Mt. St. Helens seen from space in 1973

Use VegetationAnalysis software to observe and measure changes in Mt. St. Helens before and after the great eruption of 1980,


AnalyzingDigitalImages software

3 images of Mt. St. Helens:

The Tools

1. Launch the AnalyzingDigitalImages software and open the 3 images of Mt. St. Helens as time series. Calibration procedure automatically looks for a white box in the lower right of the images. Just follow the Scale Calibration instructions that pop up automatically in the window on opening of the image files. Then familiarize yourself with the tools described in steps 2–6.

2. Use the menu to the right of ‘Visualization’ to toggle between an RGB picture and a NDVI enhancement. The following is a summary of NDVI values for common surface types:

NDVI Surface Cover
+ 1 Vegetation
0 Exposed soil or rock
- 1 Water, snow, clouds

3. With Point Analysis Tool in the menu button to the right of ‘Analysis Tool’, explore intensities of infrared, red, and green light and the vegetation index, NDVI, at the same pixel for each satellite image.

– Intensities are scaled from 0 to 100%

– Move the cross hair in three ways:

(a) Click the mouse on an area of interest

(b) Click and drag the mouse to an area of interest

(c) Use the small up and down arrows along the upper-right edge

4. With the Line Analysis Tool in the menu button to the right of ‘Analysis Tool’,

• Click and drag the cursor to draw a line.

– Move ends of line in similar way as the Point Tool.

• NDVI values for pixels along the line are graphed.

• The number of pixels along the line and length of line are automatically calculated.

• Average NDVI is calculated and color-coded by the year of each image.

– Move cursor to each satellite image to see yearly data on the graph.

– To see all data, move the cursor to the graph window.

5. With the Area Analysis Tool

• Click and drag the cursor to draw a rectangle.

Move ends of line in similar way as the Point and Line Tools.

• A histogram (graph) is created of NDVI values for all pixels inside the rectangles.

- The histogram shows the percentage of values within narrow ranges of NDVI values.

– Data can be viewed year by year when cursor is moved to each satellite image.

– Change the range of min/max NDVI values to calculate the percent of NDVI values between the selected range.

• The number of pixels within area and size of the area are automatically calculated.

5. Page Setup and Print—To print the images and graphs, first use ‘PageSetup’ in the File Menu (Mac users). Select ‘landscape’ printing and set scale to 75%.


Question 6.1.
Using the line analysis tool, measure the diameter of caldera formed by the eruption. A caldera is the crater formed by a volcanic explosion or by the collapse of a volcanic cone. Find the location (x,y coordinates) of the center of the caldera and compare this to the location of the center of the volcano as seen in 1973. Does this explain the direction where most of the volcanic ash fell?

Combine this measurement with an interesting measurement reported on the USGS Earthshots web site to estimate how large an area of solid rock was turned into volcanic debris: “Before the eruption, Mount St. Helens towered about a mile above its base, but on 18 May 1980 its top slid away in an avalanche of rock and other debris. When finally measured on 1 July 1980, the mountain’s height had been reduced by 1,313 feet— from 9,677 feet to 8,364 feet.”

[From Foxworthy and Hill, 1982, p. 11. Lipman, Peter, W., and Mullineaux, Donal, R., (ed.), 1981, The 1980 Eruptions of Mount St. Helens: Washington, U. S. Geological Survey Professional Paper 1250, Washington, D. C. (844 p.), p. 134.]

Using the 1973, 1983, and 1996 images of Mt. St. Helens, fill in the following table.

Location Year NDVI Surface Feature
X Y      
319 180 1973    
321 167 1973    
278 257 1973    
167 77 1973    
186 89 1973    
276 206 1973    


Question 7.2.
Can you make any generalizations from your observations and measurements? Explain.

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. The article and additional pictures and maps can be found on the Internet at

  • More ideas for investigations concerning temporal changes
    For another investigation concerning temporal changes, see Satellite Views of Rondonia, Brazil in the GSS book A New World View, Chapter 5. It explores deforestation of tropical rain forest in the Amazon.
  • Visit the EarthShots website ( to find many examples of investigations in temporal change.
  • Visit the EOS Webster Click n Pic web page ( to order up time series of LandSat images from any location in the USA.