2-7) IR, Red, or Green as Color or Gray: A color or gray shade image of only one set of satellite measured intensities. Gray shades allows unbiased viewing of the intensities, and color illustrates the actual contribution to the color composite being displayed on the screen.
Use the remaining six visualizations to quickly see which surface features reflect greater amounts of IR, Red, or Green light.
8-10) IR v R, IR v G, or R v G: Display the difference between two sets of measurements (A v B means Intensity A – B).
The color of the greater value is displayed, with bright colors showing large differences and dark colors indicating little difference.
IR is displayed as a shade of Gray; Red as a shade of Red; and Green as a shade of Green
Example: using IR v R, if a pixel has 10% IR and 20% Red, the difference is 10% and will be displayed in the computer’s Red.
11-13) Normalized versions of IR v R, IR v G, or R v G
The formula is (Intensity A - Intensity B) / (Intensity A + Intensity B).
The color of the greater value is displayed:
IR is displayed as a shade of Gray
Red as a shade of Red;
Green as a shade of Green
This formula tends to minimize difference in illumination of the surface caused by shadows of clouds and slope of the land surface that cause uneven illumination of the surface by the Sun.
Example: using IR v R, if a pixel has 10% IR and 20% Red, the normalized difference is 10% divided by 30% = 0.33. This value is scaled to 33 and will be displayed in the computer’s Red. Compare this to 10 displayed in previous example.
Explore the Analysis Tools
Select tools from the menu button next to “Analysis Tool”.
Pixel Tool
• Cross hair appears where click on the image
• Move by click and drag or with arrows next to x and y position
x increases from left to right & y increases from top to bottom
• Intensity of IR, red, and green light of pixel beneath center of cross hair is output
Line Tool
• Yellow line appears when click and drag on the image. A blue circle at the end of line where cursor clicked and a red circle at the end of line where released mouse click
• Adjust end of line with arrows next to x and y position
x increases from left to right & y increases from top to bottom
• Length of line in pixels is output in lower left edge of the window.
Question: What are the maximum and minimum x and y values you can find on the satellite image?
Question: Using the small white square, which represents one mile along each edge, in the lower left of the image, what is the number of pixels that represents 1 mile? Assuming the edge of one pixel touches the edge of the neighboring pixel, what is the size of one pixel? How many pixels represent 10 miles?
Question: This image is oriented so that north is up and east is to the right. The east-to-west and north-to-south extents of the satellite image are how many miles? What is the distance from the upper-left corner to the lower-right corner of the image? Hint: you will need to use the Pythagorean Theorem if you are using the pixel analysis tool or you may use the line length in pixels output from the line analysis tool.
Question: What is the greatest distance across the snow cover observed on Mt. St. Helens in the lower left corner of the satellite image?
Question: What is the greatest width across the lake observed in the left center of the satellite image? What is the greatest length across the lake?
Question: 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 by 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, Donald, 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.
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