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It’s time to apply the concepts of color, light, and images toward understanding satellite images, one important type of digital image. So far we have described land surface types, but now we’ll use analysis tools to examine detailed values and lengths of features in satellite images. As in any digital image, you can’t see anything smaller than a pixel in size. If you look at this computer screen or a photograph with a magnifying glass, you will see tiny blocks of a single color. Your eye doesn’t see these blocks as separate, but as a continuous, flowing portion of the image. Even though pixels appear to be small on the computer image, they actually represent a relatively large area of real estate on a satellite image. The latest technology is producing satellite images that can see very fine features on the Earth’s surface. As you look at these images, can you see individual trees, rooftops, or roads? Materials Download the DEW software bundle. You can use either the DigitalImageBasics "False Color" window or theAnalyzingDigitalImages program for this investigation. Use the satellite image on the right.

Download the satellite image below by right-clicking and saving to your computer. This picture was taken of Mt. St. Helens in southwestern Washington in 1973. The volcano violently erupted in 1983, creating dramatic changes to the vegetation around the volcano.

What To Do...

Explore the Visualization Tools

Use the selections within the menu button below “Satellite Image Visualization” to examine the relative intensities of near infrared (IR), and Red (R) and Green (G) light reflected from the Earth’s surface and displayed in the image on your screen.

Use the selections under the dropdown menu in 'False Color' or 'Data in Images' in the DigitalImageBasics program, or the 'Enhance Colors' tab panel in AnalyzingDigitalImages, to examine the relative intensities of near infrared (IR), and Red (R) and Green (G) light reflected from the Earth's surface and displayed in the image on your screen.

There are 13 Visualizations:

1) Standard color composite of Landsat imagery

IR displayed in the computer display’s Red 
Visible Red displayed in the computer’s Green 
Visible Green displayed in the computer’s Blue

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 analysis tools from the dropdown menus under 'Data In Images' in DigitalImageBasics or 'Spatial Analysis' in AnalyzingDigitalImages.

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.

Go To Vegetation Analysis

Back to Remote Sensing Investigations