Exploring the Spatial and Spectral Information in a Landsat Image with the SurfaceType Software
Purpose: Explore the characteristics of reflected infrared and visible red and green light from different surfaces on the Earth and make distance measurements with satellite images.
Background: Landsat
The Landsat satellite program has a long and significant history in monitoring our Earth. The first images of the Earth's land features were taken in 1972, and for over 40 years, the data has been used to study the many interconnected processes changing our landcover.
NOTE: The images that follow are not interactive. Please download the software (OSX OS9 PC) and follow along with the mini tutorial.
Getting Started
Click 'Select Satellite Image' to choose a satellite picture to examine. The program uses jpegs of Landsat images available on the United States Geological Survey’s Earthshots website: http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents. Go to this site to download satellite images to your computer. Right-click or click and hold on an image of interest, and a pop-up menu will allow you to 'Save Image As...'. Practice with these five Landsat images of Mount St. Helens, which are from the Earthshots website that has additional images, data, and articles.
Change the visual enhancement (or visualization) of the satellite image by clicking the pop up menu labeled 'Satellite Image Visualization'.
Images will first be displayed as a typical Landsat Image with the infrared data in the computer's red color and the visual red and green light mapped by the computer's green and blue colors, respectively.
Separate satellite sensor measurements may be visualized with the remaining options that either use the computer color typically associated with the sensor data or in shades of gray.
There are two analysis tools.
1) The pixel analysis tool displays the intensities at each pixel on the satellite image as you click the cursor on the image (example screen is shown above).
A crosshair appears on the screen to highlight the cursor's location.
The numerical values represent the intensity of a narrow bands of reflected light from the Earth's surface.
Intensity values are scaled from 0 to 100%.
Use the arrows next to the x and y pixel positions to move the crosshair incrementally across the image.
Explore the intensities of reflected light from different surface materials (plants, water, snow, and rock/soil/ash deposits).
2) The line analysis tool allows you to draw lines on the image and calculate the size of surface features (example screen is directly above).
To draw a line, click the cursor on the image and hold the mouse key down as you drag the cursor across the image.
When you release the mouse key, the line stops changing and stays drawn on the image.
Use the arrows next to the x and y start and stop positions to incrementally change the position of the line.
The pixel length of the line can be converted to distances using the scale provided at the lower right of each satellite image.
There are 13 visualizations to view the satellite images.
1) Standard color composite of Landsat imagery in which the intensity of the infrared measurements are displayed in the computer display's red, the visible red displayed in the computer's green, and the visible green displayed in the computer's blue.
2-7) IR, Red, or Green (Color or Gray). Display color or gray shade image of only one set of satellite measured intensities.
A gray shade image of IR displays the pixel's IR value in all three colors of the computer's display.
Gray shade images allow you to examine the intensities of values and eliminates the unequal sensitivity to red, green, or blue color by human eyes.
Use the color version to illustrate the contribution of the satellite measurements to the standard color satellite image since the IR is shown in red, and so on.
8-10) IR v R, IR v G, or R v G: For each pixel, display the difference between two listed satellite measurements.
The brighter the color displayed, the greater the difference between the two measurements; and the darker the color (or closer to black), the smaller the difference between the two measurements.
11-13) Normalized versions of IR v R, IR v G, or R v G: Used to visualize the difference between two sets of satellite measurements, the basic formula is (Intensity A - Intensity B) / (Intensity A + Intensity B).
Minimizes the effects of clouds and shadowing by topography.
Color scheme is similar to #8-10 above: shades of gray are used to indicate where IR is greater, red where Visible Red is greater, and green where Visible Green is greatest.
All visualizations may be saved as a picture on your computer.
X/Y Locations
Moving the cursor, line, or rectangle with the control buttons can be confusing unless you understand the mapping of X and Y on the satellite image.
The satellite images are 451 pixels wide and 325 pixels high.
The position of the upper left corner is (0,0) - this is very important to remember!
X controls the horizontal location of the cursor, and the maximum value on the screen is 450 (0 indicates the left edge and 450 the right edge, so 451 pixels make up each row of the image).
Y controls the vertical location of the cursor, and the value of the upper edge is where Y=0. The lower edge has Y=324.
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