ABCs of DEW Software chapter 6Introduction
The word spectral can mean “ghostly,” but we are using its other meaning which relates to the rainbow of color formed when all the energies of white light are spread out and arranged in order. Similarly, the electromagnetic spectrum is an ordered arrangement of invisible radiation with energies ranging from high energy gamma rays to the longest low energy radio waves and including visible light (see diagram at right). Electromagnetic energy is colorless—we see colors in visible light only because of functions going on in the eyes and brain.
Space satellites have cameras/sensors to detect energies invisible to our eyes, but to analyze them it’s helpful to display those energies as visible light computer monitors. For just one value of electromagnetic energy we could use shades of gray that give brightness information ranging from black representing no brightness to white representing very bright. Color monitors display all colors as combinations of three colors: red, green, and blue (RGB), so with a color monitor, each of those three colors can represent a different invisible electromagnetic energy. The displayed colors can help us identify physical properties of objects, Earth’s surface, gas components of the atmosphere, even the Earth’s magnetic field, depending on what type of camera/sensor is used.
These images show the Dallas - Fort Worth metropolis in northeast
Texas. This city grew significantly, from 2,378,000 in 1970 to
3,776,000 by 1988. These images show the urban/suburban areas
expanding into arable land in the countryside. (Note: You can click on each image to open a larger version in a new window, or right-click and save the image to your computer.)
Special challenge: Go to the EOS Webseter Landsat "Click n Pic" page http://mvh.sr.unh.edu/Landsat/ and find a more recent image of the Dallas - Fort Worth area to compare with the image on this page. Also, can you find a series of images for your own city or town?