Humans cannot see light past the visible spectrum, but satellites are able to detect wavelengths into the ultraviolet and infrared. Satellites, like Landsat 7, fly high above the earth, using instruments to collect data at specific wavelengths. These data can then be used to build an image. Satellite instruments are able to obtain many images of the same location, at the same time. Each image highlights a different part of the electromagnetic spectrum.
The Landsat 7 satellite uses an instrument that collects seven images at once. Each image shows a specific section of the electromagnetic spectrum, called a band. Landsat 7 has seven different bands. The table below shows the seven bands of Landsat 7.
| Band Number | Wavelength Interval | Spectral Response | ||
| 1 | 0.45-0.52 µm | Blue-Green | ||
| 2 | 0.52-0.60 µm | Green | ||
| 3 | 0.63-0.69 µm | Red | ||
| 4 | 0.76-0.90 µm | Near IR | ||
| 5 | 1.55-1.75 µm | Mid-IR | ||
| 6 | 10.40-12.50 µm | Thermal IR | ||
| 7 | 2.08-2.35 µm | Mid-IR |
All of the images above were obtained by Landsat 7 at the same time, and at the exact same location. If you look closely at the images, you will see that they do not all look the same. Light and dark spots in the images appear in different places. This is because different objects on earth (plants, soil, water, etc.) reflect different wavelengths of light. The bright spots on the images show where a lot of light is being reflected.
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The image above is a "true color" image of the desert around the Salton Sea and Imperial Valley in Southern California. The American/Mexican border is clearly visible. |
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In this Band 4 image, the light areas indicate strong reflection of wavelengths between 0.76 and 0.90 µm; the darker areas do not strongly reflect in those wavelengths. What do you think the light areas are? Click here for Answer. |
| Landsat 7 Band Number | Applications |
| 1 | coastal
water mapping, soil/vegetation discrimination, forest classification,
man-made feature identification |
| 2 | vegetation discrimination and health monitoring, man-made feature identification |
| 3 | plant species identification, man-made feature identification |
| 4 | soil moisture monitoring, vegetation monitoring, water body discrimination |
| 5 | vegetation moisture content monitoring |
| 6 | surface
temperature, vegetation stress monitoring, soil moisture
monitoring, cloud differentiation, volcanic monitoring |
| 7 | mineral and rock discrimination, vegetation moisture content |
| For more details see: Lillesand, T. and Kiefer, R., 1994. Remote Sensing and Image Interpretation. John Wiley and Sons, Inc., New York, p. 468. |
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