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Remote sensing

In the broadest sense, remote sensing is the measurement or acquisition of information of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object.

It is the utilization at a distance (as from aircraft, spacecraft, satellite, or ship) of any device for gathering information about the environment. The technique can make use of devices such as a camera, laser, radar, sonar, seismograph or a gravimeter. Modern remote sensing normally includes digital processes but can as well be done with non-digital methods.

This kind of data collection normally makes use of the emitted or reflected electromagnetic radiation of the examined object in a certain frequency domain (infrared, visible light, microwaves). This is possible due to the fact that the examined objects (plants, houses, water surfaces, air masses ...) reflect or emit radiation in different wavelengths and in different intensity according to their current condition. Some remote sensing systems use sound waves in a similar way, and others measure variations in gravitational or magnetic fields.

While all astronomy could be considered remote sensing (in fact, extremely remote sensing) the term "remote sensing" is normally only applied to terrestrial observations.

Examples of remote sensing are very numerous. For example:

• Topographical maps were often produced from stereographic pairs of aerial photographs. Trained personnel would then trace the shape of the land onto maps.
• Earthquakes are located (after the fact) by comparing seismograms taken at different locations; the relative intensity and precise timing yield information about the location and nature of the tremor.
• Digital elevation maps can be produced by interferometric synthetic aperture radar, a process in which an aircraft, spacecraft, or satellite passes over the target area while emitting a series of radar pulses. Combining the data from these pulses yields a detailed map containing information about ground cover and possibly elevation or movement on a scale of centimeters. The data usually covers a swath many kilometers wide.

• International Archive for stereo views
• RemoteSensing.org
• RemoteSensingOnLine
• General Aerial Photograph Information (U.S. Geological Survey)

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