Geography For Dummies. Jerry T. Mitchell

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       Radar imaging: In radar imaging, a sensor emits continuous beams of energy that bounce off Earth and return to the sensor, which records them. Because the emitted beams travel at a known and uniform speed, the time that it takes them to make the round trip is a function of the elevations of the locations where the beams reflect. For example, a beam that bounces off a mountaintop takes less time to return to the sensor than one that reflects off a valley bottom. This information can be used to produce detailed images of terrain and very exact topographic maps.Radar beams can penetrate clouds and fog with no loss of strength. Thus, radar imaging is extremely useful for monitoring and mapping Earth’s surface in regions where atmospheric characteristics inhibit aerial photography (such as characteristically cloudy equatorial areas). It may also be used at night to the same effect as day. The same, of course, cannot be said of regular film.

       Thermal imaging: Thermal scanners (a form of infrared imaging) record heat differences on Earth’s surface. This is particularly useful for mapping ocean surface currents (whose temperature variations have a major effect on weather and climate) as well as for identifying and mapping different kinds of pollution. It has also proved very useful in mapping and monitoring forest fires and other fire-related phenomena, especially in situations in which smoke prohibits analysis by means of standard photography.

       LiDAR (Light Detection And Ranging): This has really come into its own over the past two decades, primarily for its ability to make very high-resolution maps. Using laser pulses, the system has helped us better map ocean bottoms (bathymetry) and helped archaeologists find sites under vegetative cover. LiDAR highlights that mapping is never really done. Sure, there are always new things to map but as LiDAR shows us there are always new, better, and increasingly more accurate ways to do so.