Geography Reference
In-Depth Information
Non-photographic imagery
Like infrared photography, other remote sensing technologies record surface features in ways
that are beyond the capabilities of human eyesight and normal cameras and film. Virtually all
of them make use of sensors that scan the Earth and record surface information electronically.
Because they do not use film, the pictures they produce are not, technically speaking, pho-
tographs. Thus, in the lingo of remote sensing, you have aerial
photographs
and non-photo-
graphic
images.
Three image-types are widely used.
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.
Infrared imaging:
Infrared pictures of Earth may be obtained from scanners as well as
films. Other than a different way of receiving images, the basic characteristics and use of in-
frared imaging is the same as for infrared films, discussed previously.
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 tem-
perature variations have a major effect on weather and climate) as well as for identifying and
mappingdifferentkindsofpollution.Ithasalsoprovedveryusefulinmappingandmonitoring
forest fires and other fire-related phenomena, especially in situations in which smoke prohibits
analysis by means of standard photography.
