Photography has been the mainstay of imaging for the better part of two centuries.
Yet, photography has inherent disadvantages in its need for film and its limitation to
visible and near infrared wavelengths. Thus, non-film sensors have been developed.
Thermal infrared sensing does not use film because there are no film emulsions
sensitive to these wavelengths. All objectsemitin the thermal infrared portion of
the spectrum and knowing the relationship of the most plentiful emission wave-
lengths totemperature, we are able to determine temperatures from afar.
At wavelengths much longer than most of the natural electromagnetic energy
from the sun and Earth, geographers use microwave and radar; these remote sensing
types use identical wavelengths and are particularly sensitive to all the states
(phases) of water. Microwave sensors are passive sensors because they do not gener-
ate the energy they are receiving from targets. Microwaves naturally occur but are
neither powerful nor plentiful. Therefore, microwave systems usually have rela-
tively poor spatial resolution when mounted on airplanes or satellites. Radar (an
acronym for radio detection and ranging) generates its own, concentrated micro-
waves and measures the strength of their return as they are backscattered off a target.
Sonar (an acronym for sound navigation ranging) is another active sensing tech-
nique. Sound is propagated and then the nature of its backscatter is measured. In
the case of geographers, it is the distance to the bottom of the water that is being
sensed. Sonar senses through water, whichismuchdenserthanair,soitsspeed
is considerably slower than analogous radar pulses in the air, and sonar wave-
lengths are on the order of several meters. Sonar has revealed the nature of sea
and lake bottoms and has been key in provingplate tectonics.
Lidar (an acronym of light detection and ranging) is based on the technology of
lasers, which have beam coherences and densities not achievable with sonars or
radars. These are active systems that use a variety of wavelengths much shorter
Remote Sensing 285Table 3. Remote Sensing Techniques
Name Wavelengths Geographic uses
Photography .4μm–.7μm Mapping, measurement of cultural and
physical features
Color infrared photography .7μm–.9μm Vegetation type and health, surface soil moisture
Reflective infrared sensing 0.7μm–3.0μm Vegetation type and health, surface soil moisture
Thermal infrared sensing 3.0μm–14μm Temperature differences in rural and urban
systems, “night vision,” cloud temperatures
Microwave sensing 0.1 mm–1 m Soil moisture and temperature
Radar 0.1 mm–1 m Clouds, storms, landscape roughness
Sonar 3 mm–3 m Ocean bottom features
Lidar 250 nm–10.0μm Nature of vegetation canopy, aerosol, and
cloud studies
Multispectral sensing Various Land resource analysis, change detection