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. As early as 1858 technology had advanced enough to allow aerial photography from balloons, permitting top-down views of areas for various artistic, civic, and military applications.
History edit
One of the first historical instances of remote sensing were photographs of the landscape taken from hot air balloons by Félix Tournachon in 1858 [1] [2]. He is recorded to have photographed a french village from a height of 80m[3], and took the first successful aerial photograph of Paris in 1868.
Methods edit
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.
Remote sensing generally has several basic problems: scaling, alignment, resolution, interpretation and archiving.
In order to generate maps, most remote sensing systems expect to convert a photograph or other data item to a distance on the ground. This almost always depends on the precision of the instrument. For example, distortion in an aerial photographic lens or the platen against which the film is pressed can cause severe errors when photographs are used to measure ground distances.
In order to coordinate a series of observations, most sensing systems need to know where they are, what time it is, and the rotation and orinetation of the instrument. High-end instruments now often use positional information from satellite navigation systems. The rotation and orientation is often provided within a degree or two with electronic compasses. Compasses can measure not just azimuth (i.e. degrees to magnetic north), but also altitude (degrees above the horizon), since the magnetic field curves into the Earth at different angles at different latitudes. More exact orientations require gyroscopic pointing information, periodically realigned in some fashion, perhaps from a star or the limb of the Earth.
The resolution determines how many pixels are available in measurement, but more importantly, higher resolutions are more informative, giving more data about more points. However, more resolution occasionally yields less data. For example, in thematic mapping to study plant health, imaging individual leaves of plants is actually counterproductive. Also, large amounts of high resolution data can clog a storage or transmission system with useless data, when a few low resolution images might be a better use of the system.
Interpretation is the critical process of making sense of the data. Traditionally, this was a human being, perhaps with a few meaurment tools and a light table. In modern systems that produce digital data, often the tool is a family of computer programs that interpret the data to form maps, or statistical analyses.
Old data from remote sensing is often unreasonably valuable because it may provide the only long-term data for a large extent of geography. At the same time, the data is often complex to interpret, and bulky to store. Modern systems tend to store the data digitally, often with lossless compression. The difficulty with this approach is that the data is fragile, the format may be archaic, and the data may be easy to falsify. One of the best systems for archiving data series is as computer-generated machine-readeable ultrafiche, usually in typefonts such as OCR-B, or as digitized half-tone images. Ultrafiches survive well in standard libraries, with lifetimes of several centuries. They can be created, copied, filed and retrieved by automated systems. They are about as compact as archival magnetic media, and yet can be read by human beings with minimal, standardized equipment.
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:
- Stereographic pairs of aerial photographs have often been used to make Topographic maps. Trained personnel would then trace the shape of the land onto maps. Satellite imagery has also been used.
- Thematic mappers take images in multiple wavelengths of electro-magnetic radiation (multi-spectral) and are usually found on earth observation satellites, including (for example) the Landsat program or the IKONOS satellite. Maps of land cover and land use from thematic mapping can be used to prospect for minerals, measure land usage, and examine the health of plants, including entire farming regions or forests.
- seismograms taken at different locations can locate and measure Earthquakes (after the fact) by comparing the relative intensity and precise timing.
- synthetic aperture radar can produce precise Digital elevation models (another form of Topographic map). This is an interferometric 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.
- Altimeters using Radar on satellites have provided a wide range of data. By measuring the bulges of water caused by gravity, they map features on the seafloor to a resolution of a mile or so. By measuring the height and wave-length of ocean waves, the altimeters measure wind speeds and direction, and surface ocean currents and directions.
See also edit
External links edit
Notes edit
- ^ History of Remote Sensing @ NASA
- ^ National Gallery of Art Biography of Tournachon
- ^ History of Aerial Photography
Category:Computer vision
Category:Earth sciences
Category:Social sciences