Remote Sensing
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Aerial Photography
Aerial photography is the taking of photographs of the ground from an elevated/direct-down position. Usually the camera is not supported by a ground-based structure. Platforms for aerial photography include fixed-wing aircraft, helicopters, unmanned aerial vehicles (UAVs or "drones"), balloons, blimps and dirigibles, rockets, pigeons, kites, parachutes, stand-alone telescoping and vehicle-mounted poles. Mounted cameras may be triggered remotely or automatically; hand-held photographs may be taken by a photographer.
Aerial photography is used in cartography (particularly in photogrammetric surveys, which are often the basis for topographic maps), land-use planning, archaeology, movie production, environmental studies, power line inspection, surveillance, commercial advertising, conveyance, and artistic projects.
Remote Sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation. Remote sensing is used in numerous fields, including geography and most Earth Science disciplines (for example, hydrology, ecology, oceanography, glaciology, geology); it also has military, intelligence, commercial, economic, planning, and humanitarian applications.
In current usage, the term "remote sensing" generally refers to the use of satellite- or aircraft-based sensor technologies to detect and classify objects on Earth, including on the surface and in the atmosphere and oceans, based on propagated signals (e.g. electromagnetic radiation). It may be split into "active" remote sensing (i.e., when a signal is emitted by a satellite or aircraft and its reflection by the object is detected by the sensor) and "passive" remote sensing (i.e., when the reflection of sunlight is detected by the sensor)
Brief History of Remote Sensing
1826 The invention of photography
1850’s Photography from balloons
1873 Theory of electromagnetic energy by J. C. Maxwell
1909 Photography from airplanes
1910’s World War I: aerial reconnaissance
1920’s Development and applications of aerial photography and photogrammetry
1930’s Development of radar in Germany, USA, and UK
1940’s World War II: application of Infrared and microwave regions
1950’s Military Research and Development
1960’s The satellite era: Space race between USA and USSR.
1960 The first meteorological satellite (TIROS-1)
1960’s First use of term “remote sensing”
1960’s Skylab remote sensing observations from the space
1972 Launch of the first earth resource satellite (Landsat-1)
1970’s Rapid advances in digital image processing
1980’s Landsat-4: new generation of Landsat sensors
1986 Launch of French earth observation satellite (SPOT-1)
1980’s Development of hyperspectral sensors
1990’s Launch of earth resource satellites by national space agencies and commercial companies
Advantages of remote sensing for land evaluation
Relatively cheap and rapid method of acquiring up-to-date information over a large geographical area.
It is the only practical way to obtain data from inaccessible regions, e.g. Antarctica, Amazonia. At small scales, regional phenomena which are invisible from the ground are clearly visible. Cheap and rapid method of constructing base maps in the absence of detailed land surveys. Easy to manipulate with the computer, and combine with other geographic coverages in the GIS.
Disadvantages of remote sensing for land evaluation
They are not direct samples of the phenomenon, so must be calibrated against reality. This calibration is never exact; a classification error of 10% is excellent.
They must be corrected geometrically and geo-referenced in order to be useful as maps, not only as pictures. This can be easy or complicated.
Distinct phenomena can be confused if they look the same to the sensor, leading to classification error. Example: artificial & natural grass in green light (but infrared light can easily distinguish them). Phenomena which were not meant to be measured (for the application at hand) can interfere with the image and must be accounted for. Examples for land cover classification: atmospheric water vapor, sun vs. shadow (these may be desirable in other applications). Resolution of satellite imagery is too coarse for detailed mapping and for distinguishing small contrasting areas. Rule of thumb: a land use must occupy at least 16 pixels (picture elements, cells) to be reliably identified by automatic methods. However, new satellites are being proposed with 1m resolution, these will have high data volume but will be suitable for land cover mapping at a detailed scale.
Stages in remote sensing:
1) Requirement of an energy source (Sun is the main source of energy during day time.)
2) Energy interaction with the atmosphere
3) Interaction with the target
4) Recording of energy by sensor
5) Data transmission & processing
6) Image processing & analysis
7) Application
Applications of Remote Sensing
Some of the important applications of Remote Sensing technology are:
Environmental assessment and monitoring (urban growth, hazardous waste).
Global change detection and monitoring (atmospheric ozone depletion, deforestation, global warming).
Agriculture (crop condition, yield prediction, soil erosion).
Nonrenewable resource exploration (minerals, oil, natural gas).
Renewable natural resources (wetlands, soils, forests, oceans).
Meteorology (atmosphere dynamics, weather prediction).
Mapping (topography, land use. Civil engineering).
Military surveillance and reconnaissance (strategic policy, tactical assessment).
News media (illustrations, analysis).
Remote sensors
The instruments used to measure the electromagnetic radiation reflected/emitted by the target under study are usually referred to as remote sensors. There are are two classes of Remote Sensor:
1. Passive remote sensor.
2. Active remote sensor.
Passive remote sensor: Sensors which sense natural radiations, either emitted or reflected from the earth, are called passive sensors. The sun as a source of energy or radiation. The sun provides a very convenient source of energy for remote sensing. The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then reemitted, as it is for thermal infrared wavelengths. Remote sensing systems which measure energy that is naturally available are called passive sensors. Passive sensors can only be used to detect energy when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. There is no reflected energy available from the sun at night. Energy that is naturally emitted (such as thermal infrared) can be detected day or night, as long as the amount of energy is large enough to be recorded.
Active remote sensor: Sensors which carry electromagnetic radiation of a specific wavelength or band of wavelengths to illuminate the earth’s surface are called active sensors.Active sensors, on the other hand, provide their own energy source for illumination. The sensor emits radiation which is directed toward the target to be investigated. The radiation reflected from that target is detected and measured by the sensor. Advantages for active sensors include the ability to obtain measurements anytime, regardless of the time of day or season. Active sensors can be used for examining wavelengths that are not sufficiently provided by the sun, such as microwaves, or to better control the way a target is illuminated. However, active systems require the generation of a fairly large amount of energy to adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and a synthetic aperture radar (SAR).
Parameters of a sensing system
The major parameters of a sensing system which can be considered as indicators of the quality of data and which have bearing on optimum utilization for specific end use include:
Spatial resolution: The capability of the sensor to discriminate the smallest object on the ground of different sizes; usually specified in terms of linear dimension. As a general rule, higher the resolution, smaller the object that can be identified.
Spectral resolution: The spectral bandwidth with which the data is collected.
Radiometric resolution: The capability of the sensor to discriminate two targets based on its reflectance/emittance difference; it is measured in terms of the smallest reflectance/emittance that can be detected. Higher the radiometric resolution, smaller the radiance differences that can be detected between two targets.
Temporal resolution: The capability to view the same target, under similar conditions, at regular intervals.
Spectral bands and Structure
The most important criterion for the location of spectral bands is that they should be in the atmospheric window and away from the absorption bands of atmospheric constituents. Field studies have shown that certain spectral bands are best suited for specific themes. The thematic mapper bands are selected based on such investigations.
Electromagnetic spectrum: The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and x-rays) to the longer wavelengths (including microwaves and broadcast radio waves). There are several regions of the electromagnetic spectrum which are useful for remote sensing. For most purposes, the ultraviolet or UV
Portion of the spectrum has the shortest wavelengths which are practical for remote Sensing. This radiation is just beyond the violet portion of the visible wavelengths, Hence its name. Some Earth surface materials, primarily rocks and minerals, fluoresce or emit visible light when illuminated by UV radiation.
Earth observation satellite
Earth observation satellites are satellites specifically designed for Earth observation from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc.
The Earth observation satellites ERS-1, ERS-2 and Envisat of European Space Agency as well as the MetOp spacecraft of EUMETSAT are all operated at altitudes of about 800 km. The Proba-1, Proba-2 and SMOS spacecraft of European Space Agency are observing the Earth from an altitude of about 700 km. The Earth observation satellites of UAE, DubaiSat-1 & DubaiSat-2 are also placed in Low Earth Orbits (LEO) orbits and providing satellite imagery of various parts of the Earth.
