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In this article we will discuss about Remote Sensing:- 1. History of Remote Sensing 2. Principles of Remote Sensing 3. Types 4. Advantages 5. Limitations 6. Applications.
Contents:
- History of Remote Sensing
- Principles of Remote Sensing
- Types of Remote Sensing
- Advantages of Remote Sensing
- Limitations of Remote Sensing
- Applications of Remote Sensing
1. History of Remote Sensing:
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The technology of modern remote sensing has a very long history, dating back to the end of the 19th century with the invention of the camera.
Initially cameras were used to take photographs on the ground, which provided (and still does) a fascinating and exciting way to capture moments in time and keep a record of something that happened, which looked more realistic than a drawing or painting, and which could be captured much quicker than by drawing or painting.
The idea and practice of remote sensing first developed in the 1840s, when it was realized that a different and perhaps more revealing view of a particular landscape could be obtained by taking a photograph from a vantage point, such as an incline or building, and efforts were made to look down at the Earth’s surface by taking pictures with the aid of cameras secured to tethered balloons, for purposes of topographic mapping.
It was realized that the airborne perspective gave a completely different view than to what was available from the ground. The most novel platform at the end of the last century is perhaps the famed pigeon fleet that operated as a novelty in Europe.
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By the first World War, cameras were mounted on airplanes, which provided aerial views of fairly large surface areas and was used as a method of data and information acquisition, that proved invaluable in military reconnaissance. Thus, aerial photography remained the single standard tool for depicting the surface from a vertical or oblique perspective till the early 1960s.
The history of Satellite remote sensing can be traced back to the early days of the space age of both Russian and American programs. It actually began as a dual approach of imaging surfaces, from spacecraft, using several types of sensors. After World War II, in 1946, V-2 rockets acquired from Germany, containing automated still or movie camera, were launched to high altitudes from White Sands, New Mexico.
These rockets, however never attained orbit, but took pictures of the earth’s surface as the vehicle ascended. In the 1960s, with the emergence of the space program, cosmonauts and astronauts started taking photographs out of the window of their spacecraft in which they were orbiting the earth.
Today, remote sensing is carried out using airborne and satellite technology, not only utilizing film photography, but also digital camera, scanner and video, as well as radar and thermal sensors (Fig. 11.1).
Unlike in the past, when remote sensing was restricted to only the visual part of the electromagnetic spectrum i.e., what could be seen with naked eye, today through the use of special filters, photographic films and other types of sensors, the parts of the spectrum which cannot be seen with the naked human eye can also be utilized.
Thus, today remote sensing is largely utilized in environmental management, which frequently requires rapid, accurate and up-to-date data collection.
2. Principles of Remote Sensing:
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Remote sensing involves the detection and measurement of the radiations of different wavelengths reflected or emitted from distant objects or materials, which helps in their identification and categorization.
It offers four basic components to measure, which include:
(i) The energy source
(ii) The transmission path
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(iii) The target
(iv) The satellite sensor
Among these, the energy source or electromagnetic energy, is very important, as it serves as the crucial medium for transmitting information from the target to the sensor. It is described as an electromagnetic spectrum, on which, many forms exist that describe energy in a specific region of the spectrum (Fig. 11.2).
These are visible light, radio waves, microwaves, heat, UV rays, X-rays and gamma rays. This spectrum is an overview of the continuum of electromagnetic energy from extremely short wavelengths (cosmic gamma rays) to extremely long wavelengths (radio and television waves). These divisions are not absolute and definite as overlapping may occur.
The basic unit of wavelength, is measured in meters (m). Most energy in the visible (including U.V., visible, and near infrared) and infrared portions of the electromagnetic spectrum is measured in micrometers (10-6m) [1 micrometer = 10,000 angstroms]. Millimeters may be used for longer wavelengths (blue – 0.4 – 0.5 mm).
Depending on the wavelength and the nominal spectral location, principal applications can be matched with suitable satellite bands for classification.
3. Types of Remote Sensing:
Satellite remote sensing involves gathering information about features on the Earth’s surface from orbiting satellites, which may carry either of the following two types of sensor systems:
(i) Passive System:
It generally consists of an array of small sensors or detectors, which records the amount of electro-magnetic radiation reflected and/or emitted from the Earth’s surface. Thus, passive remote sensing relies on naturally reflected or emitted energy of the imaged surface.
Most remote sensing instruments fail into this category, obtaining pictures of visible, near-infrared and thermal infrared energy. A multi-spectral scanner is an example of a passive system (Fig. 11.3). Passive visible and near-infrared data are used in a variety of GIS applications, for example in the classification of vegetation and land-use, and may be performed at a variety of temporal and spatial scales.
(ii) Active System:
This type of a system propagates its own electro-magnetic radiation and measures the intensity of the return signal. Thus, active remote sensing means that the sensor provides its own illumination and measures what comes back. Remote sensing technologies that use this type of system include lidar (laser) and radar.
Synthetic Aperture Radar (SAR) is an example of an active system. Active remote sensing (radar and lidar) systems are rapidly increasing in use since the launch of the ERS-1 Synthetic Aperture Radar (SAR) satellite in 1991. In comparison to visible/near-infrared imagery, radars are sensitive to very different surface properties.
As for example, radar images are sensitive to the shape, orientation and size of leaves and their moisture content, rather than the vegetation color. Similarly, airborne lidars have been largely used for mapping surface topography in three dimensions. Existing and planned radar and lidar altimeters will also help in monitoring closely the elevation of the world’s ice caps and sea level with centimeter precision.
4. Advantages of Remote Sensing:
Remotely sensed imaging systems have several advantages over camera photography, from which it differs significantly in the following two ways:
(i) It is not just restricted to the visible part of the electromagnetic spectrum (from about 0.4 to 0.7 micrometers in wavelength), but can also measure energy at wavelengths invisible to the eye, such as near-infrared, thermal infrared and radio wavelengths.
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(ii) Most remote sensing instruments can record different wavelengths at the same time, yielding not one but numerous images of the same location on the ground, each corresponding to a different range of wavelengths called a band.
Thus, remotely sensed data acquired by the Earth observation satellites provides a number of benefits for studying the Earth’s surface, which include:
(i) Good spectral (including infra-red bands) and spatial resolution.
(ii) Allows broad regional coverage.
(iii) Allows continuous acquisition of data.
(iv) Provides up-to-date information because of the capability of regular revisit.
(v) Provides cost effective and map-accurate data.
(vi) Provides large archive of historical data.
(vii) Enables to combine satellite digital data with other digital data.
(viii) Has the ability to manipulate or enhance digital data,
(ix) Allows the possibility of stereo viewing.
5. Limitations of Remote Sensing:
Although remote sensing has many advantages over ground-based survey, yet remote sensing has not totally replaced ground-based survey methods, largely because of some limitations with this technology, which still exist.
These include:
(i) Cost of data collection and data purchase.
(ii) Problems with data analysis and interpretation.
(iii) Potential limitations with spatial, spectral and temporal resolutions of the various sensors.
(iv) Problems with all weather capability as some sensors cannot ‘see’ through cloud.
In-spite of these limitations, remote sensing has however many advantages over ground- based survey in that large tracts of land can be surveyed at any one time, and areas of land (or sea) that are otherwise inaccessible can be monitored.
The advent of satellite technology, and multi-spectral sensors has further enhanced this capability, with the ability to capture images of very large areas of land in one pass, and by collecting data about an environment that would normally not be visible to the human eye.
6. Applications of Remote Sensing:
Satellite data allows the proper management of our renewable and non-renewable resources as it provides timely and detailed information about the Earth’s surface.
Following are a few examples of some of the important uses of satellite data:
(i) Assessment and monitoring of vegetation types and their status.
(ii) Agricultural property management planning and crop yield assessment.
(iii) Soil surveys including mineral and petroleum exploration.
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(iv) Litho logic mapping.
(v) Monitoring and planning of water resources and groundwater exploration.
(vi) Geographic information
(vii) Map making and revision and production of thematic maps.
(viii) Weather and agricultural forecasts and assessment of environment and natural disasters.
(ix) Urban planning.
(x) Image processing.
(xi) Precision geo-referencing.
(xii) Laser film writing and printing.
Applications of Remote Sensing in Forest Resource Management:
(i) Satellite imagery can provide the visible boundaries of soil types, while remote sensing provide for a shallow penetration of soils. Additional physical data can be obtained from spectral signatures for the soil surfaces.
(ii) Remote sensing allows for classification of soils, which can be interpreted from the remote sensing images and the spectral signatures.
(iii) Remote sensing can provide information on the productivity of forests, meadows, wildlife habitat conditions, land-use and recreational suitability, which allows for future protection of the environment.
(iv) Multi temporal techniques can be used to map dynamical features, erosion and soil moisture.
(v) Remote sensing can also be used in combination with ground radar, to detect changes of diagnostic soil horizons such as albic, spodic and argillic horizons or soil/rock boundaries.
