GEOGRAPHIC 

INFORMATION SYSTEM(GIS)           

The Global Positioning System (GPS) is a location system based on a constellation of 24 satellites orbiting the earth at altitudes of approximately 7000 kilometres. GPS satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide accurate positioning 24 hours a day, anywhere in the world. Uncorrected positions determined from GPS satellite signals produce accuracies in the range of 50 to 100 meters. When using a technique called differential correction, users can get positions accurate to within 5 meters or less. With some consideration for error, GPS can provide any point on earth with a unique address (its precise location). A GIS is a descriptive database of the earth (or a specific part of the earth). GPS provide location of a point (X, Y, Z), while GIS gives the information at that location. In other words, GPS tells the "where", while GIS tells the "what". GPS/GIS is reshaping the way users locate, organise, analyse and map the resources.

 GPS Applications

Lange and Stenberg (1990) cited that GIS application of GPS include georeferencing of satellite imagery and navigating to sample sites for ground truth. In remote tropical areas where base maps are lacking, GPS will provide an opportunity to establish ground control points to locate field plots and rectify satellite imagery.

 Remote Sensing

Remote sensing is the science (and to some extent, art) of acquiring information about the earth's surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analysing, and applying that information.

 Process

The process of remote sensing involves an interaction between incident radiation and the targets of interest. The following seven elements are involved in this process:

Energy Source or Illumination - Remote sensing depends mainly on energy source, which illuminates or provides electromagnetic energy to the target of interest.

Radiation and the Atmosphere - As the energy travels from its source to the target, it will interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.

Interaction with the Target - Once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation.

Recording of Energy by the Sensor - After the energy has been scattered by, or emitted from the target, a sensor (remote - not in contact with the target) is required to collect and record the electromagnetic radiation.

Transmission, Reception, and Processing - The energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital).

Interpretation and Analysis - The processed image is interpreted, visually and/or digitally or electronically, to extract information about the target, which was illuminated.

Application - The final element of the remote sensing process is achieved when the information about the target is extracted from the imagery in order to better understand it, reveal some new information, or assist in solving a particular problem.

 The first requirement for remote sensing is to have an energy source to illuminate the target (unless the sensed energy is being emitted by the target). This energy is in the form of electromagnetic radiation. 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.

 There is a lot of "invisible" radiation around humans, which can be detected by other remote sensing instruments and used to their advantage. The visible wavelengths cover a range from approximately 0.4 to 0.7 mm. The longest visible wavelength is red and the shortest is violet.

Blue, green and red are the primary colours or wavelengths of the visible spectrum. They are defined as such because no single primary colour can be created from the other two, but all other colours can be formed by combining blue, green, and red in various proportions. Other than the visible spectrum, the ultraviolet or UV portion (0.2 - 0.4 mm) and the infrared (IR) region and beyond, are extremely useful for remote sensing. Some earth surface materials, primarily rocks and minerals, fluoresce or emit visible light when illuminated by UV radiation. The infrared (IR) region covers the wavelength range from approximately 0.7 mm to 100 mm - more than 100 times as wide as the visible portion. The infrared region can be divided into two categories based on their radiation properties as follows:

·         Reflected IR

·         Emitted or thermal IR.

 The reflected IR covers wavelengths from approximately 0.7 mm to 3.0 mm.  The thermal IR region is quite different than the visible and reflected IR portions, as this energy is essentially the radiation that is emitted from the earth's surface in the form of heat. The thermal IR covers wavelengths from approximately 3.0 mm to 100 mm.  The portion of the spectrum of more recent interest to remote sensing is the microwave region from about 1 mm to 1 m. This covers the longest wavelengths used for remote sensing (CCRS, 1999).

 Type of sensors

Based on how the sensors measure energy sensors are divided into two types:

 Passive sensors: 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. 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 sensors: Active sensors on the other hand, provide their own energy source for illumination. The sensor emits radiation, which is directed towards the target to be investigated. The radiation reflected from the 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. Some examples of active sensors are laser fluoro-sensor and synthetic aperture radar (SAR).

 Electromagnetic energy may be detected either photographically or electronically. The photographic process uses chemical reactions on the surface of light-sensitive film to detect and record energy variations. The difference between the terms images and photographs in remote sensing is that an image refers to any pictorial representation, regardless of wavelengths or remote sensing device that has been used to detect and record the electromagnetic energy. A photograph refers specifically to images that have been detected as well as recorded on photographic film. All photographs are images, but not all images are photographs. Therefore, unless talking specifically about an image recorded photographically, the term image is used. 

A photograph could also be represented or displayed in a digital format by subdividing the image into small equal-sized and shaped areas, called picture elements or pixels, and each area representing the brightness with a numeric value or digital number. When a digital image of the original photograph is scanned, it is subdivided into pixels with each pixel assigned a digital number representing its relative brightness. The computer displays each digital value as different brightness levels. The information from a narrow wavelength range is gathered and stored in a channel, also referred to as a band.

 Indian Remote Sensing Satellites

The successful demonstration flights of Bhaskara 1 and Bhaskara 2 launched in 1979 and 1981, respectively, began a new era in the remote sensing development program in India. The Bhaskara satellites had a two-band TV payload for land applications and a satellite Microwave radiometer (SAMIR) for oceanographic/atmospheric applications. The spatial resolution of the TV payload imageries was about 1km and SAMIR about 125km. The first two IRS spacecraft, IRS-1A (March 1988) and IRS-1B (August 1991) were launched by Russian Vostok boosters. The two identical IRS spacecraft host Linear Imaging Self-Scanning (LISS) sensors working in four spectral bands: 0.45-0.52 µm, 0.52-0.59 µm, 0.62-0.68 µm, and 0.77-0.86 µm. The IRS-1C and IRS-P2 satellites were launched in 1993 and 1994 respectively. IRS-1C spacecraft carries three cameras: Panchromatic camera (PAN), Linear Imaging Self-Scanning sensor (LISS-3) and Wide Field sensor (WiFS). Panchromatic camera provides a spatial resolution of 5.8m and operates in single panchromatic spectral band (0.5-0.75mm). The LISS-3, multispectral system operates in four spectral bands, three in visible near infrared and one in short wave infrared (as B2, B3, B4 and B5).  LISS-3 provides ground resolution of 23.5m in VINR and 70.5m in SWIR. The WiFS camera operates between spectral bands 0.62-0.68mm and 0.77-0.86mm. Very recently (May 25) IRS P-4 (or Oceansat) was launched which is to be used mainly for oceanic studies.

 Remote Sensing Applications

Remote sensing has various applications due to its multiple-view approach of the information collection, which involves multi-stage, multi-temporal and multi-spectral sensing. In multi-stage remote sensing the data is collected from various platforms such as satellite data, high altitude, low altitude aircraft data and ground observations. Multi-temporal sensing involves collection of data of the same area at different times, to study the changes occurring with time. Multi-spectral data involves getting the data of the same site in various spectral regions of the spectrum. As a result this technique has become an essential tool in many developmental programs such as ocean resources, survey mapping and monitoring of land use/land cover,  study soils and in agriculture, forestry application, geology and geomorphology, urban land use, water resources and pollution level, watershed management, multipurpose river valley projects, environmental impact assessment and natural resource management models (which has its application in studying various natural resources in integrated study for sustainable development).

 Management of the environmental problems demands information on various aspects of the environment. Remote sensing technology helps handle and solve few of the problems. However, the technique does not provide the solution but gives an accessible pool of information to help solve these problems. Remote sensing is a multi-disciplinary activity which deals with the inventory, monitoring and assessment of natural resources through the analysis of data obtained by the observations from a remote platform. It can be defined as the science of deriving information about an object or phenomenon, from measurements made at a distance from the object without actually coming in to physical contact with it. The observations are synoptic, provide repetitive coverage of large areas and the data is quantifiable.

Although GIS depends heavily on remote sensing, remote sensing on its own cannot be called as true GIS, as it lacks strong geographic data management and analytical operations.

 Associated tools

GISs are closely related to several other types of information systems, but the ability to manipulate and analyse geographic data makes GIS technology on a platform of its own. Although there are no hard and fast rules about how to classify information systems, GIS can be differentiated from desktop mapping, computer-aided design (CAD), DBMS, and global positioning systems (GPS) technologies. 

DBMS
Database management systems specialise in the storage and management of all types of data including geographic data. DBMSs are optimised to store and retrieve data and many GISs rely on them for this purpose. They do not have the analytic and visualisation tools common to GIS.

 Desktop Mapping     

A desktop mapping system uses the map metaphor to organise data and user interaction. The focus of such systems is the creation of maps: the map is the database. Most desktop mapping systems have more limited data management, spatial analysis, and customisation capabilities. Desktop mapping systems operate on desktop computers such as PCs and smaller UNIX workstations.

 Computer Aided Drawing/Design (CAD)

CAD systems have been evolved to create designs and plans of buildings and infrastructure. This activity requires the components of fixed characteristics to be assembled to create the whole structure. These systems require few rules to specify how components can be assembled and very limited analytical capabilities. CAD systems have been extended to support maps but typically have limited utility for managing and analysing large geographic databases.