GIS or Geographic Information System is a computer based tool that uses components such as satellites, GPS, software, hardware and plotters to store, analyze and visualize geographically referenced data. It gives an image mostly in the form of map with attribute data on the spatial features represented in the image. GIS uses layers of different types of information and overlay them to add tiers of information to a geographic background. Even though there are various types of data integrated in these layers, most layers in GIS are spatial data as they are associated with specific locations on earth’s surface and are linked to attributes on the specified location.
The physical data integrated in GIS is collected by remote sensing. Remote sensing is the science, technology and art of obtaining information on the physical properties of various features or phenomenon taking place on earth’s surface by a recording device called sensor without being in physical contact with the desired feature of study. The sensors used in remote sensing collect information about objects on earth surface by sensing and recording energy emitted and reflected by the target. The raw data recorded initially may contain various errors and deficiencies which are further processed and enhanced for extraction of desired information and then analyzed and interpreted using GIS. Sensors are like cameras except they use infrared, microwave and ultraviolet regions of electromagnetic spectrum along with visible light. Passive and active sensors are used in remote sensing. Passive sensors depend on sunlight or other naturally emitted energy to illuminate the target and the reflected energy is recorded by the sensor whereas active sensor provide their own source of illumination and transmits electromagnetic energy towards the target and records the time and strength of the backscattered energy from the target.
Energy received by the sensors are divided into different range of electromagnetic radiation spectrum depending on the wavelength of energy received.
Sensors provide images of various resolutions which describes the number of pixels used to display the image. Higher resolution images are of crisper and cleaner as they contain higher number of pixels than low resolution images with low clarity. Spatial, spectral, temporal and radiometric resolution are the 4 types of resolutions in remote sensing.
Spatial resolution is the measure of the smallest object that can be resolved by the sensor and measures the number of pixels in a given area. Higher spatial resolution can provide clearer and more detailed images for interpretation. Spectral resolution is the ability of sensor to identify the specific wavelength of energy received and record within the electromagnetic spectrum.
Temporal resolution is the number of times a sensor can obtain the imagery of desired area and the time period within which the sensor revisit the area.
Radiometric resolution refers to the number of possible brightness values in each band of data. This is described by the number of bits into which the recorded energy is divided. Each bit records an exponent of power 2, therefore if a sensor use 8 bits to record energy they have 256 digital values available ranging from 0 to 255 brightness value. The sensor detects even very slight differences in emitted or reflected energy. The finer the radiometric resolution, the more sensitive it is to detecting small differences in energy.
GIS is mainly used for scientific investigation, resource management and development planning. It is a means of storing, retrieving, sorting and comparing spatial data provided by remote sensing to support analytical processes involved in management of current resources and planning for their use in future for further development. The data can also be used for identifying land covers and their management for sustainable future. Remote sensing and GIS is described as a key factor of disaster management as they can be effective in reducing the risk by identifying the hazard zone and cause of disaster. GIS can be used in mapping the hazard zone. GIS offers the control points on the ground, theme and training sites for location and mapping to remote sensing data while remote sensing offers rapid updates on changes detected in the spatial data used in GIS.