Remote sensing is defined as the total processes used to acquire and measure the information of objects and phenomena of the earth's surface and atmosphere using sensors onboard airborne or space borne platforms. The objects and phenomena concerned are not in direct contact with the sensor.
Four basic classes of remote sensors have been recognised. These are radiometer, audiometer, magnetometer, and gravimeter.
The term remote sensing was first used in the early 1960s. There is a medium of transmission involved in the remote sensing.
Electromagnetic spectrum has special significance in remote sensing. Electromagnetic radiation can be described as wave form of energy. Remote sensing makes extensive use of the electromagnetic waves like ultraviolet, visible, infrared, microwave and radar portions of spectrum.
The basic processes that help in the collection of information about the properties of the objects and phenomena of the earth surface are as follows:
Source of Energy, Transmission of energy from the source to the surface of the earth, Interaction of energy with the earth’s surface, Propagation of reflected or emitted energy through atmosphere, Detection of the reflected or emitted energy by the sensor, Conversion of energy received into photographic or digital form of data, Extraction of the information contents from the data products, and Conversion of information into Map/Tabular forms.
The numerous remote sensing devices which detect and record energy may be divided into active and passive systems.
In satellite remote sensing, the Multi Spectral Scanners are used as sensors. These are of two types: Whiskbroom Scanners and Pushbroom Scanners.
Three types of resolutions are used by remote sensors for extracting information pertaining to different terrains. These are spatial resolution, spectral resolution and radiometric resolution.
Remotely sensed data products may be broadly classified into photographic images and digital images.
The images or data obtained from the sensors is used for extraction of information of the earth’s surface. The process of identifying objects by reading the images is called visual interpretation. The elements of visual interpretation are: tone or colour, texture, size, shape, shadow, pattern and association.
Remote sensing is inter-disciplinary and is concerned with several branches of science. The remote sensing techniques have proved to be of great help in the quantitative geographic works for reasons of providing accurate and precise data about special phenomena.
Remote sensing is an important tool for acquisition of data in India. Several Indian organisations make extensive use of remote sensing imageries and have made progress in their respective fields.