Remote sensing: principles, electromagnetic spectrum, components and applications

Remote sensing: principles, electromagnetic spectrum, components and applications

Remote sensing, also called earth observation, refers to obtaining information about objects or areas at the Earth’s surface without being in direct contact with the object or area. Humans accomplish this task with aid of eyes or by the sense of smell or hearing; so, remote sensing is day-today business for people. Reading the newspaper, watching cars driving in front of you are all remote sensing activities. Most sensing devices record information about an object by measuring an object’s transmission of electromagnetic energy from reflecting and radiating surfaces.

Principles of remote sensing

Detection and discrimination of objects or surface features means detecting and recording of radiant energy reflected or emitted by objects or surface material. Different objects return different amount of energy in different bands of the electromagnetic spectrum, incident upon it. This depends on the property of material (structural, chemical, and physical), surface roughness, angle of incidence, intensity, and wavelength of radiant energy.

The Remote Sensing is basically a multi-disciplinary science which includes a combination of various disciplines such as optics, spectroscopy, photography, computer, electronics and telecommunication, satellite launching etc. All these technologies are integrated to act as one complete system in itself, known as Remote Sensing System. There are a number of stages in a Remote Sensing process, and each of them is important for successful operation. Progressive stages in remote sensing are as follows:

Emission of electromagnetic radiation, or EMR (sun/self- emission)
Transmission of energy from the source to the surface of the earth, as well as absorption and scattering
Interaction of EMR with the earth’s surface: reflection and emission
Transmission of energy from the surface to the remote sensor
Sensor data output
Data transmission, processing and analysis

At temperature above absolute zero, all objects radiate electromagnetic energy by virtue of their atomic and molecular oscillations. The total amount of emitted radiation increases with the body’s absolute temperature and peaks at progressively shorter wavelengths. The sun, being a major source of energy, radiation and illumination, allows capturing reflected light with conventional cameras and films.

Electromagnetic radiation and Electromagnetic spectrum

EMR is a dynamic form of energy that propagates as wave motion at a velocity of c = 3 x 1010 cm/sec. The parameters that characterize a wave motion are wavelength (λ), frequency (ν) and velocity (c). The relationship between the above is:

c = νλ.

Electromagnetic energy radiates in accordance with the basic wave theory. This theory describes the EM energy as travelling in a harmonic sinusoidal fashion at the velocity of light. Although many characteristics of EM energy are easily described by wave theory, another theory known as particle theory offers insight into how electromagnetic energy interacts with matter. It suggests that EMR is composed of many discrete units called photons/quanta. The energy of photon is

Q = hc / λ = h ν

Where Q is the energy of quantum

h = Planck’s constant

Components of Remote Sensing System

Although, the remote sensing includes a wide array of technologies and types, but they all are based on certain common concepts with the same basic components. The basic components of remote sensing system are given below:

Target
Energy source
Transmission path, and
Satelite sensor

The target is the object or material being studied. All the components in the system work together, to measure and record the information about the target without making physical contact. The energy source illuminates or provides electromagnetic energy to the target.

The energy interaction with the target depends on the target properties and the radiation. It also acts as a medium for transmitting the information from target to the sensor. The sensor is a remote device to collect and record the electromagnetic radiation.

Sensors are also used to measure the given-off energy or emitted energy by the target; reflected-off energy of the target; or transmitted energy from the target. After recording of energy, the resulting set of data is transmitted to the receiving station.

At receiving station, the data is processed to a usable format, i.e., in the form of image. The image is then interpreted to extract the informations about target. The interpretation of image can be done visually or electronically with the help of computers and image processing softwares.

Remote Sensing Applications

Land Use Mapping

Remote sensing data is useful in obtaining up-to-date land use pattern of large areas at any given time and also monitor changes that occur from time to time. It can be used for updating road maps, asphalt conditions, and wetland delineation. This information is used by regional planners and administrators to frame policy matters for all-round development of the region.

Weather Forecasting

Remote sensing is extensively used in India for weather forecasting. It is also used to warn people about impending cyclones.

Environmental Study

It can be used to study deforestation, degradation of fertile lands, pollution in atmosphere, desertification, eutrophication of large water bodies and oil spillage from oil tankers.

Study of Natural hazards

Remote sensing can be used to study damages caused by earthquakes, volcanoes, landslides, floods and melting of ice in polar regions. Many times remote sensing will be helpful to predict the occurrence of natural hazards.

Resource exploration

Remote sensing data is helpful for updating existing geological maps, rapid preparation of lineament and tectonic maps, identifying the sites for quarrying the minerals and helpful in locating fossil fuel deposits.

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.

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.

Remote sensing is used in numerous fields, including geography, land surveying and most Earth Science disciplines for example, hydrology, ecology, oceanography, glaciology, geology.It also has military, intelligence, commercial, economic, planning, and humanitarian applications.

GIS

Geographic Information System (GIS) is a computer based application of technology involving spatial and attributes information to act as a decision support tool.

It keeps information in different layers and generates various combinations pertaining to the requirement of the decision-making. In the recent times, GIS has emerged as an effective tool in management of disasters since, geo-spatial data and socio-economic information need to be amalgamated for the better decision making in handling a disaster or to plan for tackling a disaster in a better way.

Applications:

Disaster Management

The different line departments and agencies who are stakeholders in the disaster management process could utilize GIS. Some basic hardware like computer system, printer, network systems, along with GIS software is required to set up the GIS in any organisation.

Objectives:

The prime objectives of developing the GIS database are to help disaster managers at State, District and Block level for:

i) Pre-disaster planning and preparedness
ii) Prediction and early warning

iii) Damage assessment and relief management

GIS combines layers of information on various themes to enable the managers to take the most appropriate decisions under the given circumstances. For disaster management, a GIS database could be a useful managerial tool for various reasons, some of which are as under:

Disaster Managers could generate maps both at micro and macro level indicating vulnerability to different extents under different threat perceptions.
Locations likely to remain unaffected or remain comparatively safe could be identified.
Alternate routes to shelters, camps, and important locations in the event of disruption of normal surface communication could be worked out.
Smooth rescue and evacuation operations could be properly planned.
Rehabilitation and post-disaster reconstruction works could be properly organized.
Locations suitable for construction of shelters, godowns, housing colonies, etc. can be scientifically identified.
Areas where no construction should be taken up or existing habitations require relocation could be identified.

Hydrology

Remote sensing of hydrologic processes can provide information on locations where in situ sensors may be unavailable or sparse. It also enables observations over large spatial extents. Many of the variables constituting the terrestrial water balance, for example surface water storage, soil moisture, precipitation, evapotranspiration, and snow and ice, are measurable using remote sensing at various spatial-temporal resolutions and accuracies. Sources of remote sensing include land-based sensors, airborne sensors and satellite sensors, which can capture microwave, thermal and near-infrared data or use LIDAR.

Weather forecasting and Ecology

Many ecological research projects would benefit from the creation of a GIS to explore spatial relationships within and between the data. In particular, while some projects can be done without using a GIS, many will be greatly enhanced by using it (click here for some examples of research projects which have used GIS).

The very act of creating a GIS will make you think about the spatial relationships within your data, and will help you formulate hypotheses to test or suggest new ones to explore. In addition, thinking about your data in a spatial manner will help you identify potential spatial issues and/or biases with your data.

GIS can also be used to make measurements and carry out calculations which would otherwise be very difficult. For example, a GIS can be used to work out how much of your study area consists of a specific habitat type, or how much of it is over 1,000m high, or has a gradient greater than 5º, and so on. Similarly, a GIS can be used to calculate the size of the home range of an individual or the total area occupied by a specific species or how long your survey tracks are, or how much survey effort was put into different parts of your study area.

GIS can also be used to link data together in the way that is needed for statistical analysis. For example, many statistical packages require all your data to be in a single table, with one line per sample and then information about that sample and the location where it came from in different columns or fields. A GIS provides you with a way to easily create such tables and populate it with information, such as the altitude at each location, the gradient of slope and the direction it faces, from other data sets. This makes preparing your data for statistical analysis much simpler.

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