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A picture is worth a thousand words. Have you ever wondered why?
Picture concisely convey information about positions, sizes, and interrelationships between objects-by their nature, they portray spatial information that we can recognize as objects. These objects in turn tell a story that can convey a different kind of meaning. Human beings are good at deriving information from such images, so we experience little difficulty in interpreting even scenes that are visually complex because of our innate visual and mental abilities.
If you have heard the term "remote sensing" before you may have asked, "what does it mean?" It's a rather simple, familiar activity that we all do as a matter of daily life, but that gets complicated when we increase the scale. As you view the screen of your computer monitor, you are actively engaged in remote sensing.
A
physical quantity (light) emanates from that screen, which is a source of
radiation. The radiated light passes over a distance, and thus is
"remote" to some extent, until it encounters and is captured by a
sensor (your eyes). Each eye sends a signal to a processor (your brain), which
records the data and interprets this into information.
Specialized knowledge is important because remotely sensed images have qualities that differ from those we encounter in everyday experience:
Image Presentation
Unfamiliar scales and resolutions
Overhead views from aircraft or satellites
Use of several region of the electromagnetic spectrum.
Remote Sensing refers to the group of techniques of colleting information about an object & its surroundings from a distance without being in contact with it. This gives rise to some form of imagery which is further processed & interpreted to produce useful data for application in Agriculture, Archaeology, Forestry, Geography, Geology, Planning & other fields.
The prime objective of Remote Sensing is to extract environmental and natural
resources data related to our earth. Information about the object concerned is
conveyed to the observer through electromagnetic energy, which is the
information carrier & thus provides a communication link.
Electromagnetic Radiation is the form of energy transfer in the free space,
which exhibits both wave & particle properties.
Remote sensing has been variously defined but basically it is the art or science of telling something about an object without touching it.
Remote
sensing is the term currently used by a number of scientists for the study of
remote objects (earth, lunar, and planetary surfaces and atmosphere, stellar and
galactic phenomena, etc.) from great distances. Broadly defined?remote sensing
denotes the joint effects of employing modern sensors, data-processing
equipment, information theory and processing methodology, communications theory
and devices, space and airborne vehicles, and large-systems theory and practice
for the purpose of carrying out aerial or space surveys of the earth?s
surface. (National Academy of science 1970)
Remote
sensing is the science of deriving information about an object from measurement
made at distance from the object, i.e., without actually coming in contact with
it. The quantity most frequently measured in present-day remote sensing system
is the electromagnetic energy emanating from objects of interest, and although
there are other possibilities (e.g., seismic waves, sonic waves, a and
gravitational force), out attentions? is focused upon systems which measure
electromagnetic energy. (D.A. Landgrebe, in Swain & Davis, 1978)
imagery
is acquired with a sensor other than (or in addition to) a conventional camera
through which a scene is recorded, such as by electronic scanning, using
radiation outside the normal visual range of the film and camera-microwave,
radar, thermal, infrared, ultraviolet, as well as multispectral, special
techniques are applied to process and interpret remote sensing imagery for the
purpose of producing conventional maps, thematic maps, resources surveys, etc.,
in the field of agriculture, archaeology, forestry, geography, geology, and
others. (America Society of Photogrammetry).
Remote
Sensing is a technology for sampling electromagnetic radiation to acquire and
interpret non-immediate geospatial data from which to extract information about
features, objects, and classes on the Earth's land surface, oceans, and
atmosphere (and, where applicable, on the exteriors of other bodies in the solar
system, or, in the broadest framework, celestial bodies such as stars and
galaxies).
Remotely
sensed images are formed by many interrelated processes. An isolated focus on
any single component produces a fragmented picture. Therefore, our initial view
of the field can benefit from a broad perspective that identifies the kinds of
knowledge required for the practice of remote sensing.
Consider first the physical features, including buildings, vegetation, soil, water, and the like. These are the features that applications scientists wish to continue. Knowledge of the physical features resides within such specific disciplines as geology, forestry, soil science, geography, and urban planning.
Sensor data are formed as an instrument (e.g. a camera or radar) views
the physical features by recording electromagnetic radiation emitted or
reflected from the landscape. Although the image domain can consist of pictorial
images familiar to us all, often images are most useful in their digital forms,
which present information as numerical arrays that can be displayed and analyzed
by computers. For many of us, sensor data often seem to be abstract and foreign
because of their unfamiliar overhead perspective, unusual resolutions, and the
use of spectral regions outside the visible spectrum. As a result, the effective
use of sensor data requires analysis and interpretation to convert data to
information for addressing such practical problems as siting landfills or
searching for mineral deposits.
These interpretations create extracted information, which consists of
transformations of sensors data designed to reveal specific kinds of
information. The above figure demonstrates that the same sensor data can be
examined from alternative perspective to yield different interpretations.
Therefore, a single image can be interpreted to provide information about, for
example, soils, land use, or hydrography, depending on the specific image and
the purpose of the analysis.
Finally, we can proceed to the applications, in which the analyzed remote sensing data can be combined with other data to address a specific practical problem, such as land use planning, mineral exploration, or water quality mapping. When digital remote sensing data can be combined with other digital data, applications can be implemented in the context of a geographic information system (GIS), designed to bring varied data together in a format that allows efficient statistical and geographic analysis. For example, remote sensing data may provide accurate land use information that can be combined with soils, geologic transportation, and other information to guide the siting of a new landfill.
Remote
sensing takes one of the three basic forms depending on the wavelength of energy
detected and on the purpose of the study.
The
simplest is to record the reflection of solar radiation from the earth?s
surface. This is the kind of remote sensing most nearly similar to everyday
experience. For example, film in a camera records radiation from the sun
after it is reflected from the object of interest, regardless of whether one
uses a single hand-held camera to photograph a family scene or an aerial
camera to photograph a large area of the earth?s surface. This form of
remote sensing mainly uses energy in the visible and near infrared portions
of the spectrum.
The
third class of remote sensing instruments generates their own energy, and
then records the reflection of that energy from the earth?s surface. These
are active sensors- Active in the sense that they provide their own source
of energy and are independent of solar and terrestrial radiation. As an
everyday analogy, a camera with a flash attachment can be configured as an
active sensor. Active sensors are best represented by imaging radars, which
transmit a microwave signal towards the earth?s surface from an aircraft
or satellites, then use the reflected energy to form an image.
In
order to detect electromagnetic energy, which exists in a wide range of
wavelengths & frequencies, different varieties of remote sensing
instruments are applied.
Photographic camera is designed to detect energy in the visible and near
infrared portions of the electromagnetic spectrum. This had been made
possible with the use of suitable white-black or color films sensitized to
these spectral regions.
An important component of the photographic camera is a lens system, which
can cause images of points to be displaced on the photograph if lens
aberrations are present. Another important component of photographic remote
sensing systems is photographic film, which consists of a base and a thin
layer of chemical emulsion on top.
Electro-optical
Decoders
Electro-optical Decoders are transducers that transform electromagnetic
radiation into electrons or electrical signals from a scene viewed. The
detector elements behave like the grains of photographic film & produce
an analogue electrical record of each point of the scene scanned according
to the nature of incoming electromagnetic radiation.
Electro optical decoders are subject to unpredictable fluctuations in their
electrical output known as noise, which is analogous to granularity in the
photographic film.
Vidicon
Television Camera
The vidicon camera is a form of image tube, which can be regarded as the electronic counterpart of the photographic cameras, because its principle application is sensing in the visible and near infrared portions of the electromagnetic spectrum. It makes use of an optical system (camera lens) to focus an optical image onto a photosensitive surface of an electron gun which then converts the image into the electronic form, either as charge pattern on storage device or in non storing mode, traveling electrons.
Thermal
infrared scanner
It is a good example of the optical mechanical line scanners developed for
generating imagery outside the spectral region of the photographic film. The
characteristics of the line scanner is the multifaceted rotating mirror
inclined at 45 degree to the rotation axis, which scans the ground along the
lines perpendicular to the flight direction. The radiation emitted or
reflected from the ground is projected by the rotating mirror onto the
detector, which senses the intensity of the radiation in specific part of
electromagnetic spectrum & converts it into electrical signals. These
signals are used to modulate the intensity of the signal line cathode ray
tube to expose an image line on the photographic film, which is made to
advance.
Thus, as the sensor platform moves forward, successive new scan lines of the
ground surface are swept. Together continuous scan lines give a real time
image of the ground surface sensed.
Multispectral
Scanners
Multispectral scanners can sense 24 separate spectral channels
simultaneously; built such a large amount of spectral data normally requires
the use of high speed digital computer to help in analysis.
A
good example of multispectral scanner system employed in the Landsats 1,2,3
and 4, which is designed to provide images of the earth simultaneously in
four spectral bands.
Microwave imagers make use of the antennae to collect radiation information
in the microwave wavelength from the ground & are generally classified
into passive and active types:
Passive Microwave imagers
Passive
microwave sensing collects thermal mission from the earth?s surface in the
microwave spectrum. The commonly employed spectral bands are separated into
the microwave regions; each of which is given a letter designation.
The
distinct advantage of microwave sensors is their capability to penetrate
clouds, although water vapour and oxygen can still hamper them. These
sensors can detect emitted, reflected and transmitted radiation within the 1
mm to 300 mm wavelengths. The strength of passive microwave radiation
depends on the temperature and dielectric properties of the material rather
than the surface roughness.
Active microwave imagers
Active microwave remote sensing involves the sending out of pulse of
microwave energy to a target from the sensor and then measuring the
reflected signal. This method of sensing is commonly known as Radar (Radio
Detection and Ranging), which was rapidly developed during Second World War
for military application. It was designed to measure distances and
determining location of objects. Major advantage of Radar is its
all-weather, day & night operation ability which make its application
particularly valuable in areas with perennial loud covers where conventional
aerial photography cannot be applied.
Aircraft
Aircraft of various types provide exceptionally convenient and operationally
flexible platforms for remote sensing, carrying payloads from a few tens of
kilograms to many tonnes. With a suitable choice of vehicle a range of
altitudes can be covered from a few tens of meters, were atmospheric
propagation effects are negligible, to many thousands of meters, above most
of the earth?s atmosphere. The choice of flying altitude will obviously
have an impact on the scale, spatial coverage and spatial resolution of the
data collected.
The main disadvantage of aircraft as platforms for remote sensing when
compared with spacecraft are:
A typical airborne observing mission has duration of only a few hours, as compared with few years for a space borne mission. This means that it is much more difficult to provide continuity of data for example, for ten year monitoring program.
Since airborne observations are acquired from much lower altitudes than space borne observations, the spatial coverage of the data is smaller and airborne observations are obviously unsuitable for studying large areas.
Since
aircraft necessarily operate within the earth?s atmosphere, and the
atmosphere is in motion, neither the position nor the motion of the aircraft
may be exactly what was intended.
Spacecraft
Spacecrafts are suitable sensor
platforms because they have overcome the difficulties of the ceiling limits
7-operation duration. The use of spacecraft orbiting regularly around the
earth from the heights of several hundreds of kilometers makes regular
surveillance of the earth with suitable remote sensing devices possible. The
term spacecraft refers to rockets, artificial satellites or manned space
vehicles.
Rockets
Rockets are used as platforms for photographic system, but in most cases the
photography acquired was only used for recording the
orientation of the rocket in flight. The photographs are fairly clear &
cloud free because the rockets were usually fired in the period of
fine weather. But the coverage of this rocket photographs was very
restricted because the rocket has to be fired vertically upward and it had
to land not far from its launching site. Also the rocket went up & down
so fast that the camera had at most, only 8 minutes to take photographs
above the earth.
Earth Satellite
The earth
satellite is an artificial object in speed, which revolves around the earth
following a specific orbit. Unlike an orbit, a satellite can stay aloft for
a much longer period of time, thus permitting constant surveillance of the
earth. Earth satellites can be conveniently classified, according to their
orbital characteristics, into 3 types:
1.
Those with equatorial geosynchronous orbits
2.
Those with near polar sunsynschronous orbit and
3.
Those with general orbits.
Landset, SPOT and other systems establish the technical and commercial value of the land observation satellite concept in the 1970s and 1980s. But they also revealed the high costs in the technical challenges of designing and operating a system intended to provide general-purpose data for a broad community of users who may have diverse requirements. So the need to operate smaller, special purpose earth observation satellites called micro satellites focused on the requirements of specific groups of users, thereby decreasing the costs of operation.
Manned Spacecraft
With manned spacecraft, more direct human control is possible in obtaining
imagery of the earth from space. The space shuttle is much more flexible
spacecraft and provides an ideal orbital platform for collecting remote
sensing data & for testing advanced sensor systems because of its
ability to carry the instruments into space and return them to the ground
for recalibrating and refurbishing.
For example, the
image given below taken from remote platform, represents land area with
different features like vegetation, water, buildings etc.
Tasks
The image interpreter must routinely conduct several kinds of tasks, many of which may be completed together in an integrated process.
Classification
Classification is the assignment of objects,
features or areas to classes based on their appearance on the imagery. Often the
distinction is made between three levels of confidence and precision. Detection
is the determination of the presence or absence of a feature recognition implies
a higher level of knowledge about a feature or object such that the object can
be assigned an identity in a general class. Finely, identification means that
the identity of an object or feature can be specified with enough confidence and
detail to place it specific class.
Enumeration
Enumeration is the task of listing or counting discrete
items visible on an image.
Measurement
Measurement is an important function in image
interpretation problems, which can be of two kinds. First is the measurement of
distance and height, and by extension volumes and areas as well. A second form
of measurement is quantitative assessment of image brightness.
Delineation
Finally the interpreter must often delineate or outline, regions as they are observed on remotely sensed images. The interpreter must be able to separate distinct aerial units that are characterized by specific tones and textures and to identify edges or boundaries between separate areas.
Approaches
Manual Approach
In order to extract meaningful information out of the collected data, one has to exercise one?s judgments to sieve the significant features out of the insignificance. This is the first stage of image interpretation known as detection.
The second step is recognition and identification, in which the image interpreter has to exercise general, local as well as specific levels of reference to allocate objects into known categories. In recognition & identification, the non-geometric image characteristics of tone or color, texture, pattern, shape, shadow, size and situation normally gives clues.
The result of identification & recognition is a list of objects and features in the area. These form the basis for the delineation of areas having homogeneous observable patterns and characteristics. This is analysis stage. Each area so delineated has to be classified through education. The accuracy is then evaluated by field checks.
Therefore, the final stage of interpretation is classification, producing spatial data, which can be displayed as maps or for incorporation into a geographic information system by computer.
Computer
Assisted Approach
The manual approach suffers from its inability to deal quickly with a large quantity of image data. This weakness is particularly evident when Multispectral scanner imagery or multiband photography is to be analyzed.
The computer-assisted approach involves a number of steps. First of all, the analogue image data have to be converted into digital form. This is done by means of digitizing TV scanner or a microdensitometer.
The second step is data preprocessing, which is a group of procedures to clean up the raw data, such as correcting geometric and radiometric distortions.
This is followed by feature extraction. The types of features or measurements necessary to classify the image data are selected in this stage. Possible features are spatial, spectral a temporal. For supervised classification, sample areas are selected from the image data for more detailed examination. Another approach is unsupervised classification in which no training samples are used. The partitioning of the feature space is carried out by the method of cluster analysis, which can identify natural groupings of patterns. The nature of each grouping is determined afterwards by field checks.
The result of classification from the computer assisted approach and is output as line printer maps or cathode ray tube displays. Numerical information on the area of the mapped classes or the frequency of occurrence of each class and other useful statistical data can also be displayed by the computer if required.
Elements
Image
tone
Image
tone denotes the lightness or darkness of a region within an image. For black
& white images tone may be characterized as light, medium gray, dark gray,
dark and so on. For color image it refers simply to color.
Image texture
Image texture refers to the apparent roughness or
smoothness of an image region. Usually it is caused by the patent of highlighted
of shadowed areas as an irregular surface is illuminated from an oblique angle.
Shadow
Shadow is an important clue in
the interpretation of objects. It may reveal characteristics of the size or
shape of an object.
Pattern
Pattern
refers to the arrangement of individual objects into distinctive, recurring
forms that permit recognition on aerial imagery.
Association
Association
specifies the occurrence of certain objects or features usually without the
strict spatial arrangement implied by pattern.
Shape
Shapes of features are obvious clues
to their identity. They provide basis for identification.
Size
The size of an object is relative in
relation to other objects on the image, which provides the interpreter with an
intuitive notion of the scale and resolution of an image.
Site
Site refers to topographic positions.
The
Human Population
Remote sensing data faithfully record the interactions of man with his environment at different levels of sophistication according to the scale and the sensor type applied. And outgrowth from this application is to device methods to estimate population numbering an area, which is essential information, require estimating the spatial distribution of population when planning the economic development of a region or a country.
Cloud Classification
Satellite images record cloud patterns from which one can deduce the three-dimensional structure of wind and pressure fields. If the clouds at different levels can be distinguished, variations in the direction & speed of the airflow at different levels in the troposphere can be detected.
Rainfall
Estimation
Visible and thermal infrared satellite images of clouds provide a means to estimate rainfall. This is based on the assumptions that high loud brightness as observed from visible images normally implies a greater probability of rain and low cloud-top temperatures as detected from infrared images also means a greater chance of rain.
Water
vapors and wind field analysis
The moisture content of the atmosphere is an important parameter, which can be measured from satellite images, which in turn helps in determining wind speed and wind direction. It also enables prediction of other weather phenomena as tornadoes, thunderstorms, hailstorms, hurricanes or typhoons.
The biosphere: vegetation, crops and soils
In the study of vegetation, crops and soils, there is invariably the need to carry out surveys with a view to discover their spatial distribution, structure and type. This information is indispensable for the purpose of management in agriculture and forestry, for informed decision-making in planning, for feasibility studies in land development projects and many engineering works. It permits homogeneous units to be defined and their extent delimited. All these information requirements can be effectively met with the use of conventional and modern remote sensing techniques coupled with minimal ground survey.
Land
use and Land cover
Land use and cover data are most essential to planners who have to make decisions concerning land resource management, they are strongly economic in nature.
Hydrosphere
The ocean is a dominant member of the hydrosphere. The ocean affects our climate and is in turn affected by the atmosphere, thus forming the major source of atmospheric atmosphere. It collects the detritus of man and nature and is also an important source of petroleum. Its currents are used to dispense sewage and waste. It is also an important source of food as well as delightful area of recreation for people. But it is difficult in obtaining horizontal and vertical measurements at frequent intervals. In recent years development in space remote sensing techniques have provided a better understanding of hydrosphere. It can also be used to detect seawater color, fish stock and aquatic plants, coastal water turdity, salinity and water quality.
After operating two coarse-resolution remote sensing satellites in the
1970s and 1980s, India began to develop multispectral remote sensing programmes
in the style of the Landset system. During the early 1990s, two Indian remote
sensing satellites were in service, IRS-1A (launched in 1988) and IRS-1B
(launched in 1991).
A major project has been taken
up for Biodiversity Characterization at Landscape Level to prepare Biological
zone maps and establishment of disturbance gradient for important bio-diversity
rich areas of the country using remote sensing and GIS. RRSSCs are
actively involved in the project both in database creation and providing
software solutions under image processing and GIS domain. The project is aimed
at prioritising areas for bio-prospecting and conservation.
? Rajiv Gandhi National Drinking Water Mission
This is a national mission with an objective of creating scientific database for
ground water using remote sensing technology. RRSSCs are involved in the
generation of precision products and ground water prospect maps at 1:50,000
scale for the states of Kerala, Karnataka, Andhrapradesh, Madhyapradesh and
Rajasthan.
?
Crop Acreage and Production Estimation
This is an important national mission wherein remote sensing techniques are used
in providing pre-harvest estimates on crop acreage for major crops in various
states in the country. RRSSCs have been actively involved with Space
Application Centre, Ahmedabad in providing software solutions through a package
"CAPEWORKS". The package is operationally being used in all ISRO
workcentres and various State remote sensing centres regularly during the
cropping seasons to derive the necessary information related to crop acreage.
RRSSCs are actively involved in
watershed development related studies in the country. IMSD project has paved the
way for scientific approach for planning and implementation of certain action
plans to improve the land productivity and water resources in a given watershed.
RRSSCs are actively involved at national level in monitoring/evaluation of
watersheds treated under NWDPRA scheme using multitemporal remote sensing data.
Methodology for operationally executing such a project was developed within
RRSSCs on a pilot mode and the same has been operationally utilized for the
project.
?
Disaster Management System
Flood damage Assessment
RRSSC Kharagpur, one of the regional centres, is well located to provide
quick information related to flood and cyclone related disasters. The
centre is actively involved in generating such information using remote sensing
and GIS techniques. RRSSC are actively involved in creating digital
databases for the flood-prone region of Assam and developing information
system for decision making for effective management of disaster. The
methodology can be replicated for other flood-affected areas in due course of
time.
?
Study of potential and actual area under
sericulture through Remote Sensing
Remote sensing
techniques have been proved to be useful in studies related to sericulture,
which basically refers to identification of mulberry growing areas. The
technique has proved to be very successful and cost effective in the country.
RRSSCs are currently involved in a national mission on the project.
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