GIS AND REMOTE SENSING

T]NIT.1
BASICS OF REMOTE SENSING

Conventional geological and other field expeditions will always remain the best methods of data
collection. However, modern geospatial tools like remote sensing can make such investigations
rapid and more comprehensive. In the last four decades, remote sensing and GIS tools have been
extensively used for better mapping, monitoring and decision-making tasks in many disciplines.
Therefore, a systematic study is needed to understand the physical principles behind remote
sensing technology to efficiently use it for different earth resources applications.

As the name suggests, remote sensing is a method of collecting information about any ground
object under investigation from a distance without being in contact. There are many definitions
found in the literature

Remote Sensing can be defined as "The art, science and technology of obtaining reliable
information about physical objects and the environment, through the process of recording,
measuring and interpreting imagery and digital representations of energy patterns derived from

non-contact sensor systems." Dr. MOHD. MINHAJUDDIN AQUTL

Remote Sensing is defined as "the science cs of objects

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, analyzing, and applying that information". Remote sensing is a
method for getting information about of different objects on the planet, without any physical
contacts with it.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 DN MOHD. MINHAJUDDIN AQUIL
M.E.(Irarn tlon Engg')' Pho

In much of remote sensing, the process involves an interaction between incident radiation and the
targets of interest. This is exemplified by the use of imaging systems where the following seven

elements are involved. Note, however that remote sensing also involves the sensing of emitted
energy and the use of non-imaging sensors.

D
x..

I
*
B
t
I

Figure I Remote sensing

1. Energy Source or Illumination (A) - the first requirement for remote sing is to have
an energy source which illuminates or provides electromagnetic energy to the target of
interest.

2. Radiation and the Atmosphere (B) - as the energy travels from its source to the target, it
will come in contact with and interact with the atmosphere it passes through. This
interaction may take place a second time as the energy havels from the target to the sensor.
3. Interaction with the Target (C) - 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.

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

5. Tbansmission, Reception, and Processing (E) - 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).

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

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 7. Application (G) - the final element of the remote sensing process is achieved when we
apply the information, we have been able to extract from the imagery about the target in
order to better understand it, reveal some new information, or assist in solving a particular
DT. MOHD. MINHAJUDDIN AQUI L
problem.
Ar,""i:;ymE ')'Pho

Dec
w
Although, the first space photography of the -6 in 1959, the
background of remote sensing expeditions started long before, perhaps through Aristotle's (Circa

336-323 BC) philosophy about the nature of light and Sir Isaac Newton's Principia summarizing

basic laws of mechanics. In 1826, Joseph Nicephore Niepce took the first photographic image;

however, the first aerial photograph from balloon was taken by G F Tournachon in 1958. With the

invention of airplane by Wright Brothers (1903), aerial photography received a new pair of wings.
Photoreconnaissance survey was widely used during World War I (1914-18) and II (1939-45).
During this period, the aerial photography and photogrammetric techniques had advanced
manifold and paved the way for civilian use. Meanwhile, several space photography missions were
successfully launched by the US and Russia in the 60's viz. Mercury program (1960), Gemini
Mission (1965), Apollo Program (1961), Corona (1960-72). In this period, sensors were also
developed for Earth Observations for meteorological purposes (TIROS- I , ITOS, NOAA, etc.). The
payload ofNOAA satellites were modified and the first-ever Earth Resources Technology Satellite

(ETRS-l) was launched in 1972,later renamed as Landsat-I, and thus, began a new era in the
history of remote sensing.

During the 1980's, with successive launces of Landsat Series with Multi-spectral Scanner (MSS)
and Thematic Mapper (TM), extremely valuable data about land surface were collected worldwide

and spectral behavior of rock, mineral and vegetation were thoroughly studied. At the same time,
space shuttle programs were initiated for radar imaging and applications (SIR-A, SIR-B etc.).

Hyperspectral sensing systems were also introduced with potential of lithological mapping. With
improvement of electronic technology, many other countries including Germany (MOMS), French
(SPOT series), Japan (MOS) and China-Brazil series joined hands with USA in space remote

sensing experiments. In 1988, India also joined the league with IRS - 1A satellite and the legacy

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 continued' In the 1990's, many improvements of sensor techaology were seen with better spatial,
spectral, radiometric and temporal resolutions. In 1995, IRS-IC was launched with 5 m spatial
resolution' By the end of the decade, private agencies entered the race with better spatial (Ikonos
lm; Quickbird 60cm) and radiometric resolution, of remote sensing
increasing the potential
applications. DT. MOHD. MINHAJUDDTN AQUIL
M-E.[ranaPo Engg')' PtC

As was noted in the previous section, the 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. Electro-magnetic radiation which is reflected or emitted

from an object is the usual source of remote sensing data. However, an! media such as gravity or
magnetic fields can be utilized in remote sensing. A device to detect the electro-magnetic radiation

reflected or emitted from an object is called a "remote sensor" or "sensor". Cameras or scanners
are examples of remote sensors. A vehicle to carry the sensor is called a "platform,'. Aircraft or
satellites are used as platforms. The technical term "remote sensing" was first used in the United

Statesin the 1960's, and encompassed photogrammetry photo-interpretation, photo-geology etc.

Since Landsat-l, the first earth observation satellite was launched,in 1972, remote sensing has

become widely used' The characteristics of an object can be determined, using reflected or emitted

electro-magnetic radiation, from the object. That is, "each object has a unique and different
characteristic of reflection or emission if the type of deject or the environmental condition is
different. "Remote sensing is a technology to identiff and understand the object or the
environmental condition through the uniqueness of the reflection or emission.

Figure 2 Data collection by remote sensing

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 This concept is illustrated in Figure 2 while Figure 3 shows
the flow of remote sensing, where
three different objects are measured by a sensor in a limited
number of bands with respect to their,
electro-magnetic characteristics after various factors have affected
the signal. The remote sensing
data willbe processed automatically by computer ard/or manually interpreted
by humans, and
finally utilized in agriculfure, land use, forestry, geology, hydrology,
oceanography, meteorology,
environment etc.

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Figure 3 Ftow of remote sensing

Electro-magnetic radiation is a carrier of electro-magnetic

energy by transmitting the oscillation
of the electro-magnetic field through space or matter. The transmission
of electro-magnetic
radiation is derived from the Maxwell equations. Electro-magnetic
radiation has the characteristics
of both wave motion and particle motion.
(l) Characteristics as wave motion
Electro-magnetic radiation can be considered as a transverse
wave with an electric field and a
magnetic field' A plane wave for an example as shown
in Figure 4 has its electric field and
magnetic field in the perpendicular plane to the transmission
direction. The two fields are located
at right angles to each other. The wavelength(1,), frequency(v)
and the velocity(u) have the
following relation.

.----Equation 1

L vsrqtu,
-- --

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Figure 4 Electromagnetic radiation

Electro-magnetic radiation is transmitted in a vacuum of free space with the velocity of light c,
( = 2.998 x 108 m/sec) and in the atmosphere with a reduced but similar velocity to that in a

vacuum. The frequency n is expressed as a unit of hertz (Hz), that is the number of waves which
are transmit[ed in a second.

(2) Characteristics as particle motion

Electro-magnetic can be treated as a photon or a light quantum. The energy E is expressed as

follow. Dr. MOHD. MINHAJUDDIN AQUIL

E: h,
, :can College of Engg. & Technology
Deocan Technolt

The photoelectric effect can be explained by considering the electro-magnetic radiation as

composed of particles. Electro-magnetic radiation has four elements of frequency (or wavelength),

transmission direction, amplitude and plane of polarization. The amplitude is the magnitude of
oscillating electric field. The square of the amplitude is proportional to the energy transmitted by
electro-magnetic radiation. The energy radiated from an object is called radiant energy. A plane
including electric field is called a plane of polarization, When the plane of polarization forms a
uniform plane, it is called linear polarization. The four elements of electro-magnetic radiation are
related to different information content as shown in Figrne 5. Frequency (or wavelength)
corresponds to the color of an object in the visible region which is given by a unique characteristic

curve relating the wavelength and the radiant energy. In the microwave region, information about

objects is obtained using the Doppler shift effect in frequency that is generated by a relative motion

between an object and a platform. The spatial location and shape of objects are given by the

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
linearity of the transmission direction, as well as by the amplitude. The plane of polarization is
influenced by the geometric shape of objects in the case of reflection or scattering in the microwave

region. In the case of radar, horizontal polarization and vertical polarization have different
responses on a radar image.

ip,rlixl l(riti(,n

Figure 5 Information derived from elements of electromagnetic radiation

All objects of the earth's surface (at 300 Kelvin) like, soil, rock, vegetation, etc. above absolute

zero (-273" Centigrade or 0 Kelvin) emit electromagnetic energy. And so does the Sun (at 6000

Kelvin). Sun is the major source of energy required for remote sensing pu{pose (except radar and
sonar). The energy is transferred by electromagnetic radiation through the vacuum between the

Sun and the Earth at the speed of light. It interacts with the atmosphere before coming into contact
with the earth's surface. While returning, it interacts with the atmosphere once again and finally
reaches the remote sensor. The detectors or photographic film system onboard records this
reflected or emitted energy in analogue or digital form Figure 6

Dn MOHD. MINHAJUDDIN AQUIL

tLE. ErEg.), PhO

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Figure 6 Overyiew of Remote Sensing Data Collection
The electromagnetic radiation principle conceptuali zedby James Clerk Maxwell
in 1960, refers to
all energy that moves with the velocity of light (300,000 km per second) in a'harmonic
wave
pattern. The electromagnetic wave consists of two fluctuation fields
- one electrical and the other
magnetic at the right angle to one another. Both are also perpendicular to
the direction of travel
Figure 7.The electromagnetic waves are characterized by its wavelength (i.e.
the distance from
any point on one cycle or wave to the same position on the next cycle or
wave measured in
micrometer, pm) and frequency (number of cycles or waves pass through a point per
second). The
frequency is inversely proportional to the wavelength.
UDDIN AQUIL

Figure 7 Components of Electromagnetic wave

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
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It rn':rtlX lA':rr1"l42yth ! i r ra ror -,

Figure 8 Wavelength ranges in electromagnetic spectrum

The basic principle involved in remote sensing methods is

that in different wavelength ranges of
the electromagnetic spectrum Figure 8, each type of object reflects
or emits a certain intensity of
light, which is dependent upon the physical or compositional
attributes of the object. Hence, the
spectral behavior (i'e. the intensity of light emitted or reflected
by the objects) of same ground
object in different wavelength ranges may be studied through
Spectral signature curves. Such
curyes may help to differentiate different types of objects,
e.g., soil, vegetation, waterbody,
settlements, etc' and map their distribution on the ground Figure
9. The remote sensing missions
are, thus, aprocess of collection of spectral information
of the ground objects, enhancement and
interpretation for different applications. Further discussion
on the electromagnetic energy and its
interaction with earth surface features is discussed in the
next module.

Dn, MOHD. M HAJUDDIN AQUIL

u.E .). PhD

Dep E

Dar-us-Sal ad-01, T.S

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Dr. MOHD.
MrNH, A_JUDDINAQUTT-
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Associor-'i::i-'t'o'
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gg

0.5 a.7 1 1 1.3 ?.5 1 7 1.9 2"1

Wavolength (;rm)
tr'igure e Spectrar signature
are shown in the
rt:['.'":i:[il]=-fr'rtt ;l'#]i.3:if-:T:LXorLandsaGT

An irnage refers to any pictorial representation, regardless of what wavelengths or remote sensing
device 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. The
black
and white photo to the left, of part of the city of Ottawa, Canadawas taken in the visible part
of
the spectrum. Photos are normally recorded over the wavelength range from 0.3 pm
to 0.9 pm -
the visible and reflected infrared. Based on these definitions, we can say that
all photographs are
images, but not all images are photographs. Therefore, unless we are talking specifically
about an
image recorded photographically, we use the term image.

1 7t' lat 85 22 a

1'Je, 17 17A 11lt 6A

2?. 0 I ?55

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:{r ?.-aA

fi w 1,9

85 'l'!s 221 ? 36

tr'igure 10 Figure ll Digital format

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 A photograph could also be represented and displayed in a digital format by subdividing the image
into small equal-sized and shaped areas, called picture elements or pixels, and representing the
brightness of each area with a numeric value or digital number. Indeed, that is exactly what
has
been done to the photo to the left. In fact, using the definitions we have just discussed, this is
actually adigital image of the original photographl The photograph was scanned and subdivided
into pixels with each pixel assigned a digital number representing its relative brightness. The
computer displays each digital value as different brightness levels. Sensors that record
electromagnetic energy, electronically record the energy as an a11ay of numbers in digital format

right from the start. These two different ways of representing and displaying remote sensing data,
either pictorially or digitally, are interchangeable as they convey the same information (although

some detail may be lost when converting back and forth).

In previous sections we described the visible portion of the spectrum and the concept of colours.
We see colour because our eyes detect the entire visible range of wavelengths and our brains
process the information into separate colours. Can you imagine what the world
would look like if
we could only see very narrow ranges of wavelengths or colours? That is how many sensors
work.
The information from a nalrow wavelength range is gathered and stored in a channel, also
sometimes referred to as a band. We can combine and display channels of information
digitally
using the three primary colours (blue, green, and red). The data from each channel represented
is
as one of the primary colours and, depending on the relative brightness (i.e. the
digital value) of
each pixel in each channel, the primary colours combine in different proportions to represent
different colours.
Dr. MOHD. MINHAJUDDIN AeUIL

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Figure 12 Display
When we use this method to display a single channel or range of wavelengths, we are actually
displaying that channel through all three primary colours. Because the brightness level of each
pixel is the same for each primary colour, they combine to form a black and white image,showing
various shades of gray from black to white. When we display more than one channel each as a
different primary colouq then the brightness levels may be different.

All objects whose temperature is greater than an absolute zero (273"k), emit radiation. All stars
and planets emit radiation. Our chief star, the sun is almost a spherical body with a diameter of
1 .39 x 106 km at a mean distance from the earth equal to 1 .5 x 108 km. The continuous conversion
of hydrogen to helium which is the main constituents of the sun, generates the energy that is
radiated from the outer layers. If the energy received at the edge of earth's atmosphere were
distributed evenly over the earth, it would give an ayerage incident flux density of 1367 wlm2.

,)r. MOHD. MINHAJUDDIN AQUIL

M.E.(TranBporl,ation Engg.). PhQ
Associate Professor
Department of Civil Engineering
Deccan College o log1
Dar-us-Salam, S

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Region W.volsngth RsrftNrks
Gamms ray lncom nis ryabsorbed
bythe sph nol
avaibbe rb[ renEte sensing
X{ay Cornpletaly absorbcd by atrnospfiere Not
emdoyed in remote censing.
Ulrado{Bt lengths less than 0.8 prn are
orb€d by ozone in lh€ upp€r

Photographh TransmittEd hrough strnosphe,€ Drtecuabb

UV band with fi lm and photodGtectors,but atmoeBheft
Bcallerin0 is s€v6re
VErbts lmsged \uith film and photodetsctors locludes
rerleited energy peak o, earli at O_S pm
lnfrared
ngth

sePgrded,
Reftected Reflected solar radiation hal oonhalfts
lR b6nd inforhation 6tDln }iermal properties of
maleriab The band from 0.7 to 0 9 pm is
dateclEble witt ftlm and is csil€d the
photographb lR band
Th€rrnal lR 3tagpm Principal affiospheric windo$6 in the
b61d . tth€se

s con
sy6tem5 but not 5y lilnr. Microwave

Radsr 0-1 to 30 cm mic

Active torm ot otegensing.
Radarimageg€re various
lr,ayeren0ul band$
Redio >30 cm Longestwave kngh poti:n ofetectofiragn€{tc
epedrurn. Some da$ifi€d raders with tery
longwavEleil$ihs operato in this.egion

This is known as the solar constant. 35 percent of the incident radiant

flux is reflected back by the
earth' This includes the energy reflected by clouds and atmosphere.
l7 percent of it is absorbed by
atmosphere and 48 percent is absorbed by the earth's surface
materials (Mather, lgg7).If the sun
were a perfect emitter, it would be an example of an ideal black
body. A black body transforms
heat energy into radiant energy at the possible maximum rate
consistent with planck,s law which
defines the spectral exitance of a black body as follows (Henderson
,1970):

Where, Ct= 3.742 x 10-16Wm,2, C, = 1.43gg x 10-2 mok

l" = Wavelength (pm)
T = Temperature (in ok)
M, = Spectral exitance per unit wavelength.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
Curves showing the variation of spectral exitance for black bodies at different temperatures are
shown in Figure 13. The wavelength at which the maximum spectral exitance is achieved is

reduced as the temperature increases. The dashed line in Figure 13 which joins the peaks of the

spectral exitance curves, is described by Wien's displacement law. The law gives the wavelength

of maximum spectral exitance (Am) in the following form:

1,,=c:/T --Equation 3

C::2.898 x 10 -3
mk, T: temperature of body

16000K _

Wavslength, Im

Figure 13 Spectral Exitance curves for black bodies

The distribution of the spectral exitance for a black body at 59000k closely approximates the sun's

spectral exitance curve (Mather, 1987), while the earth can be considered to act like a black body

with a temperature of 2900 k Figure 14 The solar radiation, maximum of which occurs atO.47 llm,
is within the visible spectrum. Wavelength dependent mechanisms of atmospheric absorption alter

the solar irradiance that actually reaches the surface of the earth. Figure 15 shows the spectral
irradiance from the sun at the edge of the atmosphere (solid curve) and at the earth's surface
(dashed line).
Dr. MOHD. MINHAJUDDIN AQUlt.
lt-E. Engg.), Phu

College of Engg. & Technolo-i,

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Visible
spectrum
l.---r.| . Thermal infrared sDectrum

I
E
.T

E
o
ts
o-
o
6
c
Erl)
E
o
o
CL
-
a 01

Figure 14 spectral exitance curves at temperatures ofSun and Earth

c t,s
9
'S
or
2, irradiation curue outside atmosphere
.!E1t.t irradiation curve at sea level
-Pnr t\So,.,
OE
B.B'o
U)Y
-E 4l
o
a
o s2 0S rq f,s ta z.I ,o
Wavelength pm

Figure 15 Solar irradiance curves

The characteristics of radiation soruces impose some limitations on the range
of wavebands that
can be utilized in remote sensing. Generally, the selection of wavebands for
use depends on (a) the
characteristics ofthe radiation source, (b) the effects ofat d
(c) the nature of the target.

All electromagnetic radiation detected by a re

twice, before and after its interaction with earth's atmosphere. This passage
will alter the speed,
frequency, intensity, spectral distribution, and direction of the radiation.
As a result atmospheric
scattering and absorption occur (Curran, 1988). These effects are most
severe in visible and
infrared wavelengths, the range very crucial in remote sensing. During the
transmission of energy
through the atmosphere, light interacts with gases and particulate matter in process
a called
atmospheric scattering' The two major processes in scattering are selective scattering
and non-
ie and Raman scattering are of selective Non selective
Prepared by: Dr. Mohd. Minhajuddin Aquil
Associate Professor, CED, DCET
 DT. MOHD. MINHAJUDDIN AQUII-
M.E.Ort Engg.)' Pht

De g)

scattering is independent of wavelength. It is pro ceed l0 Ilm,

such as, water droplets and ice fragments present the clouds. This type of scattering reduces the

contrast of the image. While passing through the atmosphere, electromagnetic radiation is scattered

and absorbed by gasses and particulates. Besides the major gaseous components like molecular

nitrogen and oxygen, other constituents like water Vapour, methane, hydrogen, helium and
nitrogen compounds play an important role in modifuing the incident radiation and reflected
radiation. This causes a reduction in the image contrast and introduces radiometric errors. Regions
of the electromagnetic spectrum in which the atmosphere is transparent are called atmospheric
windows. The atmosphere is practically transparent in the visible region of the electromagnetic
spectrum and therefore many of the satellite based remote sensing sensors are designed to collect

data in this region. Some of the commonly used atmospheric windows are 0.38 - O.72llm (visible),

0.72 -3.00IIm (near infrared and middle infrared) and 8.00 -14.00 Ilm (thermal infrared). Figure

l6 Shows relative scatter as a function ofwavelength from 0.3 to I I lm of the spectrum for various
levels of atmospheric haze. The characteristics of all the four types of scattering in the order of
their importance in remote sensing are given in Table 2.2.
Table 2 Types of Atmospheric Scatter in order of importance (Curran. 19gg)

Typ6 size of Type of Scatt€r of Effect of ecatter

ol rffective effective partlcles rn visible and
Scatter of rtmospheric atmospherlc tear visiblo
particl6s Darticl6s particles vavelength
Rayle0h Smaller than the Gas molecules Moleculo absofts Affects short
uavelength of high energy visiblewave
radiation. radiation and lengths, resulting
re-emits. skylight in haze in
scatter is inversely photography,
proportionai to and blue skies
fourth power
oa wave length
Mie Same size as Spherical Physical scattering Affects alt visible
the wavelength particles, under overcast wave lengths
of radiation fumes and dust skies
Non- Largerthan the Water droplets Physical scattering After all visible
select ve wavelength of and dusl by fog and clouds wave lengths
radiation. equally, resulting
white fog and
clouds
Ram€n \ny Any Photon has elastic Variable
coflision \nith
molecule resulting
an a loss or a gain
in energy; this can
decrease or
tncrease
wave length.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 s

Figure 16 Relative scatter for various levels of atmospheric haze

The main part of the radiance measured from high flying airqaftor satellite stems form multiple
scattering in the atmosphere. Therefore, the remaining signal can be interpreted in terms of
suspensions only after a careful correction for the atmospheric contribution. For this reason the

varying optical parameters of atmosphere must enter the radiative hansfer calculations (Fischer J,
1989). Before we study the effects of solar radiation and atmospheric properties, we shall
consider
the mass quantities which determine the spectral upward radiance. The source of the
shortwave
radiation field in atmosphere is the Sun emitting in a broad spectral range. The extraterrestrial
irradiance at the top of the atmosphere, the solar constant, depends on the black body
emission of
the Sun's photosphere and on the scattering and absorption process in the Sun's chromosphere.
Important Fraunhofer lines caused by the strong absorption in the Sun's chromosphere
show some
prominent drops in the spectral distribution of the solar radiation. Figure 17 shows
the solar
irradiance at the top of the earth's atmosphere to be between 0.4 and 0.8 j..lm
as determined by
Necked and Labs. Da MOHD. MINHAJUDDIN AQUIL
E .I PhD
q 15oo
t
s 12M

e00 l, T.g
:E,

!c ooo

E 3oo

Figure 17 Solar irradiance at the top of the atmosphere illuminating the Earth between
0.4 J.lm - 0.g J.lm.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 DT. MOHD. MINHAJUDDIN AQUTL
M.E.(rilrtl ')' PltD

r)e gY
Ozone is a trace gas in the atmosphere mainly confi 20 and,40
km with a maximum concentration near 25 km. At these levels, ozone dominates the short wave
radiation budget, while at other heights its influence is nearly negligible. The Chappuis band of
ozond in the visible spectrum is the only ozone band used to detect the oceanic constifuents from

space. The transmission of the chlorophyll fluorescence to the top of the atmosphere is hindered
through the absorption by water vapour and molecular oxygen in their vibration action bands. In
order to study the selective gaseous absorption in the radiative transfer calculations the
transmission functions of 0z and Hz} are computed from absorption line parameters by explored
through areas of Lorentz's theory of collision broadening. The contribution from resonance
broadening is negligible in the spectral region considered. Also, the Doppler line broadening,
which is small when compared with Lorentz line widths, is neglected since the area absorption
takes placein the atmosphere below 40 km (Barrow 1962). The transmission functions are
averaged/Over I nm wavelength intervals. The reduction in the solar flux due to absorption and

scattering by a clear mid-latitude summer atmosphere. Response studies for the temperafure and
pressure dependence of the transmission function have been performed and show only a weak
influence for the temperature effect. The pressure impact is not negligible and has to be accounted
for. Air molecules are small compared to the wavelength of the incoming sunlight. Hence, the
extinction through molecular scattering can be determined with Rayleigh theory. The necessary
properly for the determination of the scattering coeffrcient of the vertical profile of the atmospheric
pressnre has been estimated (Hunt, et. aI., 1973). Since molecular scattering within the atmosphere

depends mainly on pressure, the scattering coefficient can be estimated by climatological

measurement.

Atmospheric spectral turbidity variations are caused by variations in aerosol concentration,

composition and size distribution. The vertical distribution of the aerosols is taken from (Adler
and Ken, 1963). The phase functions of aerosols are nearly wavelength independent within
the
visible and near infrared. For the radiative transfer calculations, the scattering functions are
estimated by Mie theory. The range of atmospheric turbidity values used to study the effects of

Prepared by: Dr. Mohd. Minhajuddin Aquil

1.8
Associate Professor, CED, DCET
 aerosol scattering on the measured spectral radiances correspond to
horizontal visibilities at the
surface between 6 and 88 km.

The energy recorded by a sensor is always modified by the atmosphere

between the sensor and the
ground' As shown in Figure 18, the atmosphere influences the radiance recorded
by a sensor in
two ways, namely, (a) it attenuates or reduces the energy illuminating a ground
object and (b) the
atmosphere acts as a reflector itself adding the path radian to the signal
detected by the sensor.
These two atmospheric effects are expressed mathematically as follows (Lillesand,
and Kiefer
t97e):

ga sensor
PET
;*h

Figure 18 Atmospheric effects influencing the Spectral Radiance.

oET
Lror=]**to
Equation 4

Lp = Path radiance from the atmosphere

p: Reflectance of object
E : Irradiance on object or incoming ene

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 T: Transmitted energy
The inadiance (E) is caused by directly reflected 'sunlight' and diffi-rsed
'skylight,, which is the
sunlight scattered by the atmosphere. The amount of irradiance depends on
seasonal changes, solar
elevation angle, and distance between the earth and sun.

When electromagnetic energy is incident on any feature of earth's surface, such

as a water body,
various fractions of energy get reflected, absorbed, and transmitted as shown
in Figure 19.
Applying the principle of conservation of energy, the relationship can be expressed
as:

E,(1,1
' ER ()') + EA (I) + ET (I)

Where, and, EI: Incident energy Dr. MOHD. MINHAJUDDTN AeUtl

En: Reflected energy
Ee:Absorbed energy Dg)
Er : Transmitted energy

E! (i,) = ER (1,) + Ee G) + Er (I)

Er (X) = lncidenl energy

Ea (tr
r
) = Absrobed energy E1 ()") = Transmitted energy

Figure l9 Basic interactions between Electromagnetic Energy

and a water body

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Equation 5 is the fundamental equation by which the conceptual
design of remote sensing
technology is built. If 6 0') is a zero, then p(1"), that is, the reflectance
is one, which means, the
total energy incident on the object is reflected and recorded by sensing
systems.

p(h.) ='t * 4(r) Equation 5

The classical example of this type of object is snow (white object). If €(1,) is one, then (),) is a zero
indicating that whatever the energy incident on the object, is completely
absorbed by that object.
Black body such as lamp smoke is an example of this type of object. Therefore,
it can be seen that
the reflectance varies from 0 (black body) to I (white body). when
we divide the incident energy
on both sides of the balance equation, we get the proportions of energy
reflected, absorbed and
transmitted which vary for different feahlres of the earth depending
on the material type. These
differences provide a clue to differentiate between features of an
image. Secondly, from the
wavelength dependency of the energy components, it is evident that
even within a given feature
type, the proportion ofreflected, absorbed, and transmitted energies may vary at different
wavelengths. Thus, two features which are indistinguishable in
one spectral range, may exhibit a
marked contrast in another wavelength band. Because many remote
sensing systems operate in the
wavelength regions in which reflected energy predominates, the reflectance
properties of terrestrial
features are very important.

For any given material, the amount of solar radiation that it reflects,
absorbs, transmits, or emits
varies with wavelength.

Da MOHD. MINHAJUDDTN AeUtL

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Figure 20 EMR
When that amount (usually intensity, as a percent of maximum) coming from the material
is plotted
over a range of wavelengths, the connected points produce a curve called the material,s
spectral
signature (spectral response curve). Here is a general example of a reflectance plot
for some
(unspecified) vegetation fype (bioorganic material), with the dominating factor
influencing each
interval of the curve so indicated; note the downfurns of the curve that result
from selective
absorption:

&
f6
&
*
*o
ta
T
#
&
,s
ra

-_ ffi4ilw
-}
i

Figure 21 Spectral response curye

This important property of matter makes it possible to identifi different substances
or classes and
te them by their individual as shown in the figure below

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 a

-\'o-a,--
plnewoods
-"'Grclmds
\
ned Ssnd Plt

srtly lYdet
'{rm} 0-4 0.6 0.E I0 12
#{K*.rser Wse6lBngth ('rm)

Figure22
For example, at some wavelengths, sand reflects more energy than green vegetation but at other
wavelengths it absorbs more (reflects less) than does the vegetation. In principle, we can recognize

various kinds of surface materials and distinguish them from each other by these differences in
reflectance. Of course, there must be some suitable method for measuring these differences as a

function of wavelength and intensity (as a fraction [normally in percent] of the amount of
irradiating radiation). Using reflectance differences, we may be able to distinguish the four
common surface materials in the above signatures (GL: grasslands; PW : pinewoods; RS : red
sand; SW : silty water) simply by plotting the reflectances of each material at two wavelengths,
commonly a few tens (or more) of micrometers apart.

Speckal signature is a set of characteristics by which a material or an object may be identified on

any satellite image or photograph within the given range of wavelengths. Sometime, spectral
signatures are used to denote the spectral response ofa target.

Dr. MOHD. MINHAJUDDIN AQUTL

Associate ol
Deccan College of Engg. & Tccbnology

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Dr. MOHD. MINHAJUDDTN AeUIL

. '... .Water
60
vegetation
Soil
50 - -
-Vigorous
c@ 40
r._- _
o
o
o
o 30
o
E
o 20
o
o
o
x. 10

0
0.3 0.5 0.7 0.9 ,.1 1.3 .1.5 1 7 1.9 2.1 2.3 2 5
Wavelength, gm

Figure 23 Spectral Signatures ofTypical Features ofearth's

Surface.

Remote Sensing techniques have several advantages

over the conventional fieldbased
investigations. These are
-
a) Synoptic overview: The remote sensing images provide synoptic
a overview or bird,s eye
view of a larger area, enabling us to study the relationship among
different ground objects
and delineation of regional features/trends.

b) Feasibility Aspects: Due to inaccessibility to ground survey

in many parts of the terrain,
remote sensing is the onry scientific method for
data collection.
c) Time saving: Remote sensing saves time and manpower
as larger area can be covered by
this technique.
d) Unobtrusiveness: If
the remote sensors collect the information passively
by recording the
electromagnetic energy reflected or emitted by the ground
object, the area of interest is not
disfurbed. It also ensures collection of information
in its natural state.
e) systematic Data collection: Remote Sensing devices
collect the information of the ground
stuface in a systematic manner with a specific time interval,
removing the sampling bias
introduced in some in situ investigations.
f) Derivation of Biophysical Data: under controlled conditions,
remote sensing can provide
fundamental biophysical information, e.g., location, elevation,
temperafure, moisture
content, etc.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 s) Multi-disciplinaryApplications: The same remote sensing data may be used by researchers
or workers from different disciplines, e.g., geology, forestry agriculture, hydrology,
planning, defense, etc. and therefore, increase the overall benefit-to-cost ratio.
Although,
there are many advantages making the technique an enormously popular tool, it
has some
limitations, too.
h) Understanding limit of application: The greatest limitation of this technique is that its utility
is often oversold. Remote Sensing alone cannot provide all the information needed for any

scientific study. The applicability of these tools and techniques are limited to selection of
appropriate sensors, its resolutions, time of data collection and appropriate post-processing

operations
i) Expensive technique: The collection and interpretation of remote sensing data is expensive,
as it requires specific instrumentation and skills. However, the enonnous advantages of this
technique ovemrle this limitation' Dr. MoHD. MINHAJUDDIN AeuIL
M.E. lon Engg.), PhO

DeecanCollegcof E . &Technology

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET