This document discusses remote sensing platforms and sensors. It describes the three main categories of remote sensing platforms: ground-based, airborne, and satellite (space-borne). It provides details on each type of platform, including examples. The document also discusses remote sensing systems and their characteristics, focusing on spatial, spectral, temporal, and radiometric resolution. It provides information on imaging systems and scanning systems, such as whiskbroom and pushbroom scanning. Finally, it discusses Landsat satellites as an example of resource satellites.

1. PLAT FORMS AND SENSORS
Remote sensing is defined as a science which deals with
obtaining information about objects on earth surface by
analysis of data received from remote platform.
In this context, information flows from an object to a
receiver or sensor in the form of radiation transmitted
through the atmosphere.
The interaction between the radiation and the object of
interest conveys information required on the nature of the
object.

2. In order for a sensor to collect and record energy reflected or emitted
from a target or surface, it must reside or placed on a stable platform
Platforms
Platforms refer to the structures or vehicles on which remote
sensing instruments are mounted.
Platform is the carrier for remote sensors for which they are borne.
Determines a number of attributes, which may include:
distance the sensor is from the object of interest.
periodicity of image acquisition and
location and extent of coverage.

3. As the platform height increases the spatial resolution
decreases and observational area increases.
High altitude platforms are good for acquiring large
areal coverage with typically lower spatial resolutions.
The types or characteristics of platform depend on the
type of sensor to be attached and its application.
There are three broad categories of remote sensing
platforms: ground based, airborne, and satellite
(space borne).

4. 1. Ground borne Platforms
 Ground borne platforms are used to record detailed information
about the surface which is compared with information collected from
aircraft or satellite sensors i.e. for ground observation.
Figure 3.1 Ground platform
 E.g. to study properties of a single plant or a
small patch (minor) of grass, it would make
sense to use a ground based instrument.
 Ground observation includes both the laboratory and field study,
used for both in designing sensors and identification and
characterization of land features.

5.  Ground based sensors may be placed on a ladder, scaffolding, tall
building, cherry-picker, crane, etc.
2. Airborne Platforms
Aerial platforms are primarily stable wing aircraft and in addition
helicopters, balloons and drone are occasionally used.
Aircraft are often used to collect very detailed images and facilitate the
collection of data over virtually(almost all) any portion of the Earth's
surface at any time.
An aircraft that carries a camera or scanner
needs a hole at the floor of the aircraft.
Air born observations are possible from 100m
up to 30-40km height.
Figure 3.2 Aerial platform

6. 3. Space borne Platforms
The most stable platform aloft is a satellite, which is space borne. It
also includes the space shuttle(regular place to travel).
Space-borne or satellite platform are onetime cost effected but
relatively lower cost per unit area of coverage, can acquire imagery
of entire earth without taking permission.
Satellites are any objects man made or natural which revolve
around another object in a circular or elliptical path around a planet.
For example, the moon is a natural satellite, whereas man-made
satellites include: communication, and telemetry (location and
navigation) purposes.

7. 2. Remote
Sensing
Systems

8. Characteristics of Remote Sensing Systems:
 Imaging remote sensing systems are characterized
by their resolution and the scanning system they
employ.
Remote Sensing Systems Resolutions:
 Resolution: is the ability of the system to render
the information at the smallest discretely
separable quantity in terms of
 distance (spatial),
 wave length band of EMR (Spectral),
 time (temporal) and
 radiation quantity (radiometric).
 Imaging remote sensing instruments operating in
the visible and infrared spectral region are
characterized by their spatial, spectral and

9. Quiz 5%
1, Define platform and its categories of remote sensing
platforms:
2, platform height decreases the spatial resolution
decreases and observational area increases.
True/ False
3, Write types of resolution
4, A scanning system used to collect data over a variety of
different wavelength ranges is called______________
You have 10min.

10. a) Spatial Resolution/ Ground Resolution
• The smallest possible object/area a sensor can detect or
the ground segment sensed at any instant.
• Equals to IFOV (the angular cone of visibility of the sensor
at an instant)
• Relates to the pixel (picture element) size of an image.
• Has 2 effects:
 The ability to identify
spatial features
 The ability to quantify
spatial features
IFOV
Ground Cell
Resolution

11. b) Spectral Resolution
• Refers to the ability of a sensor to define fine
wavelength intervals
• The higher the no of bands for a sensor, the
higher will be its spectral resolution
• Many R. sensing systems record energy over
several separate wavelength ranges at various
spectral resolutions are called multi-spectral
sensors
• Advanced multi-spectral sensors called hyper
spectral sensors, detect hundreds of very
narrow spectral bands throughout the visible,
NIR & MIR portions of the EMS.

12. c)Temporal Resolution
Refers to the frequency of obtaining images from an
area by a sensor or the length of time it takes for a
satellite to complete one entire orbit cycle.
It is the capability of the satellite to image the exact
same area of the same viewing angle at different
periods of time.
•It also called the repetivity of a satellite
Depends on factors of:
 the satellite/sensors capability
 the swath overlap and
 latitude

13. • Refers to the data range or quantization level
with in which images are encoded by a sensor.
The higher the level of quantization the
higher the details captured by a sensor & the
higher the radiometric resolution is.
• a measure of sensor to differentiate the
smallest change in the spectral/ reflectance/
emittance b/n various targets.
d) Radiometric Resolution

14. Remote sensors acquire data using scanning systems,
which employ a sensor with a narrow field of view that
sweeps over the terrain to build up and produce a two-
dimensional image of the surface.
Scanning systems can be used on both aircraft and
satellite platforms and have essentially the same
operating principles.
A scanning system used to collect data over a variety
of different wavelength ranges is called a
multispectral scanner (MSS), and is the most
commonly used scanning system.
There are two main methods of scanning employed to
acquire multispectral image data –
across-track scanning (Whisk-broom), and
along-track (push-broom) scanning.
Multispectral Scanning Systems:

15. Multispectral Scanning Systems:
I. Across Track /Whiskbroom scanners
 Scan the Earth in a series of lines
 The lines are oriented perpendicular to the direction of
motion of the sensor platform /across the swath.
• Each line is scanned from one side of the sensor to the
other, using a rotating mirror (A)
(A) rotating mirror
(B) bank of
detectors
(C) Instantaneous
field of (IFOV)
(D) ground resolution
Cell
(E) angular FOV
(F) Swath

17.  Employs a single detector per band of the multispectral
signal
 As the platform moves forward over the Earth, successive
scans build a two-dimensional image.
 A bank of detectors (B), each sensitive to a specific ℓ range,
detects & measures each spectral band energy and then, as
an electrical signal, convert it to digital data & recorded for
subsequent computer processing.
 The IFOV (C) & platform altitude determine the ground
resolution Cell (D), and the spatial resolution.
 The angular FOV(E) is the mirror sweep, used to record a
scan line & determines imaged swath (F) width.
• Ex. Landsat Satellite Series (MSS, TM & ETM+)

18. II) Along- Track /Push broom Scanning System:
• Use the forward motion platform to record successive
scan lines & build a 2-D image, perpendicular to the flight
direction.
• Use a linear array of detectors (A)
called CCD’s located at the focal
plane of the image (B) formed by
lens systems (C), which are
"pushed" along in the flight track
direction (i.e. along track).
• Each detector measures the
energy for a single ground
resolution cell (D)
• Ex. The SPOT Satellite

19. Advantage of Along-track over Across-track
scanners
1. Records strong signals & thus builds strong
radiometric resolution, b/c each detector has
longer dwell time over each ground resolution.
2. Better geometric integrity b/c of fixed r/s among
detectors.
3. Requires less power for operation b/c of their
smaller size & weight.
4. More reliable and have longer life

20. Satellite is an object that has been intentionally placed into
orbit.
Satellite is a space borne remote sensing platform placed and
stabilized in an orbit in which it moves.
Images taken using sensors that are mounted on board satellites
are called satellite images.
Since the first launch of Civilian Earth Resource technology
satellites in 1972, a lot more satellites have launched and
supplied high amount of information.
Satellite orbits are matched to the capability and objectives of
the sensor/s they carry.

21. Because of their orbits, satellites permit repetitive coverage of the Earth's
surface on a continuing basis.
Satellites for earth observation are positioned in between 150-36,000km.
Cost is often a significant factor in choosing among the various platform
options.
Figure 3.3 Space platform
 Space borne remote sensing provides the
following advantages:
Large area coverage;
Frequent and repetitive coverage of an area of
interest;
Quantitative measurement of ground features
using radiometrically calibrated sensors;
Semi-automated computerized processing and
analysis;
Relatively lower cost per unit area of coverage.

22. Types of Satellites
There are various ways of classifying satellites.
Some of the classification ways are based on their orbit and
on their purpose.
Based on their orbits, there are 2 types of satellites:
 the sun synchronous /near polar/ orbit satellites and
 the geosynchronous /Geostationary /orbital satellites.
The sun synchronous or near polar orbit satellites are those
that follow an orbit (basically north-south) which in
conjunction with the Earth's rotation (west-east).

23. This allows them to cover most of the Earth's surface
over a certain period of time.
They are so named for the inclination of the orbit
relative to a line running between the North and South
poles.
synchronous:- acting together (at the same time)
Many satellite orbits are sun-synchronous such that
they cover each area of the world at a constant local
time of day called local sun time.

24. Figure : Near Polar Orbit (Sun-Synchronous Satellites)
• Follows the ascending/north wards &
descending/south ward passes.
• Maintain a constant equatorial crossing
time.
• Altitude varies in b/n 450 – 900Km above
the earth.

25. Geosynchronous orbital satellites
 Those Satellites which are at very high altitudes and that view the same
portion of the Earth's surface at all times has geostationary orbits.
 Geostationary satellites, at altitudes of approximately 36,000 km, revolve at
speeds which match the rotation of the Earth so they seem stationary, relative
to the Earth's surface.
 This allows the satellites to observe and collect information continuously
over specific areas.
Weather and communications satellites commonly have these types of orbits.
Due to their high altitude, some geostationary weather satellites can
monitor weather and cloud patterns covering an entire hemisphere of the
Earth.

26. Figure : Geostationary Orbit Satellites
As a satellite revolves around the Earth, the sensor "sees" a certain
portion of the Earth's surface.
The area imaged on the surface, is referred to as the swath.

28. If we start with any randomly selected pass in a
satellite's orbit, an orbit cycle will be completed
when the satellite retraces(coming back on the
same way) its path, passing over the same point on
the Earth's surface directly below the satellite
(called the nadir point) for a second time.
The exact length of time of the orbital cycle will
vary with each satellite.
The interval of time required for the satellite to
complete its orbit cycle is not the same as the
"revisit period".

30. I.Communication Satellites:
 Communication Satellites are used basically for
telecommunication and broadcasting.
 They possess very high resolution radiometers (VHRR) and
has geostationary orbit. Examples: INSAT, Orb view.
Based on their purpose, there are 4 categories of
satellites:
 Communication Satellites,
 Meteorological Satellites,
 Resource Satellites and
 military satellites.

31. II. Meteorological Satellites
• Designed specifically to assist weather prediction and monitoring
• Incoroporate sensors with coarse spatial resolutions
• High Temporal resolutions
• Reduced volume of data
• Launched by different countries
• Some are:
o NOAA (National Oceanic and Atmospheric Administration) satellite series
 Sun-Synchoraneous orbital
(refer Lillesand pp. 465-6 )
o GOES (Geostationary Operational Environmental Satellites)
 Equatorial Synchronous
III. Ocean Monitoring Satellites
Ex: Seasat, Nimbus-7 Satellite

32. IV. Resource Satellites Includes:
 The Land sat series
 The SPOT series
 The IRS
A) The LANDSAT Series
 Originally named as Earth Resource Technology
Satellite(ERTS) launched by NASA in 1972
 The first satellite designed specifically to monitor the Earth's
surface,
 Launched 7 series (Landsat 1 – 5, 7 & 8)
 Employs whiskbroom/across track/ scanning system

33.  The first three satellites (Landsats 1-3) are at
 altitudes around 900 km and
 revisit periods of 18 days
 Landsat 4-5 satellites are at
o around 700 km and
o have revisit periods of 16 days.
 All Landsat satellites have equator crossing times in the
morning to optimize illumination conditions.

34.  A number of sensors have been on board Landsat series of
satellites:
o the Return Beam Vidicon (RBV) camera systems,
o the MultiSpectral Scanner (MSS) systems,
o the Thematic Mapper (TM).
 The most popular instrument in the early days of Landsat
was the MultiSpectral Scanner (MSS) and later the
Thematic Mapper (TM).
o MSS:
 collected data over a swath width of 185 km, with a full
scene being defined as 185km x 185km.
 senses the EMR from the Earth's surface in four spectral
bands.
Landsat Satellite sensors

35.  Each band has
o a spatial resolution of approximately 60 x 80 metres
o a radiometric resolution of 6 bits, or 64 DN.
 Use a line scanning device /oscillating mirror.
o Six scan lines are collected simultaneously with each
west to east sweep of the scanning mirror.
Channel Wavelength Range (µm)
Landsat 1,2,3 Landsat 4,5
MSS 4 MSS 1 0.5 - 0.6 (green)
MSS 5 MSS 2 0.6 - 0.7 (red)
MSS 6 MSS 3 0.7 - 0.8 (near infrared)
MSS 7 MSS 4 0.8 - 1.1 (near infrared)

36. TM (Landsat 4&5)
 Launched in 1992
 Improved over the MSS sensor:
o higher spatial and radiometric resolution;
o finer spectral bands; 7 as opposed to 4 spectral bands;
o Sixteen scan lines are captured simultaneously for each
non-thermal spectral band (four for thermal band),
o uses an oscillating mirror which scans during both the
forward (west-to-east) and reverse (east-to-west)
sweeps of the scanning mirror.
o improved the geometric and radiometric integrity of the
data /having longer dwell time.
o Spatial resolution of TM is 30 m for all but the thermal
infrared band which is 120 m.

37. Chan
nel
Wavelength
Range (µm)
Application
TM 1 0.45 - 0.52
(blue)
soil/vegetation discrimination; bathymetry/coastal
mapping; cultural/urban feature identification
TM 2 0.52 - 0.60
(green)
green vegetation mapping; cultural/urban feature
identification
TM 3 0.63 - 0.69
(red)
vegetated vs. non-vegetated and plant species
discrimination (plant chlorophyll absorption);
cultural/urban feature identification
TM 4 0.76 - 0.90
(n- IR)
identification of plant/vegetation types, health, and
biomass content; water body delineation; soil moisture
TM 5 1.55 - 1.75
(s-IR)
sensitive to moisture in soil and vegetation;
discriminating snow and cloud-covered areas
TM 6 10.4 - 12.5
(thermal IR)
vegetation stress & soil moisture discrimination
r/ted to thermal rad/n; thermal mapping
TM 7 2.08 - 2.35
(s-IR)
discrimination of mineral and rock types; sensitive to
vegetation moisture content

38. Landsat 6:
 carrying the Enhanced Thematic Mapper (ETM) was
launched in October 1993, but failed.
 aimed at providing continuity of Landsat 4 &5 TM data.
 It had a similar planned orbit as Landsat4 or 5 and sensor
configuration as 7 bands as in TM.
 It carried an 8th panchromatic band (0.5 – 0.9µm) with
spatial resolution of 15m.
ETM+(Landsat 7)
 launched in April 1999, carrying (ETM+).
 Aimed to provide continuity of Landsat 4 &5 TM data.
 has similar orbit and repeat pattern as Landsat 4 or 5.
 has 8 spectral bands.
o Bands 1 to 6 are in blue, green, red, near-IR & SWIR and
are exactly the same as those in Landsat TM with 30m
ground resolution.
o Its thermal-IR band has an improved ground resolution
60m (as compared to 120m of TM & ETM).

39. o Has panchromatic 8th band with 15m resolution.
 Landsat 7 carries an on board solid state recorder for
temporary storage of remote sensing data. This allows
acquisition of remote sensing data over areas out side the
reach of ground receiving stations.

40. Summary of LandSat Series Characteristics
Feature Landsat 1-3 Landsat 4 - 5 Landsat 7
Altitude 907– 915km 705km 705km
Orbital Period 103mi 99mi 99mi
Temporal 18 days 16 days 16 days
Eq. Cross Tim 9:30 am 10:00 am 10:00am
Sensors MSS TM TM, ETM+
Swath 185X185Km 183X170Km 183X170Km
Resolution 80m 30m-ms
120m Th
30m – ms
60m-Th
15m-Pan

41.  Land sat 8:
 Launched on February 11, 2013 by NASA
 Has Sun-synchronous orbit with altitude of 705 km
 covers the entire globe every 16 days (except for the
highest polar latitudes)
 Circles the Earth every 98.9 minutes
 Equatorial crossing time is 10:00 a.m. +/- 15 minutes
Has 9 reflective spectral bands, including a pan band:
Band 1 Visible (0.43 - 0.45 µm) with 30 m resolution
Band 2 Visible (0.450 - 0.51 µm) with 30 m
Band 3 Visible (0.53 - 0.59 µm) with 30 m
Band 4 Red (0.64 - 0.67 µm) with 30 m
Band 5 Near-Infrared (0.85 - 0.88 µm) with 30 m
Band 6 SWIR 1(1.57 - 1.65 µm) with 30 m
Band 7 SWIR 2 (2.11 - 2.29 µm) with 30 m
Band 8 Panchromatic (PAN) (0.50 - 0.68 µm) with 15 m
Band 9 Cirrus (1.36 - 1.38 µm) with 30 m

42. And Two spectral bands in the Th..Infrared Sensor (TIRS)
Band 10 TIRS 1 (10.6 - 11.19 µm) with 100m resolution
Band 11 TIRS 2 (11.5 - 12.51 µm) with 100 m resolution

44. B) The SPOT Series
• Has sun synchronous orbit with
o altitude of 832 km,
• Has pushbroom /along track/ scanning system
• The field of view of the HRV sensor is 4.130 and the
resulting swath width is 60 km and total swath width is 117
km (with 3 km overlap when both sensors point at nadir).
• The orbital repeat period is 26 days with equatorial crossing
time of 10:30am.
• have twin high resolution visible (HRV) imaging systems.
The SPOT (Satellite Pour l’Observation de la Terre) programme
is funded by the governments of France, Belgium, and Sweden
and is operated by the French Space Agency, CNES (Centre
National d’Etudes Spatiales).

45.  Has 5 series:
o SPOT-1
• launched in 1986 & is still nominally operational.
• carries an imaging sensor,High Resolution Visible (HRV)
capable of measuring upwelling radiance in:
 three channels (0.50–0.59 µm, 0.61–0.68 µm and 0.79–
0.89 µm)
o at a spatial resolution of 20 m in or
 in a single panchromatic channel (0.51–0.73 µm) at a
spatial resolution of 10 m.
• HRV does not use a scanning mirror. Instead, it uses a
linear array of charge-coupled devices (CCDs),
• all the pixels in an entire scan line are imaged at the same
time.

46.  SPOT 2 was launch in January 1990.
 SPOT-3 followed in September 1993. but lost following a
technical error.
 SPOT-4 was successfully launched on 24 March 1998.
o The HRV instrument on SPOT-4 is extended to provide an
additional 20 m resolution channel in the mid-infrared
region (1.58–1.75 µm) & the new instrument is called the
HRVIR.
• This sensor can be used in multispectral mode (X), or
panchromatic mode (M), or in a combination of X and M
modes.
o SPOT-4 carries an onboard tape recorder, which provides
the capacity to store 20 scenes for downloading to a
ground station.
 SPOT-5 satellite was launched in May 2002.
 It carries a new stereo instrument, an enhanced
panchromatic channel, and a four-band multispectral
imager.

47. o The spatial resolution is:
 10 m (20 m in the short wave infrared (SWIR) band), and
 5 or 2.5 m spatial resolution at panchromatic images.
o A new instrument, called HRS (High Resolution Stereoscopic)
simultaneously collects images from two different angles.
Advantage of the SPOT over the Landsat:
 As it has no moving parts the CCD pushbroom system
last longer than the electro-mechanical scanners
carried by Landsats-1 to -7.
 As the individual CCD detector has a longer ‘look’ at the
pixel area on the ground, and collects more ground-
leaving photons per pixel.
 This increased dwell time means that the signal is
estimated more accurately and the image has a
higher signal-to-noise ratio.

48. Band Wavelength
1, Green 0.50 to 0.59 µm
2, Red 0.61 to 0.68 µm
3, (near-IR) 0.78 to 0.89 µm
4, (mid-IR) 1.58 to 1.75 µm
Panchromatic 0.61 to 0.68 µm
Band Characteristics of SPOT Series

49. C) The IRS:
The Indian Government has an active and successful remote-sensing
Programme since the first launch in 1989.
The Indian Remote Sensing (IRS) satellite series, combines features
from both the Landsat MSS/TM sensors and the SPOT HRV sensor.
The third satellite in the series, IRS-1C, launched in December, 1995 has three
sensors:
 a single-channel panchromatic (PAN) high resolution camera,
 a medium resolution four-channel Linear Imaging Self-scanning Sensor
(LISS-III), and
 a coarse resolution two-channel Wide Field Sensor (WiFS).
The accompanying table outlines the specific characteristics of each sensor.

50. Sensor Wavelength Range
(µm)
Spatial
Resolution
Swath
Width
Revisit Period (at
equator)
PAN 0.5 - 0.75 5.8 m 70 km 24 days
LISS-II (Linear Imaging Self Scanning Sensor)
Green 0.52 - 0.59 23 m 142 km 24 days
Red 0.62 - 0.68 23 m 142 km 24 days
Near IR
0.77 - 0.86 23 m 142 km 24 days
Shortwave
IR
1.55 - 1.70 70 m 148 km 24 days
WiFS (Wide Field Sensor)
Red 0.62 - 0.68 188 m 774 km 5 days
Near IR 0.77 - 0.86 188 m 774 km 5 days
Table : Characteristics of IRS

51. The first Indian Remote Sensing (IRS) satellite was, and carried the
LISS-1 sensor, a multispectral instrument with a 76 m resolution in
four wavebands.
A more advanced version of LISS, LISS-2, is carried by the IRS-1B
satellite, which was launched in 1991.
LISS-2 senses in the same four wavebands as LISS-1 in the optical
and near infrared (0.45–0.52 µm, 0.52–.59 µm, 0.62–0.68 µm and
0.77–0.86 µm) but has a spatial resolution of 36 m.
IRS-1C (launched 1995) carries an improved LISS, numbered 3, plus
a 5 m panchromatic sensor.
LISS-3 includes a mid-infrared band in place of the blue-green band
(channel 1 of LISS-1 and LISS-2).
The waveband ranges for LISS-3 are 0.52–0.59 µm, 0.62–0.68 µm,
0.77–0.86 µm and 1.55–1.70 µm.

52. The spatial resolution improves from 36 m to 25 m in comparison
with LISS-2.
The panchromatic sensor (0.50– 0.75 µm) provides imagery with a
spatial resolution of 5 m.
An example of an IRS-1C panchromatic image of Denver airport is
given in Figure 2.8.
IRS-1D carried a similar payload, but did not reach its correct orbit.
However, some useful imagery is being obtained from its sensors.
The panchromatic sensor is, like the SPOT HRV instrument,
pointable so that oblique views can be obtained, and off-track
viewing provides the opportunity for image acquisition every 5 days,
rather than the 22 days for nadir viewing.

53. IRS-1C also carries the Wide Field Sensor, WIFS, which produces
images with a pixel size of 180 m in two bands (0.62–0.68 µm and
0.77–0.86 µm).
The swath width is 774 km. Images of a fixed point are produced
every 24 days, though overlap of adjacent images will produce a view
of the same point every 5 days.
V. Strategic/military Satellites Includes:
FLIR (Forward Looking Infrared) systems: operate in a similar
manner to across-track thermal imaging sensors, but provide an
oblique rather than nadir perspective of the Earth's surface.
provide relatively high spatial resolution imaging.

54. Thank You!

55. 4. Remote Sensing
in the Thermal
Infrared &
Microwave
Spectrum

56. RADAR:
RADAR stands for RAdio Detection And Ranging
Possesses active sensors which provide their own source of
electromagnetic energy and emit microwave radiation in a series of
pulses from an antenna then detected backscattered microwave
radiation.
3.4. Image Data Characteristics
 An image is a digital picture or representation of an object.
 Remotely sensed image data are digital representations of the Earth.
 Digital images are arrays of numbers, i.e. an image is represented
logically as a matrix of rows and columns.

57.  The location of each data value (or picture element called ‘pixel’) is
implied by its position in the array (Fig. 23).
Raster Data, a) Pixels of a sample b) Pixel values in DN values
image of a Landsat Satellite

58.  Though images appear to be continuous tone photograph, it is
actually composed of a two-dimensional array of discrete picture
elements or pixels.
 The intensity of each pixel corresponds to the average brightness or
radiance measured electronically over the ground area corresponding
to each pixel.
 The measured values are called digital numbers (DN) and lies
commonly in the range of 0–255.
 The value 0 (black) indicates lack of the associated color (red, green
or blue), and the value 255 is the brightest level at which that color is
displayed.

59. Raster/Grid Cells
 Pixels are referenced in terms of (row,
column) coordinates, so that pixel (5, 6) in
the above figure lies at the junction of row 5
and column 6.
 The origin of the (row, column) coordinate
system is the upper left corner of the grid, at
cell (row 1, column 1).
 The grid cell (pixel) size or ground
resolution is usually expressed in units of
ground distance such as meters.
• Digital images are typically displayed as additive color composites using three
primary colors: Red, Green and Blue (RGB). Different combinations of RG and
B produce different colors of the spectrum.
• Note that RGB combinations in which the levels of red, green and blue are equal
produce shades of grey.

60.  The values stored in the cells making up a digital image (the ‘pixel
values’ or ‘pixel intensities’) are represented electronically by a series
of binary (base two) digits.
 A color image is generated on the screen using three arrays held in
graphics memory.
 These three arrays hold numbers that show shades of red (top).
 The center array shows the distribution of shades of green, and the
bottom array holds the numbers corresponding to shades of blue.
 Each array holds integer (whole) numbers in the range of 0 (black or
lack of color) to 255 (the brightest shade of red, green or blue).

61. • These values are converted from digital to analogue form by one of
three Digital-to-Analogues Converters (DAC) (centre) before being
displayed on the screen (right).
• If eight binary digits are used to record the value stored in each pixel,
then 0 and 255 are written as 00000000 and 11111111.
• Thus, a total of eight binary digits (bits) is needed to represent the
256 numbers in the range 0–255.
• The range of pixel intensities is termed the dynamic range of the
image.