Introduction to Remote Sensing, Class 11 Practical Work in Geography

What Introduction to Remote Sensing is about

Key concepts

Meaning of remote sensing

NCERT section Introduction to Remote Sensing (introduction)

• Human eyes and photographic systems respond to only a minute portion of the energy received and responded to by object surfaces.
• Remote sensing devices respond to a much wider range of radiation reflected/emitted, absorbed and transmitted by all object surfaces above 0 K (-273°C).
• The term remote sensing was first used in the early 1960s.
• Definition: all the processes by which information about a property of objects and phenomena is acquired and measured using a sensor that does not touch what it studies.
• Three components (Figure 6.1): object surface, sensor, and information-carrying energy waves.

Useful glossary terms: band (a specific wavelength interval), digital number (DN) (intensity value of a pixel), digital image (array of DNs in rows and columns), reflectance and absorptance (ratios of energy reflected or absorbed to energy received), scene (ground area covered by an image), sensor (device that receives EMR and converts it to a recordable signal), gray scale (black to white calibration).

Worked example

A thermometer pushed into soil, and an infrared sensor on a satellite that records the same field's heat: which of these is remote sensing? Name the three components involved. (2 marks)

Try it first, then show the solutionSolution

1. Remote sensing acquires information with a sensor that does not touch the object; the thermometer is in contact with the soil, so it is not remote sensing, while the satellite sensor is.
2. The three components are the object surface (the field), the sensor (the satellite's recording device) and the information-carrying energy waves (radiation emitted or reflected by the field).

Answer: Only the satellite sensor; components: object surface, sensor and energy waves.

Stages in remote sensing data acquisition (a-d)

NCERT section Stages in Remote Sensing

(a) Source of energy: the sun is the most important; energy can also be artificial (flashguns, energy beams of radar, radio detection and ranging).

(b) Transmission from source to surface: energy travels as waves at the speed of light (300,000 km/s): electromagnetic radiation (EMR). Plotting variation by size and frequency gives the electromagnetic spectrum (Figure 6.3): gamma, X-rays, ultraviolet, visible, infrared, microwaves, radio waves. Remote sensing uses the visible, infrared and microwave regions.

(c) Interaction with the earth's surface: energy is absorbed, transmitted, reflected or emitted. Objects differ in composition, so their responses differ, and one object responds differently in different spectral regions (spectral signatures, Figure 6.4). Example: fresh water absorbs strongly in red and infrared and appears dark/black; turbid water reflects more in blue and green and appears light (Sambhar Lake images, Figure 6.5).

(d) Propagation through the atmosphere: reflected energy meets gases, water molecules and dust. CO2, hydrogen and water molecules absorb in the middle infrared; dust scatters blue. Absorbed or scattered energy never reaches the sensor, so those properties go unrecorded.

Worked example

Why does a clear lake appear black but a muddy river appears light in a satellite image? (2 marks)

Try it first, then show the solutionSolution

1. Fresh, clear water absorbs most energy in the red and infrared regions, so little is reflected to the sensor: dark tone.
2. Turbid water, with suspended sand and silt, reflects more in the blue and green regions: light tone.

Answer: Clear water absorbs red/infrared (dark); turbid water reflects blue/green (light).

Stages (e-h) and satellite platforms

NCERT section Stages in Remote Sensing

(e) Detection by the sensor: remote sensing sensors are carried on satellites in a near-polar sun-synchronous orbit at 700-900 km (e.g. Indian Remote Sensing (IRS) series). Weather and telecommunication satellites are geostationary, moving with the earth's rotation at about 36,000 km (e.g. INSAT).

Characteristic	Sun-synchronous	Geostationary
Altitude	700-900 km	About 36,000 km
Coverage	81°N to 81°S	One-third of the globe
Orbital period	About 14 orbits a day	24 hours
Resolution	Fine (182 m to 1 m)	Coarse (1 km × 1 km)
Uses	Earth resources	Telecommunication and weather

A camera records a photograph at an instant of exposure; satellite sensors work differently and give images in digital format.

(f) Conversion into data: radiation received is converted electronically into a digital image of digital numbers in rows and columns (convertible to an analogue picture). Data are transmitted to an Earth Receiving Station; in India at Shadnagar near Hyderabad.

(g) Extraction of information: data are corrected for errors, then information is extracted by digital image processing (digital data) or visual interpretation (analogue products).

(h) Conversion into maps or tables: interpreted information becomes thematic map layers and tabular data.

Worked example

A disaster agency needs (a) a picture of a cyclone over the whole Bay of Bengal every half hour and (b) a detailed image of flood damage to fields in one district. Which kind of satellite suits each need, and why? (3 marks)

Try it first, then show the solutionSolution

1. (a) A geostationary satellite such as INSAT, at about 36,000 km, moves with the earth's rotation, so it watches the same one-third of the globe continuously, which suits weather monitoring and communication.
2. (b) A sun-synchronous satellite such as IRS, at 700-900 km in a near-polar orbit with about 14 orbits a day and coverage from 81°N to 81°S, gives fine resolution suited to earth-resource mapping.
3. Geostationary resolution is coarse (about 1 km × 1 km), too coarse for field-level damage, while the sun-synchronous satellite cannot watch one area continuously.

Answer: (a) Geostationary (INSAT): continuous view of a large area; (b) sun-synchronous (IRS): fine resolution for detailed mapping.

Sensors: whiskbroom and pushbroom scanners

NCERT section Sensors

Sensor: a device that gathers EMR, converts it into a signal and presents it in a usable form. By output: photographic (analogue): a camera recording at an instant of exposure; and non-photographic (digital): scanners recording bit by bit.

Multispectral scanners (MSS): obtain images while sweeping across the field of view; a reception system of a mirror and detectors. The motor oscillates the mirror across the sensor's angular field of view, which sets the length of scan lines, the swath. Each scene is made of cells that set the spatial resolution. Detectors convert energy into electrical signals and then DN values, recorded on magnetic tape.

	Whiskbroom scanner	Pushbroom scanner
Parts	Rotating mirror + single detector	Many detectors in a linear array
Working	Mirror sweeps the detector across the field of view between 90° and 120°, in many narrow bands from visible to middle infrared	Each detector records one ground cell (pixel) at nadir
Terms	Total Field of View (TFOV): full extent of oscillation; Instantaneous Field of View (IFOV): dimension of the optical head at any moment	Number of detectors = swath ÷ spatial resolution
Example	(Figure 6.7)	SPOT HRV-1: 60 km ÷ 20 m = 3,000 detectors

Worked example

A pushbroom sensor carries 6,000 detectors in a linear array and images a swath 36 km wide. What is its spatial resolution? (2 marks)

Try it first, then show the solutionSolution

1. Number of detectors = swath ÷ spatial resolution, so spatial resolution = swath ÷ number of detectors.
2. Spatial resolution = 36,000 m ÷ 6,000 = 6 m, so each ground cell is 6 m × 6 m.

Answer: 6 m (6 m × 6 m ground cells).

Resolving powers: temporal, spatial, spectral, radiometric

NCERT section Resolving Powers of the Satellites; Sensor Resolutions

Temporal resolution (revisit time): the sun-synchronous orbit lets a satellite image the same area at a fixed interval, so changes can be studied: Himalayan vegetation in May and November (IRS images: conifers in May; extra deciduous trees and crops in November); Banda Aceh before (June 2004) and after the December 2004 tsunami.

Spatial resolution: the sensor's ability to distinguish two closely spaced objects as separate (like spectacles helping the eye separate close letters). Higher resolution identifies smaller objects.

Spectral resolution: the sensor's power to record in different bands of EMR; multispectral images use a device that disperses radiation and detectors for specific ranges, extending the principle of dispersion of light (rainbow, prism; Box 6.2). Example: IRS P-6 (Resourcesat-1) images show fresh water absorbing strongly in band 4 (infrared) and dry surfaces reflecting in band 2 (green).

Radiometric resolution: the ability to discriminate between two targets by brightness: the higher it is, the smaller the radiance differences detected.

Table 6.1 (selected): Landsat MSS 80 m, 4 bands, 0-64; IRS LISS-I 72.5 m, 4 bands, 0-127; IRS LISS-II 36.25 m; Landsat TM 30 m, 0-255; IRS LISS-III 23 m; SPOT HRV-I 20 m, 3 bands, 0-255; SPOT HRV-II 10 m, 1 band; IRS PAN 5.8 m, 1 band, 0-127.

Worked example

From Table 6.1, which sensor can separate the smallest objects, and which records the most grey levels among Indian sensors? (2 marks)

Try it first, then show the solutionSolution

1. Smallest pixel = finest spatial resolution: IRS PAN at 5.8 m × 5.8 m.
2. Indian IRS sensors listed all have a radiometric range of 0-127 (128 grey levels); Landsat TM and SPOT have 0-255.

Answer: IRS PAN (5.8 m); Indian sensors listed record 0-127 grey levels.

Data products and visual interpretation

NCERT section Data Products; Interpretation of Satellite Imageries

Image vs photograph: an image is any pictorial representation regardless of the spectral region used; a photograph is an image recorded on photographic film. So all photographs are images, but not all images are photographs.

Photographic images	Digital images
Acquired in the optical region, 0.3-0.9 μm	Made of pixels, each with an intensity value and an address
Four film types: black and white, colour, black and white infrared, colour infrared (aerial photography normally black and white)	DN = average intensity of a pixel, depending on energy received and intensity levels
Can be enlarged without losing information or contrast	Smaller pixels keep more detail; zooming too far shows only pixels and loses information

Interpretation: visual interpretation (manual reading of images) or digital image processing (hardware and software). NCERT discusses visual methods.

Elements of visual interpretation: image characteristics: tone/colour, shape, size, pattern, texture, shadow; terrain characteristics: location and association.
1. Tone/colour: depends on reflected energy; smooth, dry surfaces reflect more than rough, moist ones; healthy vegetation reflects strongly in infrared (multi-layered leaves) and appears bright red in standard FCC; fresh water dark; turbid water light blue.
2. Texture: minor variations in tone from unresolved small features; dense housing fine, sparse housing coarse; sugarcane or millet coarse, rice or wheat fine; scrub coarse, evergreen forest fine.
3. Size: separates industrial complexes from dwellings, a stadium from brick kilns, settlement hierarchy.
4. Shape: Sansad Bhawan; a railway (long, gradually curving) versus a road (sharp bends); temples and mosques.
5. Shadow: depends on sun angle and object height; identifies Qutub Minar, minarets, water tanks, power lines; can hide features in city centres; more useful in large-scale aerial photographs than satellite images.
6. Pattern: repetitive arrangements: planned housing, orchards and plantations, drainage and settlement patterns.
7. Association: relationship with surroundings: a school near housing with a playground; stadium, race course and golf course in large cities; industries along highways at city edges; slums along drains and railway lines.

Standard FCC colours (Table 6.2, selected): evergreen red to magenta; deciduous brown to red; cropped land bright red; fallow light blue to white; clear water dark blue to black; turbid water light blue; high-density built-up dark blue to bluish green; sand and salt-affected land light blue to white; waterlogged land mottled black. (In standard FCC, blue is assigned to green radiation, green to red, and red to near infrared.)

Worked example

In a standard FCC image, one field is bright red and the next is light blue to white. Interpret both. (2 marks)

Try it first, then show the solutionSolution

1. Healthy crops reflect strongly in near infrared, which is shown as red in standard FCC: the bright red field is cropped land.
2. Light blue to white indicates fallow land (bare soil) according to Table 6.2.

Answer: Bright red = cropped land; light blue-white = fallow land.

Introduction to Remote Sensing in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Remote Sensing: Meaning, the Eight Stages of Data Acquisition, Satellite Platforms and Sensors
• Resolutions of Satellite Sensors, Photographic and Digital Data Products and Visual Interpretation

Introduction to Remote Sensing: key facts

Speed of EMR

300,000 km per second

Energy travels from source to surface at the speed of light.

Spectrum regions

$Gamma,X-rays,ultraviolet,visible,infrared,microwave,radio$

Remote sensing uses visible, infrared, microwave.

Eight stages

$Source\to transmission\to interaction\to propagation\to detection\to conversion\to extraction\to maps/tables$

Shadnagar (Hyderabad) receiving station.

Satellites

$Sun-synchronous:700-900km,14orbits/day,81°N-81°S,fine|Geostationary:36,000km,24h,1/3globe,coarse$

IRS vs INSAT.

Pushbroom detectors

$Numberofdetectors=swath÷spatialresolution$

SPOT HRV-1: 60 km ÷ 20 m = 3,000.

Pixels in a scene

$Pixels=(scenewidth÷resolution)\times (scenelength÷resolution)$

60 km × 60 km at 20 m = 3,000 × 3,000 = 9 million.

Grey levels

$Range0-255=256levels;0-127=128levels;0-64asprintedforLandsatMSS$

Higher radiometric resolution = finer brightness steps.

Photographic range

Optical region 0.3-0.9 μm

Four film types.

Interpretation elements

$Tone/colour,texture,size,shape,shadow,pattern(image)+location,association(terrain)$

Standard FCC: vegetation red.

Introduction to Remote Sensing: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Tricks to remember Introduction to Remote Sensing

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Eight stages

  'Sun Travels In, Passes, Detected, Converted, Extracted, Mapped'.

  Use it when: Stages of data acquisition.

• Satellites

  'IRS: Low and Lean (700-900 km, fine); INSAT: High and Hazy (36,000 km, coarse)'.

  Use it when: Satellite comparison.

• Interpretation elements

  'Tall Tigers Seldom Show Shadows; Pattern Lovers Associate': tone, texture, size, shape, shadow, pattern, location, association.

  Use it when: Elements of visual interpretation.

• Scanners

  'Whisk = one detector sweeping; Push = many detectors in a row'.

  Use it when: Scanner types.

Where students lose marks in Introduction to Remote Sensing

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying X-rays are used in remote sensing

  All spectrum regions sound useful.

  Wrong working

  Remote sensing uses X-rays and gamma rays.

  Right working

  It uses the visible, infrared and microwave regions.

  ✓Do this: V-I-M only.

  Usually costs: Marks in an MCQ.

• ✗Mistake: Swapping sun-synchronous and geostationary heights

  Two altitudes to remember.

  Wrong working

  IRS satellites orbit at 36,000 km.

  Right working

  IRS (sun-synchronous) at 700-900 km; INSAT (geostationary) at about 36,000 km.

  ✓Do this: Resources low, weather high.

  Usually costs: Marks in comparison questions.

• ✗Mistake: Thinking all images are photographs

  Everyday usage.

  Wrong working

  A satellite image is a photograph.

  Right working

  All photographs are images, but not all images are photographs; satellite scanner images are digital.

  ✓Do this: Photograph = film only.

  Usually costs: Marks in data-product questions.

• ✗Mistake: Expecting vegetation to look green in FCC

  Natural colour assumption.

  Wrong working

  Healthy crops appear green in the FCC image.

  Right working

  In standard FCC, near infrared is shown as red, so healthy vegetation appears red.

  ✓Do this: FCC: plants are red.

  Usually costs: Marks in interpretation.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.When was the term remote sensing first used?Show answer

   In the early 1960s.

2. 2.Which is the most important source of energy in remote sensing?Show answer

   The sun.

3. 3.Which spectral regions are used in remote sensing?Show answer

   Visible, infrared and microwave.

4. 4.Where is India's Earth Receiving Station?Show answer

   Shadnagar, near Hyderabad.

5. 5.How many detectors does SPOT HRV-1 have?Show answer

   3,000 (60 km swath ÷ 20 m resolution).

6. 6.What is temporal resolution?Show answer

   The revisit time of a satellite over the same area.

7. 7.In what colour does healthy vegetation appear in standard FCC?Show answer

   Red (bright red for cropped land, red to magenta for evergreen forest).

8. 8.What does a digital number represent?Show answer

   The average intensity value of a pixel.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Practical exam chapter: 5 marks.
• ☐Remote sensing: measuring object properties with a sensor not in contact; term from the early 1960s.
• ☐Stages: source, transmission (EMR, 300,000 km/s), interaction, atmosphere, detection, conversion, extraction, maps/tables.
• ☐Spectrum: gamma to radio; remote sensing uses visible, infrared, microwave.
• ☐Sun-synchronous IRS 700-900 km, 14 orbits/day, fine; geostationary INSAT 36,000 km, coarse.
• ☐Scanners: whiskbroom (one detector, rotating mirror, TFOV, IFOV) and pushbroom (detectors = swath ÷ resolution).
• ☐Resolutions: temporal, spatial, spectral, radiometric.
• ☐Photographs are images on film (0.3-0.9 μm); digital images are pixels with DNs.
• ☐Interpretation: tone/colour, texture, size, shape, shadow, pattern, location, association; FCC: vegetation red, clear water black.

Introduction to Remote Sensing: frequently asked questions