Exam-useful questions from this chapter, grouped by marks, each with a full model answer. Try writing the answer yourself first, then check it against this one.
1 1-Mark Questions
Q. What is meant by Remote Sensing?
Answer: Remote sensing is the total process used to acquire and measure information about objects and phenomena on the earth’s surface by a recording device (sensor) that is not in physical contact with the object or phenomenon under study.
Q. What is a Sensor?
Answer: A sensor is a device that gathers electromagnetic radiation, converts it into a signal, and presents it in a form suitable for obtaining information, which can be recorded as a photographic or digital image.
Q. What is a Pixel?
Answer: A pixel is the smallest discrete picture element of a digital image; each pixel has its own intensity value (Digital Number) and an address in two-dimensional image space.
Q. What is a Digital Number (DN)?
Answer: A Digital Number is the average intensity value of a pixel, dependent on the electromagnetic energy received by the sensor and the intensity levels used to describe its range.
Q. What is a False Colour Composite (FCC)?
Answer: A False Colour Composite is an artificially generated colour image in which blue, green and red are assigned to wavelength regions to which they do not belong in nature, so that surface features can be told apart more easily.
Q. What is IFOV (Instantaneous Field of View)?
Answer: IFOV is the particular, fixed dimension of the sensor’s optical head at which it views the scene at any one instant while scanning across the entire field.
Q. What is TFOV (Total Field of View)?
Answer: TFOV is the entire extent of the field that the sensor covers while scanning; it is the total angular field of view of the oscillating scanner.
2 2-Mark Questions
Q. What is the difference in altitude between sun-synchronous and geostationary satellites?
Answer: Sun-synchronous satellites are placed at an altitude of 700-900 km, while geostationary satellites are placed at an altitude of about 36,000 km.
Q. State one key difference each between whiskbroom and pushbroom scanners.
Answer: A whiskbroom scanner has a single rotating mirror and a single detector, whereas a pushbroom scanner has a linear array of many fixed detectors and no rotating mirror.
Q. What is the difference between spatial resolution and temporal resolution?
Answer: Spatial resolution is the capability to distinguish two closely spaced objects as separate objects, whereas temporal resolution is the time interval after which the same place is imaged again.
3 3-Mark Questions
Q. Distinguish between active and passive sources of energy in remote sensing.
Answer: A passive (natural) source is one such as the sun, the most important source of energy in remote sensing, whose radiation the object reflects or emits. An active (artificial) source generates its own energy and directs it at the object, such as flashguns or the energy beams used in radar (radio detection and ranging).
Q. Briefly explain the role of the atmosphere in remote sensing.
Answer: Gases, water molecules and dust particles present in the atmosphere interact with the reflected/emitted energy: carbon dioxide (CO2), hydrogen (H) and water molecules absorb energy in the middle infrared region, and dust particles scatter the blue energy. This lost energy never reaches the sensor, so the true characteristics of the objects are not fully recorded.
Q. Write any three differences between the whiskbroom and pushbroom scanners.
Answer: (1) A whiskbroom scanner has a rotating mirror and a single detector; a pushbroom scanner has a linear array of many fixed detectors. (2) In a whiskbroom scanner the field of view is created by the mirror oscillating between 90° and 120°; in a pushbroom scanner it is created by the number of detectors deployed. (3) A pushbroom scanner has no moving mechanical part (no rotating mirror), so its construction is comparatively simpler.
Q. Write any three differences between photographic and digital images.
Answer: (1) Photographic images are recorded on light-sensitive film at an instance of exposure; digital images are obtained by scanners bit by bit and are made of pixels, each with a Digital Number. (2) Photographs are acquired only in the optical region, 0.3 to 0.9 µm. (3) Photographs may be enlarged to any extent without losing information content or contrast, whereas zooming a digital image beyond a certain extent produces loss of information and the appearance of pixels only.
Q. Explain any three elements of visual interpretation with examples.
Answer: Tone/Colour: dense healthy vegetation appears red in a standard FCC. Texture: rice and wheat give a fine texture while sugarcane gives a coarse texture. Shadow: structures such as the Qutub Minar can be identified only from their shadow.
Q. Using Table 6.1, compare the spatial resolution of IRS LISS-I and IRS LISS-II.
Answer: IRS LISS-I has a spatial resolution of 72.5 m x 72.5 m while IRS LISS-II has 36.25 m x 36.25 m, i.e. LISS-II gives twice as fine (better) a resolution as LISS-I; both carry 4 bands and the same 0-127 radiometric range.
Q. Write any three uses/advantages of sun-synchronous satellites.
Answer: (1) Useful for Earth Resources Applications (crop, forest and water-body studies). (2) Their fine spatial resolution (182 m to 1 m) allows detailed mapping. (3) Completing about 14 orbits a day gives frequent data, allowing the study of temporal change (such as season-to-season vegetation change).
Q. Write any three uses/advantages of geostationary satellites.
Answer: (1) Useful for telecommunication. (2) Useful for weather monitoring, since they stay positioned over the same part of the globe (e.g. INSAT series). (3) Each covers about one-third of the globe, so a very large area can be watched at once.
Q. Explain the relationship between a spectral band and a multispectral scanner.
Answer: A band is a specific wavelength interval in the electromagnetic spectrum. A multispectral scanner records images in several different spectral bands at the same time, so that the different reflectance behaviour of the same object in different bands gives more information, as with the IRS-P6 images acquired in four different bands.
Q. How does the principle of the dispersion of light help in obtaining spectral resolution?
Answer: Just as water droplets in the atmosphere naturally disperse sunlight into the seven colours of a rainbow, the same principle, reproduced artificially with a prism, is used to split the combined energy received by the sensor into separate spectral ranges, producing multispectral images.
Q. Why are a flashgun and radar considered examples of an artificial source of energy?
Answer: A flashgun and radar generate their own energy and direct it at the target object, i.e. they do not depend on a natural source such as the sun; this is why they are treated as examples of an artificial source of energy in remote sensing.
Q. Explain the statement: “All photographs are images, but all images are not photographs.”
Answer: An image is a broad term for any pictorial representation of the energy detected and recorded from an area, regardless of whether it was recorded on photographic film or by a digital sensor, whereas a photograph refers specifically to an image recorded on photographic film, so a photograph is a subset of images.
4 4-Mark Questions
Q. Briefly explain the first four of the eight stages of remote sensing (from the source of energy to atmospheric propagation).
Answer: (1) Source of Energy: the sun, or a self-emitted source. (2) Transmission of energy from the source to the earth: as electromagnetic radiation at the speed of light. (3) Interaction of energy with the earth’s surface: as absorption, reflection, emission and transmission. (4) Propagation of the reflected/emitted energy through the atmosphere (subject to partial absorption/scattering).
Q. Differentiate between photographic and digital data products, giving one distinctive property of each.
Answer: Photographic products are recorded on light-sensitive film in the optical region (0.3-0.9 µm); a distinctive property is that a photograph may be enlarged to any extent without losing information content or contrast. Digital products are made of pixels, each with a Digital Number (DN) and an address in image space; a distinctive property is that the DN values can be processed with digital image processing techniques, though zooming beyond a certain extent shows only pixels.
5 5-Mark Questions
Q. Explain, in order, the eight stages (a-h) of remote sensing.
Answer: (a) Source of Energy: the sun, or a self-emitted/artificial source. (b) Transmission of energy from the source to the earth: as electromagnetic radiation at the speed of light. (c) Interaction of energy with the earth’s surface: absorption, reflection, emission and transmission. (d) Propagation of the returning energy through the atmosphere (subject to atmospheric absorption/scattering). (e) Detection of the energy by the sensor (sensors mounted on the satellite receive it). (f) Conversion of the energy received into photographic/digital data. (g) Extraction of thematic information from the data product (through visual or digital processing). (h) Final presentation as maps and tables, which brings the information to the user.
Q. Using Box 6.1, give a detailed comparison of sun-synchronous and geostationary satellites.
Answer: Altitude: sun-synchronous 700-900 km versus geostationary about 36,000 km. Coverage: 81°N-81°S latitude versus one-third of the globe. Orbital period: about 14 orbits per day versus 24 hours (synchronised with the direction of the earth’s rotation). Resolution: fine (182 m-1 m) versus coarse (about 1 km x 1 km). Use: Earth Resources Applications (e.g. IRS) versus telecommunication and weather monitoring (e.g. INSAT). The two categories are thus designed for entirely different needs.
Q. Explain the working of the two types of multispectral scanner, whiskbroom and pushbroom, with the help of a description, and show the detector-count calculation for the SPOT HRV-I example.
Answer: A whiskbroom scanner has a single rotating mirror and a single detector; as the mirror rotates, the detector sweeps across the field of view between 90° and 120° (the Total Field of View, TFOV), recording bit by bit, one scan line at a time, and at every instant the sensor’s Instantaneous Field of View (IFOV) is fixed. A pushbroom scanner uses a linear array of many detectors instead of a moving mirror; the number of detectors equals the swath divided by the spatial resolution. SPOT HRV-I has a swath of 60 km (60,000 m) and a resolution of 20 m, so 60,000 ÷ 20 = 3,000 detectors are deployed in it. A pushbroom scanner is therefore mechanically simpler, since it has no moving mirror.
Q. Explain all seven elements of visual interpretation (tone/colour, texture, size, shape, shadow, pattern, association) with examples.
Answer: Tone/Colour: objects appear in different colours/tones according to their reflectance, e.g. dense vegetation appears red in an FCC. Texture: repetition of small features, e.g. rice/wheat give a fine texture and sugarcane a coarse texture. Size: an object’s size, dependent on scale and resolution, helps distinguish, e.g. industrial from residential areas. Shape: an object’s outline, e.g. the linear, gently-curving shape of a railway line. Shadow: reflects the sun’s angle and the object’s height, e.g. identifying the Qutub Minar. Pattern: the repetition of the relationship between objects, e.g. the layout of planned residential areas. Association: the relationship between an object and its geographical surroundings, e.g. a playground alongside an educational institution.
Q. Using Table 6.1, compare the four Indian sensors (IRS LISS-I, LISS-II, LISS III and PAN).
Answer: IRS LISS-I: 72.5 m x 72.5 m spatial resolution, 4 bands, radiometric range 0-127. IRS LISS-II: 36.25 m x 36.25 m, 4 bands, 0-127. IRS LISS III: 23.00 m x 23.00 m, 4 bands, 0-127. IRS PAN: 5.80 m x 5.80 m, 1 band, 0-127. So all four have the same radiometric range (0-127); the spatial resolution becomes finer from LISS-I to PAN, and PAN, with the finest spatial resolution of all the sensors in the table, records only one band.
6 6-Mark Questions
Q. Using Table 6.1, give a comparative account of any four satellites/sensors (name, country, spatial resolution, number of bands, radiometric resolution) and name the sensors that have the widest radiometric range.
Answer: (1) Landsat MSS (USA): 80.0 m x 80.0 m, 4 bands, radiometric range 0-64. (2) IRS LISS-II (India): 36.25 m x 36.25 m, 4 bands, 0-127. (3) Landsat TM (USA): 30.00 m x 30.00 m, 4 bands, 0-255. (4) SPOT HRV-I (France): 20.00 m x 20.00 m, 3 bands, 0-255. The widest radiometric range in the table, 0-255 grey levels, belongs to Landsat TM, SPOT HRV-I and SPOT HRV-II; Landsat MSS has the narrowest (0-64).
7 Case Studies
(A) 2 marks Convert the total swath of the HRV-I sensor into metres and calculate the total number of detectors it carries.
Answer: 60 kilometres = 60,000 metres. 60,000 metres ÷ 20 metres (spatial resolution) = 3,000. So the HRV-I sensor carries 3,000 detectors.
(B) 2 marks If this same sensor’s spatial resolution were improved from 20 metres to 10 metres (the swath staying at 60 km), how many detectors would now be needed?
Answer: 60,000 metres ÷ 10 metres = 6,000 detectors. That is, making the resolution twice as fine requires the number of detectors to double as well.
(C) 2 marks State one practical advantage of a pushbroom scanner not having a rotating mirror, compared to a whiskbroom scanner.
Answer: Without a rotating mirror, the number of moving mechanical parts is reduced, making the construction simpler and more reliable (with less wear), and the data for an entire line is obtained at the same instant.
(A) 2 marks Which of these two categories of satellite would be more suitable for mapping crop areas and forest surveys, and why?
Answer: IRS (sun-synchronous) satellites would be more suitable, because their spatial resolution is very fine (182 m to 1 m), which allows small crop parcels and forest types to be identified clearly, and this is also their main advantage for Earth Resources Applications.
(B) 2 marks Which of these two categories of satellite would be more suitable for cyclone forecasting and weather monitoring, and why?
Answer: INSAT (geostationary) satellites would be more suitable, because, synchronised with the earth’s rotation, they remain fixed continuously over one large area (about one-third of the globe) for 24 hours, allowing continuous monitoring of fast-changing weather events such as cyclones.
(C) 3 marks If a single satellite needed both fine spatial resolution and continuous 24-hour monitoring at the same time, explain, using the data in the text, why this is not possible.
Answer: According to the text, fine spatial resolution (182 m-1 m) is available only in the low-altitude (700-900 km) sun-synchronous orbit, in which the satellite completes 14 orbits a day, i.e. it does not stay fixed over one place. Continuous 24-hour monitoring, on the other hand, requires the satellite to be in a geostationary orbit (about 36,000 km altitude) synchronised with the earth’s rotation speed, where the resolution necessarily becomes coarse (about 1 km x 1 km). Since altitude, resolution and orbital fixity are all linked together, a single satellite cannot have both properties at once.