Geographical Techniques II: Remote Sensing — Electromagnetic Spectrum, Sensors, Platforms and Resolution, Image Interpretation, Photogrammetry, Digital Image Processing and Big Data — and GPS
2. Image interpretation, photogrammetry and types of aerial photographs
Visual interpretation of air photos and satellite images rests on a standard set of elements: tone or colour (relative brightness), texture (the frequency of tonal change — smooth grassland against rough forest), pattern (spatial arrangement — the grid of a planned town, the regular rows of an orchard), shape, size, shadow (which reveals height and profile), site (topographic position) and association (objects found together — a thermal power station with cooling towers, a rail siding and a coal yard). In a standard false colour composite, the near-infrared band is displayed in red, the red band in green and the green band in blue, so healthy vegetation appears bright red, water dark blue or black, and urban areas bluish grey.
Photogrammetry is the science of making reliable measurements from photographs. The scale of a vertical photograph is S = f ÷ (H − h), the camera's focal length divided by the flying height above the terrain: with f = 152 mm and a height of 3,040 m above the ground, S = 0.152 ÷ 3,040 = 1 ÷ 20,000, a scale of 1:20,000. Successive photographs along a flight line overlap by about 60 per cent (forward overlap) and adjacent strips by 20–30 per cent (side overlap), so that each point appears in two photographs and can be viewed stereoscopically in three dimensions; heights are measured from parallax, and tall objects show relief displacement, leaning radially away from the photograph's principal point. Aerial photographs are classified by the tilt of the camera axis — vertical (within about 3° of vertical, the basis of mapping), low oblique (the horizon is not visible) and high oblique (the horizon is visible) — and by film: panchromatic black-and-white, black-and-white infrared, natural colour and colour infrared (false colour).
3. Digital image processing, developments in technology and big data, and applications in India
Digital image processing has four stages. Pre-processing corrects the raw data: radiometric correction for sensor faults and atmospheric effects (for example dark-object subtraction), and geometric correction using ground control points and resampling (nearest neighbour, bilinear or cubic convolution). Enhancement improves visual interpretability: contrast stretching (linear, or histogram equalisation), spatial filtering (low-pass filters smooth, high-pass filters sharpen edges), band ratios such as NDVI and principal components. Classification assigns pixels to classes: unsupervised classification lets an algorithm find natural clusters (ISODATA, k-means) that the analyst labels afterwards; supervised classification starts from training sites of known cover and uses a decision rule — parallelepiped, minimum distance or, most commonly, maximum likelihood. Accuracy assessment compares the result with reference data in an error (confusion) matrix, reporting overall accuracy and the kappa coefficient. Object-based analysis and machine-learning classifiers (random forests, deep neural networks) are the newer methods, and comparing dates gives change detection.
Developments in remote-sensing technology include hyperspectral imaging (India's HySIS, 2018), synthetic aperture radar that sees through cloud and at night (RISAT-1, 2012; Europe's Sentinel-1; and NISAR, the NASA–ISRO SAR mission launched in 2025), LiDAR for detailed elevation and forest structure, very-high-resolution commercial imagery below half a metre, constellations of small satellites imaging the Earth daily, and drones. The result is big data: petabytes of imagery that are processed in the cloud rather than downloaded — Google Earth Engine (2010) — made possible by open data policies (the free Landsat archive from 2008 and the Copernicus Sentinels from 2014) and, in India, the liberalised geospatial guidelines of 2021 and the National Geospatial Policy of 2022. Applications in natural resources management in India are coordinated by the National Remote Sensing Centre (Hyderabad) since the first Indian Remote Sensing satellite, IRS-1A, in 1988: crop acreage and production forecasting (the FASAL programme of the Mahalanobis National Crop Forecast Centre), the Forest Survey of India's biennial forest-cover assessment, wasteland and land-degradation atlases, groundwater prospect maps, watershed planning, flood and drought monitoring, and INCOIS's potential fishing zone advisories derived from ocean colour and sea-surface temperature; the Bhuvan geoportal serves much of this publicly.
4. GPS: space, ground control and receiver segments, and applications
| GPS segment | Components and role |
|---|---|
| Space segment | A constellation of at least 24 satellites (about 31 in service) in six orbital planes inclined at 55°, at about 20,200 km altitude, each circling the Earth in about 12 hours and broadcasting its position and precise time |
| Control (ground) segment | A master control station in Colorado, a worldwide network of monitor stations and ground antennas that track the satellites, correct their clocks and orbits and upload navigation data |
| User (receiver) segment | Receivers that decode the signals and compute position, velocity and time — from phones to survey-grade instruments |
A receiver measures the travel time of each satellite's signal, multiplies by the speed of light to get a pseudo-range, and fixes its position by trilateration. Three ranges would locate a point in three dimensions if the receiver's clock were perfect; because it is not, a fourth satellite is needed to solve for the clock error, so at least four satellites are required for a three-dimensional fix. Accuracy is limited by satellite clock and orbit errors, ionospheric and tropospheric delay, multipath reflection from buildings, and satellite geometry, expressed as the dilution of precision (DOP); the United States switched off the deliberate degradation called Selective Availability in May 2000. Accuracy is improved by differential GPS and satellite-based augmentation — India's GAGAN (GPS Aided GEO Augmented Navigation, by ISRO and the Airports Authority of India) serves aviation over the region. GPS is one of several global navigation satellite systems, alongside Russia's GLONASS, Europe's Galileo and China's BeiDou; India operates its own regional system, NavIC (the Indian Regional Navigation Satellite System). Applications include surveying and mapping, navigation, precision agriculture, disaster management, fleet tracking, the geotagging of assets under government schemes, time synchronisation for power grids and telecommunications, and the measurement of crustal movement.
Key takeaways
- Visible light is 0.4–0.7 µm, near infrared 0.7–1.3 µm, thermal infrared about 3–14 µm, microwaves 1 mm–1 m; Rayleigh scattering varies as λ⁻⁴; vegetation absorbs red and reflects NIR, so NDVI = (NIR − Red)/(NIR + Red).
- Passive sensors record sunlight or heat, active ones (radar, LiDAR) their own energy; whiskbroom scanners sweep across track, pushbroom arrays image along track; geostationary orbits (35,786 km) suit weather, sun-synchronous ones (600–900 km) resource mapping.
- Resolution is spatial, spectral, radiometric (2 to the bit depth: 10 bits = 1,024 levels) and temporal; interpretation uses tone, texture, pattern, shape, size, shadow, site and association; in a standard FCC vegetation is red.
- Photo scale = f ÷ (H − h); 60 per cent forward overlap allows stereoscopy; vertical, low-oblique and high-oblique photographs; image processing runs from correction and enhancement to supervised (maximum likelihood) or unsupervised classification and accuracy assessment (kappa).
- GPS: space (24+ satellites at 20,200 km), control and user segments; at least four satellites for a 3-D fix; errors from clocks, the atmosphere, multipath and DOP; GAGAN augments GPS and NavIC is India's regional system.
Practice questions (13)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The visible part of the electromagnetic spectrum extends approximately from:
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Answer: D — 0.4 to 0.7 micrometres
Visible light runs from about 0.4 µm (violet-blue) to 0.7 µm (red). 0.7–1.3 µm is the near infrared, 8–14 µm the main thermal-infrared window, and 1 mm–1 m the microwave region used by radar.A pixel has near-infrared reflectance 0.6 and red reflectance 0.2. What is its NDVI? Type a number.
Numerical answer — type the value.
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Answer: 0.5
NDVI = (NIR − Red) ÷ (NIR + Red) = (0.6 − 0.2) ÷ (0.6 + 0.2) = 0.4 ÷ 0.8 = 0.5, indicating moderately dense green vegetation. Water would give a negative value and bare soil a value near zero.Healthy green vegetation is most easily distinguished on satellite imagery by its:
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Answer: D — High reflectance in the near infrared and strong absorption of red light
Chlorophyll absorbs red (and blue) light for photosynthesis, while the cell structure of leaves scatters near-infrared strongly — the "red edge" that NDVI and false colour composites exploit. Strong NIR absorption is typical of water.Weather satellites such as INSAT that view the same hemisphere continuously are placed in:
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Answer: C — Geostationary orbit about 35,786 km above the equator
A geostationary satellite completes one orbit per day over the equator and so stays above one point, giving continuous coverage. Sun-synchronous orbits (about 700 km) are used for resource satellites, and about 20,200 km is the GPS orbit.A sensor records data with a radiometric resolution of 10 bits. How many distinct brightness levels can it record? Type a number.
Numerical answer — type the value.
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Answer: 1024
The number of levels is 2 to the power of the bit depth: 2¹⁰ = 1,024 (from 0 to 1,023). An 8-bit sensor records 256 levels and an 11-bit sensor 2,048.A scanner that uses a linear array of detectors to image a whole line across the swath at once, as the satellite moves forward, is a:
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Answer: B — Pushbroom (along-track) scanner
Pushbroom sensors such as SPOT HRV and the Indian LISS cameras use CCD line arrays and have no moving mirror. Whiskbroom scanners, like early Landsat, sweep a mirror across the track; SAR is an active microwave system.A vertical aerial photograph is taken with a camera of focal length 152 mm from a height of 3,040 m above the terrain. The scale is 1 : x. What is x? Type a number.
Numerical answer — type the value.
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Answer: 20000
Scale = f ÷ (H − h) = 0.152 m ÷ 3,040 m = 1 ÷ 20,000, so the photograph is at 1:20,000. Both quantities must be in the same unit before dividing.In a standard false colour composite, healthy vegetation appears:
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Answer: A — Bright red
A standard FCC displays the near-infrared band in red, the red band in green and the green band in blue. Because vegetation reflects strongly in the near infrared, it appears red; water appears dark blue or black.Which of the following are elements of visual image interpretation? Select all that apply.
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Answer: A — Shadow; B — Texture; D — Tone
Tone, texture and shadow are among the classic elements, with shape, size, pattern, site and association. The kappa coefficient is a statistic of classification accuracy computed from an error matrix, not an interpretation clue.A classification in which the analyst first selects training sites of known land cover and the computer then assigns every pixel using a maximum likelihood rule is:
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Answer: D — Supervised classification
Supervised classification depends on training data; maximum likelihood is its most widely used decision rule. Unsupervised classification clusters pixels first (ISODATA, k-means) and labels clusters afterwards; stretching and geometric correction are enhancement and pre-processing steps.What is the minimum number of GPS satellites a receiver needs to compute a three-dimensional position, allowing for its own clock error?
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Answer: A — Four
Four unknowns — latitude, longitude, height and the receiver clock bias — need four pseudo-ranges. Three satellites would suffice only with a perfect clock, or for a two-dimensional fix with known height.Assertion (A): GPS satellites orbit the Earth at an altitude of about 20,200 km. Reason (R): GPS satellites are geostationary, remaining fixed above one point on the equator. Choose the correct option.
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Answer: C — A is true, but R is false
A is true. R is false: GPS satellites are in medium Earth orbit in six inclined planes, each completing an orbit in about 12 hours, so their positions in the sky change constantly. Geostationary satellites orbit at about 35,786 km.India's own regional satellite navigation system is:
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Answer: C — NavIC (the Indian Regional Navigation Satellite System)
NavIC is India's independent regional system. GAGAN is a satellite-based augmentation system that improves GPS accuracy for aviation rather than an independent constellation; GLONASS is Russian and BeiDou Chinese.