Chapter 5: Remote Sensing
In the 1960s the Aral Sea, straddling the border of what are now Kazakhstan and Uzbekistan in Central Asia, was among the largest inland lakes on Earth. A fisherman on its shore could see water to the horizon in every direction. What he could not see was the shore itself moving.
Over the following decades the two rivers that fed the sea, the Amu Darya and the Syr Darya, were increasingly diverted to irrigate cotton fields. The sea began to retreat. But it retreated slowly, unevenly, and over an area so wide that no one standing anywhere on the shore could take in the whole change. The water simply seemed a little farther out each year.
The retreat was seen whole only from above. Satellites passing over the same ground every few days recorded the same lake again and again, image after image, year after year. Laid side by side, those pictures showed what no eye on the ground ever could: a body of water the size of a small country splitting, shrinking, and in places drying to bare salt. Fishing boats were left stranded on sand, kilometres from any water.
No camera on the ground took those pictures. The instrument that recorded them never touched the sea, the salt or the stranded boats. It sensed them from hundreds of kilometres away, and that is the whole idea behind remote sensing.
Diagram
Fig. 5.0-1 Caption: Remote sensing in one picture: energy travels from a source to the surface, and the surface sends part of it back to a sensor that never touches it. Brief: Draw a clean schematic (not a map, no country boundaries). Top left, the Sun as the energy source. Show rays travelling downward through a shaded band labelled ‘atmosphere’ to a patch of ground showing water, vegetation and bare soil. From that patch, show arrows travelling back upward through the atmosphere to a sensor on a satellite platform at top right. Number and label four stages: (1) energy source, (2) path through the atmosphere, (3) interaction with the surface, (4) sensor on its platform. Arrows point down from Sun to surface, then up from surface to sensor.
5.1 What Remote Sensing Is
Geography asks what is where, and why it is there. To answer either question it must first observe, and much of what a geographer needs to observe is too large, too distant or too dangerous to reach and measure by hand.
A flood spreads across a whole river basin in a night. A forest fire burns behind a wall of smoke. A glacier retreats over decades across ground no one visits. Remote sensing is the set of methods that let us study such things without going to them.
5.1.1 Sensing Without Touching
Remote sensing is the act of acquiring information about an object or phenomenon without being in physical contact with it. The information reaches us as energy, most often light or heat, that travels from the object to a sensing device and is recorded there.
We already do this without thinking. Our eyes, ears and skin are sensors, organs that detect energy and turn it into information the brain can use. We see the Sun without touching it, hear thunder from kilometres off, and feel the warmth of a fire across a room. In each case energy travels from the source to us, and we read the object from that energy alone.
But every natural sensor works within narrow limits. The eye responds to only a minute slice of the electromagnetic spectrum, which is the whole family of radiation, from extremely short waves to very long ones. That slice, the visible band, runs from about 0.4 to 0.7 micrometres. A micrometre (µm) is a millionth of a metre; the same band is 4,000 to 7,000 angstroms, one angstrom being a hundred-millionth of a centimetre. The ear registers sound only between roughly 16 and 20,000 vibrations a second, and the skin only fairly large changes in temperature.
The important idea is that instruments can be built to sense far beyond these natural limits. An ordinary camera is already one such instrument. It detects and records light reflected from objects without touching them, exactly as the eye does, which makes even a phone camera a simple remote sensor.
This marks off two ways of observing the world. In direct, or contact, observation the instrument must reach the object: a thermometer is dipped into the water, a lump of soil is dug up and weighed. In remote sensing nothing reaches the object; a property is measured from a distance, through the energy the object gives off. The strength of the second way is reach. In its working sense, remote sensing means gathering information from great distances and over broad areas, usually through instruments carried on aircraft or orbiting vehicles.
5.1.2 The Instruments That Sense
Every man-made remote sensor rests on a single presumption: that all substances reflect, absorb and emit energy, and that each does so in its own characteristic way. Measure that energy carefully, and the substance can be read, its type, temperature, moisture or density, without any contact.
Instruments built on this idea fall into four basic classes, each tuned to a different kind of energy (Fig. 5.1-1).
Flowchart
Fig. 5.1-1 Caption: The four basic classes of remote sensor, grouped by the kind of energy each one measures. Nodes: A remote sensor measures energy that a surface reflects, absorbs or emits → this branches into four classes: radiometer → measures energy in chosen bands of the electromagnetic spectrum; audiometer → measures the intensity of reflected sound waves; magnetometer → measures the strength of the Earth’s magnetic field; gravimeter → measures variations in the acceleration of gravity
The radiometer is the most important of the four for geography. It measures the energy level of chosen portions of the electromagnetic spectrum, that is, how much radiation of a given wavelength a surface sends back. Because water, crops and bare rock each return different amounts, the radiometer’s readings can tell one surface from another.
The audiometer senses sound rather than radiation. It measures the intensity of sound waves; for instance, a microphone lowered from a research vessel picks up sound reflected off the ocean floor, and the pattern of returns is assembled into a detailed map of the sea bed. This is the echo-sounding principle later built into sonar.
The magnetometer measures variations in the strength of the Earth’s magnetic field, meaning how strongly the field registers from place to place. Towed behind a ship, it detects the magnetic ‘stripes’ frozen into the basalt of the ocean floor, and from their spacing the rate of sea-floor spreading can be worked out.
The gravimeter measures minute variations in the acceleration due to gravity, the rate at which a falling body gains speed, which rises slightly where dense rock lies below. Over land it can reveal buried masses of heavy rock, helping to locate ore bodies or petroleum-bearing structures. Carried on satellites, it measures the slight flattening, or oblateness, of the Earth itself.
All four instruments do the same essential thing: they turn energy arriving from a distance into a number. To see how the most important of them, the radiometer, reads a surface, we must look more closely at the energy it measures, which is radiation.
5.2 Radiation, the Carrier of Information
The radiometer, the chief instrument of remote sensing, does one thing: it turns radiation into a number. To read that number we must first understand what radiation is, and in particular why it, and not sound, can carry a message across the emptiness of space.
Sound is a vibration of matter, and it needs air, water or rock to travel through. In the near-vacuum between the Earth and a satellite there is almost nothing to carry it. Electromagnetic radiation needs no such medium; it moves through empty space unaided. That is why a satellite can sense the ground below but could never hear it.
5.2.1 Wavelength and Frequency
Electromagnetic radiation is energy that travels as waves through space at the speed of light. Sunlight, heat, radio signals and X-rays are all the same kind of energy, differing only in the size of their waves. It is convenient to picture this energy as a smooth train of waves, like ripples crossing water.
Such a wave is described first by its wavelength, the distance between two successive crests, or two successive troughs. Wavelength is a length, so it is measured in metres, or, for the shorter waves, in micrometres and angstroms. It is written L, or with the Greek letter λ (lambda).
These waves all travel at the same enormous speed, the speed of light, about 300,000 km per second. The frequency of a wave is the number of crests that pass a fixed point in one second. Frequency is measured in hertz (Hz); one hertz is one wave, or cycle, per second.
Wavelength and frequency are locked together. Because every electromagnetic wave travels at the same speed, they are tied by one relationship:
\[c = \nu \times \lambda\]
where c is the speed of light (about \(3 \times 10^{8}\) metres per second), ν (the Greek letter nu) is the frequency in hertz, and λ is the wavelength in metres.
Rearranged, frequency equals the speed of light divided by wavelength, so a shorter wave is forced to have a higher frequency. Green light, for example, has a wavelength near 0.5 µm, that is \(0.5 \times 10^{-6}\) m. Its frequency is:
\[\nu = \frac{c}{\lambda} = \frac{3 \times 10^{8}\ \text{m/s}}{0.5 \times 10^{-6}\ \text{m}} = 6 \times 10^{14}\ \text{Hz}\]
That is six hundred million million crests passing every second. Long waves, then, have low frequency; short waves have high frequency (Fig. 5.2-1).
Diagram
Fig. 5.2-1 Caption: A wave is measured by its wavelength; because all these waves travel at the same speed, a shorter wave crowds more crests past a point each second and so has a higher frequency. Brief: Draw three or four horizontal sine waves stacked vertically, each drawn to the same overall length. Top wave: long wavelength, few crests; mark the wavelength L between two successive crests with a double-headed arrow and label the wave ‘low frequency’. Each wave below has a progressively shorter wavelength and more crests; label the bottom wave ‘high frequency’ and mark L on it too. No numbers needed; the point is the visual contrast between long/low and short/high.
5.2.2 The Electromagnetic Spectrum
If all electromagnetic waves are the same kind of energy, then the whole family can be laid out in order of wavelength, from the shortest to the longest. That ordered arrangement is the electromagnetic spectrum. The scale used to draw it is logarithmic, meaning each step multiplies the wavelength, because the family spans an enormous range (Fig. 5.2-2).
At the short, high-frequency end lie the gamma rays, with wavelengths shorter than about 0.03 angstroms. Next come the X-rays: the shorter ‘hard’ X-rays from about 0.03 to 0.6 angstroms, and the longer ‘soft’ X-rays out to nearly 100 angstroms. These grade into the ultraviolet, which extends to about 4,000 angstroms, or 0.4 µm.
At 0.4 µm begins the narrow band of visible light, the only part the eye can see. Across it the colours grade in order of increasing wavelength, from violet through blue, green, yellow and orange to red, ending near 0.7 µm.
Beyond red lies the infrared region, from about 0.7 µm to roughly 300 µm. These are longwave radiations, given off as ‘heat rays’ by warm surfaces, including the Earth itself; they cannot be seen but can be felt. Longer still are the microwaves, and within them the radar band, running from about 0.1 cm to nearly 100 cm. The longest waves of all, exceeding 300 m, carry radio and television signals.
Remote sensing draws on the ultraviolet, visible, infrared, microwave and radar portions of this spectrum. Each region reveals something the others cannot, and later in the chapter we take them one at a time.
Diagram
Fig. 5.2-2 Caption: The electromagnetic spectrum, from the shortest gamma rays to the longest radio waves; remote sensing uses the ultraviolet, visible, infrared and microwave–radar portions. Brief: Draw a horizontal band on a logarithmic scale, shortest wavelength (highest frequency) at the left to longest at the right. Label the regions in order: gamma rays, X-rays (hard, then soft), ultraviolet, visible light, infrared, microwaves (with the radar band marked inside), then radio and TV waves. Below the visible portion, expand it into its colours in order, violet, blue, green, yellow, orange, red, with boundaries 0.4 µm to 0.7 µm. Add approximate wavelength markers (angstroms at the short end, micrometres in the middle, centimetres and metres at the long end). Draw the visible band deliberately narrow, to show how little of the spectrum the eye detects.
5.2.3 Energy and the Black Body
Where does the radiation a sensor detects come from? Some is sunlight bouncing off a surface, but surfaces also make radiation of their own. Any object with a temperature above absolute zero, which is 0 K, about −273°C, emits energy, and the amount rises as the object grows hotter.
To compare how surfaces emit, physicists use an ideal standard called the black body, a theoretical surface that converts heat perfectly into radiant energy. A black body does not radiate the same at all wavelengths. Its emission is weak at very short and very long waves and reaches a peak at some wavelength in between.
The position of that peak depends on temperature. As a black body is heated, the peak shifts to shorter wavelengths (Fig. 5.2-3). A bar of iron shows this plainly. Heated a little, it first glows dull red, because its radiation has crept into the long-wave (red) end of the visible band. Heated more, the peak slides shorter and the colour passes to orange, then yellow, and finally to white heat.
This shift explains a fact from the previous subsection. The Sun, with a surface near 5,800 K, is so hot that its emission peaks in the visible band, which is why sunlight looks white. The Earth, far cooler at around 300 K, peaks well out in the infrared, which is why its own radiation is felt as heat rather than seen as light.
For remote sensing this is a gift. Because the peak wavelength is fixed by temperature, a sensor that finds where a surface’s radiation peaks can work back to how hot that surface is, without ever touching it. Thermal sensing, met later in the chapter, rests entirely on this link.
Graph
Fig. 5.2-3 Caption: Black-body radiation curves: the hotter the surface, the more energy it emits and the shorter the wavelength at which that emission peaks. Brief: Line graph. X-axis: wavelength in micrometres (µm), logarithmic, from about 0.1 to 100. Y-axis: energy emitted (relative units). Plot several smooth curves, one per temperature (about 5,800 K, 1,200 K, 600 K and 300 K), each rising to a single peak and then falling. Higher-temperature curves sit higher overall and peak farther to the left (shorter wavelength). Mark the visible band (0.4–0.7 µm) as a shaded strip. Source: standard black-body (Planck) radiation curves.
5.3 The Sensing System and the Atmosphere
We now hold the two essentials of a remote-sensing system: radiation to carry the information, and a sensor to record it. Two further questions decide whether the system will actually work.
Where does the energy come from in the first place? And what happens to it on the journey, as it crosses the atmosphere, in most cases twice, once coming down to the surface and once going back up to the sensor?
5.3.1 Active and Passive Systems
Remote-sensing devices fall into two families, divided by where their energy comes from.
A passive system senses natural energy that it does not itself supply. That energy is either sunlight reflected from a surface, or radiation the surface emits on its own, such as the heat given off by warm ground. An ordinary camera is a passive sensor: it records reflected sunlight, usually in the visible band. Because it takes what it is given, a reflected-light passive sensor can work only where and when that energy exists, which for reflected sunlight means daylight (Fig. 5.3-1).
An active system makes its own energy, aims it at the target, and records what returns. Radar is the familiar example, sending out microwave pulses and timing their echoes. Carrying its own source frees an active system from the Sun; it can operate by day or night, and its longer waves can reach through cloud.
The line between the two is not absolute. Photography is passive by nature, but it turns active the moment an artificial source, such as a camera flash, lights the scene. What matters is not the instrument but the origin of the energy it reads.
Diagram
Fig. 5.3-1 Caption: Two ways to light the target: a passive system borrows natural energy, while an active system supplies its own. Brief: Two side-by-side panels, each showing a sensor on a platform above a patch of ground. Left panel ‘passive’: the Sun sends rays down to the surface, and arrows show reflected energy travelling up to the sensor; add a second arrow showing heat emitted directly by the surface also reaching the sensor. Right panel ‘active’: no Sun shown; the sensor itself emits a beam downward to the surface, and the echo returns up to the same sensor. Label the energy as ‘natural’ in the left panel and ‘sensor-generated’ in the right. Arrows travel down then up in both panels; only the source differs.
5.3.2 Atmospheric Windows
Between the surface and the sensor lies the atmosphere, and the energy must pass through it. The air is not empty: it holds carbon dioxide, water vapour, dust, ozone, pollen and pollutants. These block, absorb and scatter radiation, so the energy that finally reaches the sensor is not quite the energy that left the ground.
This loss of signal on the way is called attenuation, the weakening of radiation as it travels through the atmosphere. Crucially, attenuation is not the same at every wavelength. At some wavelengths the air is nearly opaque, swallowing the radiation almost entirely; at others it is largely transparent and lets the energy through.
The wavelength bands where attenuation is slight, and through which the ground can therefore be sensed, are the atmospheric windows (Fig. 5.3-2). Remote sensing is confined to these windows; outside them the atmosphere itself hides the surface.
Absorption is the chief culprit, especially for infrared energy. Water vapour and carbon dioxide soak up radiation strongly at particular infrared wavelengths, so those bands never reach a sensor above the air. As a result, instruments are designed to look only through the open windows. The principal infrared window runs from about 8 to 14 µm; a second window at 3.5 to 5.5 µm is often used because the instruments for it are cheaper to build. Beyond a wavelength of about 1 cm the atmosphere is nearly transparent, which is why microwave and radar sensing can reach through cloud that would blind an optical sensor.
Graph
Fig. 5.3-2 Caption: How much energy the atmosphere lets through at each wavelength; the peaks are the windows used for remote sensing, the troughs the bands it blocks. Brief: Filled line graph. X-axis: wavelength, logarithmic, from about 0.3 µm to 1 m (mark µm, then cm, then m). Y-axis: percentage of energy transmitted through the atmosphere, 0 to 100. Draw a curve that rises to near 100 per cent in the visible and near-infrared, drops to near zero across several infrared absorption bands, rises again across the 8–14 µm thermal window, and stays high beyond about 1 cm into the microwave and radio region. Shade and label the main windows: visible and near-infrared, the 8–14 µm thermal window, and the microwave–radar region. Source: standard atmospheric transmission spectrum.
With the source settled, the atmosphere crossed and its windows mapped, we can now walk the spectrum window by window, asking what each zone of remote sensing lets us see.
5.4 The Zones of Remote Sensing
Each window in the atmosphere opens onto a different part of the spectrum, and each part tells a different story about the ground. A surface that looks identical to the eye may stand out sharply in the infrared, or on radar. This section takes the zones of remote sensing in turn, from the shortest waves to the longest, asking what each one reveals and where each falls short.
5.4.1 Gamma Ray and Ultraviolet Regions
The two shortest-wave zones used in remote sensing are also the two most limited, and both are used chiefly to identify minerals.
The gamma ray region serves geology. Uranium, thorium and potassium are naturally radioactive and give off gamma rays, so a gamma-ray spectrometer flown over the ground can estimate how much of each lies at the surface, a real help in geological mapping. But the reach is short. These radioactive elements can be sensed only from a low altitude, about 130 to 160 metres or closer, not from any great distance, because the radiation is quickly lost in the air.
The ultraviolet region faces the opposite problem. Most ultraviolet from the Sun is absorbed high in the atmosphere, and ultraviolet is only about a tenth of the solar energy that reaches the ground, so sensors work mainly with the small amount reflected back. Certain minerals, such as fluoride and calcite, reflect ultraviolet distinctly, and the ultraviolet reflectance of rocks like granite, basalt and quartz has been recorded to help tell them apart. The atomic absorption lines of vapours such as iodine and mercury also fall in this region, so their presence can be detected, which is useful in searching for petroleum and other minerals.
5.4.2 The Visible Region
The visible region is by far the best understood and most widely used zone of the whole spectrum. A large stock of cameras, scanners, films and filters exists for it, carried on both aircraft and spacecraft, and its use has grown greatly over the last few decades.
Its great advantage is familiarity. Because this is the band the eye itself reads, visible imagery looks natural, and a photograph of fields, rivers, roads and towns can be interpreted almost at sight. Visible sensing is passive, working on reflected sunlight, so it needs daylight and reasonably clear air.
Those same conditions are its limits. Visible sensing fails at night, is blocked by cloud and haze, and cannot separate two surfaces that happen to look alike to the eye. This last shortcoming is important, and it is the reason sensing does not stop at the colours we can see.
5.4.3 The Infrared Region
Infrared sensing detects the heat that surfaces emit. As we saw with the black body, every object warmer than absolute zero gives off infrared radiation, so all solid things, from animals to trees to bare rock, are sources of it. This makes the infrared a way of mapping temperature.
The region is used in two parts. The shorter, near-infrared can be captured by ordinary photographic film, giving what is called photo-infrared. The longer, thermal infrared cannot; it is registered instead by an optical-mechanical scanner, in which a rotating mirror sweeps the ground in strips across the line of flight and feeds the readings to a detector.
Its uses follow from the fact that it maps heat. A forest on fire is hotter than the forest around it, so a thermal image picks out the burning front sharply, even where smoke hides it from the eye, and so guides the effort to fight it. In farming, the temperature of leaves read from the infrared tells of a crop’s health and age, and thermal images have been used to identify crop species, detect disease and judge soil moisture. In geology the same imagery maps fault lines and keeps watch on volcanoes; in hydrology it traces water courses and reveals hot effluents discharged into rivers, a clear marker of thermal pollution.
5.4.4 Multispectral Sensing
Suppose two surfaces send back exactly the same amount of energy in one band. In that band their images share the same tone, and no sensor can tell them apart. But the two may differ sharply in another band. Multispectral sensing is the practice of sensing several wavelength bands at once, so that features which match in one band can be separated in another. As a rule, the more bands recorded, the more reliably each surface can be named (Fig. 5.4-1).
Graph
Fig. 5.4-1 Caption: Different surfaces reflect different amounts of energy in each band; that difference is what lets a multi-band sensor tell them apart. Brief: Spectral reflectance curves. X-axis: wavelength from 0.4 to 1.1 µm, marking the visible band (0.4–0.7 µm) and the near-infrared (0.7–1.1 µm). Y-axis: reflectance (per cent), 0 to 100. Plot three labelled curves: clear water (highest in blue-green, falling steadily to near zero in the near-infrared); healthy green vegetation (moderate in the visible with a small green rise, a dip in the red, then a sharp climb to high reflectance in the near-infrared); and dry bare soil (rising gently and steadily across the whole range). Source: standard spectral reflectance curves for water, vegetation and soil.
The graph explains the method. Clear water reflects best in the blue-green and fades away in the near-infrared; healthy vegetation is dull in the visible but reflects strongly in the near-infrared; bare soil rises steadily throughout. Record all these bands and compare them, and a surface that could hide in one band gives itself away in another.
Two instruments do this. The multispectral, or multiband, camera carries several lens-filter-film combinations, each set for one band; the shutters fire together, yielding several photographs of the same scene at once, over a range from about 0.4 to 0.9 µm. The multispectral scanner does the same electronically: as the platform moves, a rotating mirror sweeps each band across the ground line by line, and the readings are stored as electronic signals that are later built into a picture (Fig. 5.4-2).
Diagram
Fig. 5.4-2 Caption: A scanner builds an image line by line; a rotating mirror sweeps a scan line across the ground at right angles to the flight path, and forward motion lays the next line beside it. Brief: Perspective sketch of an aircraft or satellite flying along a ‘ground track’. From the platform, show a ‘scan path’ fanning down to the ground as a narrow line running across the direction of flight (perpendicular to it). Show the rotating mirror inside the sensor directing energy to detectors. Indicate that as the platform advances, successive scan lines lie side by side, building up a strip image. Label ‘scan path’, ‘ground track’, ‘line of flight’ and the swath width. Arrow shows the direction of flight.
The colours in a multispectral image are usually false colour, meaning deliberately assigned rather than natural. A band the eye cannot see is displayed in a visible colour so that it can be read. In the common scheme, the near-infrared band is shown as red. Healthy, actively growing vegetation, which is bright in the near-infrared, therefore appears deep red; water shows dark blue to black, built-up areas a blue-grey, and dried or mature vegetation yellow or brown.
5.4.5 The Microwave Region and Radar
At the long-wave end lies the microwave region, with wavelengths from about 0.1 cm to 100 cm, where the atmosphere hardly interferes at all. These waves are invisible and are picked up by an antenna. A passive microwave sensor, reading the microwaves a surface naturally emits, is used to judge soil moisture, map ocean-surface conditions, gauge the water held in snowfields and locate the boundary between ice and water.
The best-known microwave tool, however, is active. Radar, short for radio detection and ranging, generates its own microwave pulses, with wavelengths from about 1 cm to 1 m, and records their echoes. Because it supplies its own energy, radar works by day or night, and its long waves reach through cloud and haze. This makes it invaluable for mapping parts of the world where weather has always defeated the camera.
Early radar, developed in the 1940s, was of the Plan Position Indicator type: a rotating antenna swept a full 360°, and each echo appeared as a spot of light on a screen, fixing storms, aircraft or ships by their direction and distance. To make picture-like imagery of the ground, a different design is used, side-looking airborne radar. Its antenna sends and receives a narrow beam aimed to one side of the aircraft, sweeping a swath of terrain alongside the flight line; the time an echo takes to return fixes each feature’s position, and as the aircraft advances, a new strip is imaged (Fig. 5.4-3).
Diagram
Fig. 5.4-3 Caption: Side-looking radar images a strip of ground to one side of the aircraft, timing the echo of its own beam to place each feature. Brief: Perspective sketch of an aircraft flying along a ‘ground track’. From the aircraft, show a radar beam directed downward and to one side, not directly below, illuminating a swath of terrain that runs parallel to the flight line but is offset to the side. Show the echo returning to the antenna. Indicate that as the aircraft moves forward a new strip is imaged. Label ‘scan path’ and ‘ground track’, and make clear the beam looks to one side only. Arrow shows the direction of flight.
Radar imagery is remarkably sharp, and it carries a distinctive look, because tone depends on how a surface is angled to the beam.
From these tonal differences, surfaces such as forest, rangeland and cropland can be told apart by their pattern and shade, even in country the camera could never photograph.
5.5 Photography and the Aerial Photograph
Before satellites carried scanners into orbit, remote sensing meant one thing: the aerial photograph. It was the first remote-sensing device, and for a long time the only one.
It is also the easiest to grasp. A photograph looks like the ground it shows, and its scale can be found from a single measurement of the camera. This section traces how aerial photography grew, and how to read the scale of the picture it produces.
5.5.1 A Short History of Aerial Photography
Remote sensing has been used to gather geographical information for close to two centuries, and for most of that time the tool was the camera lifted above the ground. The idea of photographing the land for mapping was proposed as early as the 1840s, soon after photography itself was invented. To get the camera aloft, people used balloons, kites, rockets and even carrier pigeons.
The first photographs from an aeroplane date from 1909, taken in Italy while Wilbur Wright was demonstrating his machine there. By 1915 cameras built specially for aerial work were in use, and their value was proved by the end of the First World War. The real stimulus to aerial photo interpretation, however, came with the Second World War.
Before that war, aerial photographs were mostly black and white, and geographers, geologists and environmental scientists relied on them heavily. Colour photographs, prized for their clarity, came later; Kodachrome film appeared in 1935, and by the early 1940s colour reversal films for aerial use had been developed. But slow film speeds and awkward processing kept colour from spreading quickly.
The Second World War also produced a special film that mattered greatly for geography. The same false-colour principle met earlier, of showing an unseen band as a visible colour, appears in film form as false-colour, or camouflage-detection, film. Its three emulsion layers respond to infrared, green and red, in place of the usual blue, green and red, so most objects print in false colours. It was first used to tell live vegetation from dead vegetation or artificial camouflage, and later for land use, crop mapping, forest surveys, plant disease and water pollution.
5.5.2 The Scale of an Aerial Photograph
To measure anything from a photograph, you must first know its scale, and the scale of an aerial photograph is set by the camera itself. It is expressed as a representative fraction, the ratio of a distance on the photograph to the matching distance on the ground, written as a fraction such as 1 : 10,000.
The fraction depends on just two things: the focal length of the camera lens, and the elevation of the camera above the ground. As long as both are measured in the same units, the ratio between them is a pure number with no units:
\[\text{Representative Fraction (R.F.)} = \frac{\text{camera focal length}}{\text{elevation of camera}}\]
Here the camera focal length is the short distance from the lens to the film, and the elevation of camera is its height above the ground; both are lengths, in metres (Fig. 5.5-1).
Diagram
Fig. 5.5-1 Caption: The scale of an aerial photograph comes from two lengths: the camera’s focal length and its height above the ground. Brief: Vertical cross-section. At the top, a camera with its lens; mark the small image AB across the film plane and label the focal length (the short distance from lens to film) as C.F. Below the lens, draw two straight lines fanning downward from the lens through A and B to the ground, meeting it at A’ and B’, the much larger ground image; label the height from lens to ground as the elevation of camera, H. The two triangles (lens-to-AB and lens-to-A’B’) are similar, which is why R.F. equals focal length divided by elevation. Label AB (photo), A’B’ (ground), C.F. and H.
An R.F. of 1 : 10,000 means one centimetre on the photograph stands for 10,000 cm, that is 100 m, on the ground. The fraction also carries a trade-off. Raise the camera and the fraction shrinks: from a satellite, far higher up, an R.F. near 1 : 50,000 is usual, so each centimetre now covers 500 m. The higher platform captures far more ground in a single frame, but every feature on it is smaller and fine detail is lost. Detail and coverage pull against each other, and the height of the platform decides the balance between them.
5.6 Remote Sensing from Space
An aeroplane can lift a camera only so high, and can cover only so much ground before it must land and refuel. The decisive leap in remote sensing came when the sensor was mounted not on an aircraft but on a spacecraft, high enough to take in a subcontinent in one frame and to return to the same spot day after day.
This section asks what the view from space adds, and traces the satellites that first provided it.
5.6.1 The Advantages of the View from Space
Photographs from instruments mounted on spacecraft greatly widened the scope of remote sensing. Space photography holds three major advantages over the view from an aircraft.
The first is greater perspective. A single image can take in whole physiographic zones and entire drainage basins at a glance, a mosaic that contact prints from an aircraft would need many hours to piece together.
The second is synoptic coverage, meaning a view that grasps a very large area at one instant, as a single whole. From satellites in polar orbits, which pass near both poles while the Earth turns beneath them, such coverage can in time be extended over the entire world.
The third is rapid repetitive coverage. The same ground is imaged again and again at short intervals, which is exactly what is needed to study things that change. Geography is a dynamic subject, forever concerned with movement and change, and it gains greatly from being able to watch one place over time.
5.6.2 The Sensing Satellites
The step into space can be dated. Space photography began with the captured V-2 rockets that first carried cameras above the atmosphere in 1946. In the years that followed, sounding rockets, missiles and manned spacecraft all flew cameras aloft, but the real advance came with satellites built for the purpose (TL 5.1).
The first was Tiros I, launched in 1960, a weather satellite carrying television cameras to watch cloud systems, cyclonic storms and air fronts. From 1964 the Nimbus series went further, recording the vertical temperature profile of the atmosphere and the global spread of ozone, water vapour, albedo (the share of sunlight a surface reflects), cloud and precipitation. Its orbit was arranged so that it scanned the whole globe twice a day, a boon to weather forecasting and to watching tropical cyclones.
The turning point for the land came in July 1972, when NASA launched the Earth Resources Technology Satellite, ERTS-1, renamed Landsat-1 in 1975. Orbiting about 900 km up and circling the Earth roughly every 103 minutes, it completed some fourteen orbits a day and covered the globe between 81°N and 81°S once every eighteen days. It carried three systems together: the Multispectral Scanner, a Return Beam Vidicon camera, and a Data Collection System that relayed readings from ground stations. Landsat-2 followed in 1975 and Landsat-3 in 1978, extending an unbroken record of the land surface drawn on across agriculture, hydrology, geology and mapping.
A different tool arrived in 1991 with ERS-1, launched from Kourou. Fitted with radar and microwave instruments, it was free of the troubles of mist, fog and cloud, and proved especially good at mapping oceans and ice, and at tracking forest loss, fires and climatic change.
India did not stay a buyer of other countries’ imagery. It built its own satellites and receiving stations, and to that story, and to what remote sensing now does for the country, the final section turns.
5.7 Remote Sensing at Work
The satellites gather; someone must use what they gather. Remote sensing earns its place not as a gadget but as a source of evidence, and nowhere more than in geography, and in a country as large and varied as India.
5.7.1 Remote Sensing in Geography
Remote sensing is interdisciplinary by nature, feeding many sciences at once, among them meteorology, geology, pedology (the study of soils), hydrology, botany and zoology. Its great gift is speed and reach: what older ground surveys took months to gather, a sensor supplies in minutes, over areas no field party could ever cover on foot.
Yet it holds a special place in geography, and the reason lies in what geography studies. Geography’s subject is the spatial pattern of the whole system in which people and their environment act upon each other, and that pattern draws on every one of those sciences. Over the centuries the discipline has grown from describing places to analysing them, and remote sensing hands it exactly what analysis needs: accurate, precise, up-to-date data on where things are and how they are changing.
Married to cartography, the map-making that has always been geography’s cornerstone and is now digital, remote sensing lets the geographer do more than map the world. It lets them watch it change, frame by frame.
5.7.2 Remote Sensing in India
India is not behind the advanced countries in this field. Its national centre for remote sensing sits at Hyderabad, long known as the National Remote Sensing Agency and now the National Remote Sensing Centre, part of ISRO, the Indian Space Research Organisation.
Nor does India depend on other countries for its images.
The work is spread across many institutions, among them the Geological Survey of India, the Forest Survey of India, the Survey of India, the All India Soil and Land Use Survey, the Space Applications Centre at Ahmedabad and the Indian Council of Agricultural Research. Much of the applied effort runs under a single national programme, the Remote Sensing Applications Mission (Map 5.1).
Map
Map 5.1 Caption: Some of the places behind India’s remote sensing, from its national centres to one flagship application region. Brief: Outline map of India showing its full extent as officially defined by the Government of India, including Jammu and Kashmir and Ladakh, with no boundaries shown as disputed. Mark and label Hyderabad (National Remote Sensing Centre) and Ahmedabad (Space Applications Centre). Shade the Satluj and Beas river basins in the north-western Himalaya (Himachal Pradesh and Punjab), where snow-melt runoff is estimated. Include a simple legend.
In agriculture, satellite data estimate the area under major crops, forecast their yields, and track shifting cultivation in the north-east. They also identify and give early warning of drought, which matters because around half of India’s cropland is rain-fed and so hangs on the monsoon.
In water management, remote sensing maps erosion-prone watersheds, charts flooded areas in near-real time, and finds groundwater potential in dry districts. In the Himalaya it estimates the snow-melt runoff of the Satluj and Beas basins, so that the water and power those rivers will yield can be planned in advance.
In the search for minerals, Project Vasundhara joins satellite imagery to ground data to read the pattern of faults and rock structures and point to where minerals may lie. Along the coast, satellite measurements of sea-surface temperature are turned into fishery charts that guide boats towards the richer waters.
Forestry shows the method at its clearest.
A single national percentage is convenient, but it hides as much as it shows. That same 2023 assessment reports that while total green cover grew, dense forest has in places thinned into open forest, and forest cover in parts of the north-east declined. A rise in the headline figure, then, does not by itself mean the forest is healthier; only the breakdown by canopy density and by region can say that. Reading the number well means asking at what scale, and in what category, it was drawn.
From a fisherman who could not see his own sea retreat, to a nation counting its forests from orbit, remote sensing has done one thing throughout: it has let us see the whole, and see it again and again. That is why a subject about where things are, and how they change, could hardly do without it.
5.8 Summary
Remote sensing is acquiring information about an object without touching it; the information arrives as energy the object reflects or emits. Even eyes, ears and skin are sensors, but each works within narrow limits, and instruments are built to reach beyond them.
Four classes of sensor read four kinds of energy: the radiometer (electromagnetic energy), the audiometer (sound), the magnetometer (magnetic field) and the gravimeter (gravity). The radiometer matters most for geography.
Radiation is the carrier. It travels as waves at the speed of light, and because that speed is fixed, wavelength and frequency are inversely linked (\(c = f \times \lambda\)): long waves have low frequency, short waves high.
The electromagnetic spectrum runs from short-wavelength gamma rays to long-wavelength radio waves. Remote sensing uses mainly the ultraviolet, visible, infrared and microwave bands, whose boundaries are zones of transition, not sharp lines.
Every object above absolute zero radiates, and the hotter it is, the shorter the wavelength of its peak. The Sun peaks in the visible; the cooler Earth peaks in the infrared, so the Earth glows in heat the eye cannot see.
A passive system records natural energy, reflected sunlight or emitted heat; an active system, such as radar, makes its own. Active systems work by day or night and see through cloud.
Radiation must cross the atmosphere, which attenuates it unevenly. Remote sensing works only through atmospheric windows, the bands where attenuation is slight.
Each band suits a different task. Gamma and ultraviolet identify minerals but carry only short distances or survive only as reflected energy; the visible band is the most used, since its images look natural, though it needs daylight and clear skies.
The infrared reads temperature: thermal imagery finds forest fires through smoke, judges crop health, and detects thermal pollution in rivers.
Multispectral sensing records several bands at once, so features that share a tone in one band can be told apart in another; each surface has its own spectral signature. False colour shows the near-infrared as red, so healthy vegetation glows red.
Microwave and radar sense through cloud, day and night; side-looking airborne radar builds a relief-like image, bright where surfaces face the beam and dark where they turn away.
The aerial photograph was the first remote-sensing device, and false-colour film its chemical way of showing the near-infrared as a visible colour. A photograph becomes a measuring tool through its scale, the representative fraction RF = focal length ÷ elevation; the higher the camera, the smaller the scale, and the printed figure is only an average, because relief distorts it.
Space photography adds what aircraft cannot: greater perspective, synoptic coverage (a whole region at one glance) and rapid repetitive coverage (the same ground again and again), which is what reveals slow change.
A few decades carried the field from rocket-borne cameras (1946), through the first weather satellites (Tiros 1960, Nimbus 1964), to the first Earth-resources satellite (ERTS-1, later Landsat-1, 1972) and radar satellites (ERS-1, 1991).
Remote sensing feeds many sciences but holds a special place in geography, which studies spatial pattern and change and needs accurate, repeated, up-to-date data. India runs its own through ISRO and the National Remote Sensing Centre, with the IRS satellites from 1988, applied to agriculture and drought, water and snow-melt, minerals, fisheries and the biennial forest-cover assessment.
Flowchart
Fig. 5.8-1 Caption: The remote sensing process from end to end: energy leaves a source, is shaped by the atmosphere and the surface, and returns to a sensor to become information. Nodes: Energy source → branches: a passive system uses the Sun’s reflected light or the ground’s emitted heat; an active system supplies its own pulse, as radar does → radiation travels down through the atmosphere, passing only through its windows → strikes and interacts with the target surface → the surface reflects and emits radiation according to its own spectral signature → radiation travels back up through the atmosphere to a sensor on a platform (aircraft or satellite) → recorded, stored and processed into an image → interpreted into information about the surface
What This Chapter Cannot Tell You
A satellite image records energy, not meaning; it shows that a surface reflects in a certain way, not why, and the interpreter must still supply the ground truth. A single national figure, such as forest cover, hides the differences inside it, dense forest against open, one region against another. And a sensor reaches only through the atmosphere’s open windows, and reads only what a chosen band can carry; where the window is shut or the wavelength wrong, even the sharpest instrument is blind.
5.9 Key Terms
- absolute zero — the lowest temperature there can be, 0 K or about −273°C, at which all thermal motion stops; every object above it radiates energy.
- active system — a remote sensing system that makes its own energy, aims it at the target, and records what returns, as radar does.
- atmospheric window — a band of wavelengths in which the atmosphere weakens radiation only slightly, so remote sensing from above can work through it.
- attenuation — the weakening of radiation as it passes through a medium such as the atmosphere.
- audiometer — a sensor that measures the intensity of sound waves, as in the echo-sounding of the sea floor.
- black body — an idealised surface that absorbs all radiation falling on it and re-radiates it perfectly, used as the standard for comparing emission.
- electromagnetic spectrum — the whole continuous family of radiation, ordered by wavelength from very short (gamma) to very long (radio) waves.
- false colour — an image whose colours are assigned deliberately unlike those the eye sees, usually to show an invisible band such as the near-infrared as a visible colour.
- frequency — the number of complete waves that pass a fixed point in one second.
- gravimeter — a sensor that measures tiny variations in the acceleration due to gravity, revealing buried rock masses of differing density.
- hertz — the unit of frequency; one hertz is one wave, or cycle, per second.
- infrared — the band of radiation just beyond red, about 0.7 to 1000 µm, largely the heat emitted by surfaces.
- magnetometer — a sensor that measures variations in the strength of the Earth’s magnetic field.
- microwave — the band of radiation beyond the infrared, long enough to pass through cloud, and used by radar.
- multispectral remote sensing — sensing in several wavelength bands at the same time, so that features alike in one band can be told apart in another.
- passive system — a remote sensing system that records natural energy it did not produce, whether reflected sunlight or emitted heat.
- polar orbit — a satellite orbit whose path passes near both poles while the Earth turns beneath, giving worldwide coverage.
- radar — an active system (radio detection and ranging) that sends out its own microwave pulses and times their echoes.
- radiometer — the chief remote sensing instrument for geography, measuring the energy in chosen bands of the electromagnetic spectrum.
- remote sensing — acquiring information about an object or phenomenon without being in physical contact with it, through the energy it reflects or emits.
- representative fraction — a scale written as a ratio, such as 1 : 10,000; for an aerial photograph it equals focal length ÷ elevation of the camera.
- sensor — a device or organ that detects energy and turns it into information.
- side-looking airborne radar (SLAR) — a radar that scans a strip of terrain to one side of the aircraft to build a photograph-like image.
- spectral signature — the characteristic pattern by which a particular surface reflects and emits radiation across the wavelengths.
- synoptic — giving a broad view of a large area at a single instant.
- wavelength — the distance between two successive crests, or two successive troughs, of a wave.
5.10 Points to Ponder
A satellite can show a whole forest at one glance, but a forester walking through it can tell a sick tree from a healthy one. If the two disagree about the state of the forest, whose account should a planner trust, and does the answer change with the decision being made?
Before satellites, geographers mostly described places as they found them. Does a technology that lets us watch the whole Earth change, again and again, alter the questions geography asks, or only the speed with which it answers the old ones?
Remote sensing shows us bands the eye was built to ignore, and paints them in false colours so we can read them. Is an image that reveals what the eye cannot see a truer picture of the world, or a less true one?
India’s forest cover can be given as a single national percentage. What is gained, and what is lost, when a whole country’s forests are compressed into one number, and who should decide which of those losses matters most?
A single image freezes one moment; only repeated images reveal a trend. Can a landscape honestly be called ‘improving’ or ‘declining’ from one picture, or does every fair verdict about a place need a second look?
5.11 Objective Questions
- The unit of frequency is:
- angstrom
- micrometre
- hertz
- kelvin [1 mark]
- Which instrument is described as the most important remote sensor for geographical purposes?
- audiometer
- radiometer
- magnetometer
- gravimeter [1 mark]
- On a false-colour composite image, healthy, vigorous vegetation usually appears:
- green
- blue
- bright red
- yellow [1 mark]
- Assertion (A): Radar can sense the ground through cloud, by day and by night. Reason (R): Radar is an active system that supplies its own microwave energy.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Assertion (A): The Earth’s surface glows in the infrared rather than in visible light. Reason (R): A black body’s peak emission shifts to shorter wavelengths as its temperature falls.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Match each sensor in Column A with what it measures in Column B.
| Column A | Column B |
|---|---|
| a. radiometer | i. acceleration due to gravity |
| b. magnetometer | ii. intensity of sound waves |
| c. gravimeter | iii. electromagnetic energy in chosen bands |
| d. audiometer | iv. strength of the magnetic field |
[1 mark]
Fill in the blank: A band of wavelengths in which atmospheric attenuation is slight, and through which remote sensing can work, is called an __________. [1 mark]
State whether true or false: The wavelength and the frequency of an electromagnetic wave increase together. [1 mark]
5.12 Check Your Concepts
Define remote sensing, and name any two natural sensors of the human body. [2 marks]
What is an atmospheric window, and why does it matter for remote sensing? [2 marks]
Differentiate between an active and a passive remote sensing system, giving one example of each. [3 marks]
Explain why healthy vegetation appears bright red on a false-colour image. [3 marks]
Explain, with an example, how thermal infrared imagery can detect a forest fire even through smoke. [4 marks]
Describe the three advantages of remote sensing from space over aerial photography, and explain why they suit the study of change. [4 marks]
Describe how radiation carries information from a surface to a sensor, following the process from the energy source to the recorded image, with reference to the atmosphere and to the surface’s spectral signature. [6 marks]
Discuss the role of remote sensing in India, with examples of its uses in agriculture, water and forestry, and consider what a single national figure such as forest cover can and cannot tell a planner. [6 marks]
5.13 Map, Diagram & Data Interpretation
- Study the electromagnetic spectrum (Fig. 5.2-2) and answer:
- Name the regions lying on either side of the visible band, in order of increasing wavelength.
- Which regions of the spectrum are mainly used in remote sensing?
- The diagram is drawn on a logarithmic scale. State one thing this scale does not show about the boundaries between the bands. [4 marks]
- An aerial camera with a focal length of 0.15 metres photographs the ground from a height of 3,000 metres.
- Using RF = focal length ÷ elevation of camera, calculate the representative fraction of the photograph, showing your working.
- If the same camera is then flown twice as high, state what happens to the scale of the photograph. [4 marks]
- Look at the spectral reflectance curves (Fig. 5.4-1) and answer:
- In which band does green vegetation reflect most strongly?
- How does this difference help a sensor tell vegetation from water?
- State one thing the curve does not tell you about a real field of that crop. [3 marks]
- Study the remote sensing process flowchart (Fig. 5.8-1). Redraw it as a simple sequence of boxes and arrows, and mark clearly the one stage that differs between an active and a passive system. [3 marks]