Chapter 22 — Satellite Imagery and Topographical Map

A topographical map is a large-scale map that represents a small part of the earth's surface in great detail. The word comes from the Greek topos, meaning place, and graphein, meaning to draw. Such a map draws the place as it is, showing both the natural features and the man-made features together on one sheet. The natural features include relief (hills, plateaus, valleys, slopes), drainage (rivers, streams, lakes, tanks, wells) and vegetation (forests, scrub, grassland). The man-made or cultural features include settlements (villages, towns, cities), transport lines (roads, railways, footpaths, cart tracks), boundaries, temples, mosques, post offices, police stations, embankments and canals.

In India topographical maps are prepared and published by the Survey of India, which was set up in 1767 and has its headquarters at Dehradun. The Survey of India conducts field surveys, takes aerial photographs, fixes triangulation points and then draws the sheets, which are commonly called toposheets. These sheets are printed in a standard set of colours and use a standard set of conventional signs so that any trained person anywhere in India can read them in the same way.

The main scale used for the general topographical series is 1:50,000, which means one unit on the map represents 50,000 of the same unit on the ground. One centimetre on the map therefore equals 50,000 cm, or 500 metres, on the ground; two centimetres equal one kilometre. Larger scales such as 1:25,000 are used for cities and important areas, and smaller scales such as 1:250,000 are used for regional planning.

Why is a topographical map important? First, it lets the geographer study the relationship between relief, drainage, vegetation and settlement of a region without visiting it. Second, it is the base map on which engineers plan roads, canals, dams and railway lines. Third, it is used by the army, by foresters, by disaster-management authorities and by town planners. Fourth, at the Madhyamik level it trains the student to read scale, direction, grid references and contour patterns, which is a compulsory part of the geography examination. Because every feature is drawn to scale and in its true position, distances, directions, areas and heights can be measured directly from the sheet.

The Survey of India uses an orderly system so that every toposheet has a unique number and any sheet can be located quickly. The system starts with the India and Adjacent Countries series, in which the whole region between 4°N and 40°N latitude and 44°E and 124°E longitude is divided into squares of 4° latitude by 4° longitude. Each such square is called a million sheet because it is drawn on the scale of 1:1,000,000. These million sheets are numbered from 1 to 136, running from north-west to south-east. West Bengal falls mostly within million sheets 72, 73, 78 and 79.

Each million sheet, covering 4° × 4°, is further divided into 16 smaller squares of 1° latitude by 1° longitude. These are called degree sheets and are lettered A to P, moving column-wise from north to south. A degree sheet is drawn on the scale 1:250,000, and its number is written as the million-sheet number followed by the letter, for example 73 M.

Each degree sheet of 1° × 1° is again divided into 16 parts, each of 15' latitude by 15' longitude (15 minutes, or a quarter degree). These are the ordinary toposheets on the scale 1:50,000 and are numbered 1 to 16, again column-wise from north to south. So the full number 73 M/13 means million sheet 73, degree sheet M, and quarter-degree sheet 13. Each 1:50,000 sheet covers roughly 27 km east-west and 27 km north-south, an area of about 750 square kilometres.

For even more detail, a 15' × 15' sheet may be divided into four sheets of 7½' × 7½' on the scale 1:25,000, marked NW, NE, SW and SE, for example 73 M/13 NE.

Since 2005 the Survey of India also publishes the Open Series Maps on the WGS-84 datum for civilian use, in which the numbering is based on the 1:1,000,000 International Map of the World scheme (for example sheet F45 divided into further sub-sheets). For the Madhyamik examination, however, the older numbering with million sheets, letters and quarter-degree numbers is the one students must know.

Knowing the sheet number allows a student to say approximately where in India a given area lies, which neighbouring sheets to join to it, and which scale it belongs to, merely by reading the margin of the map.

Every toposheet carries three ways of stating its scale: the representative fraction (R.F.), such as 1:50,000; the statement of scale, such as 2 cm to 1 km; and the linear or graphical scale, a bar divided into kilometres and metres printed at the bottom of the sheet. The graphical scale is the most useful because it remains correct even if the sheet is photocopied larger or smaller.

To measure a straight distance between two points, place the edge of a paper strip along the line, mark both points, and lay the strip along the linear scale to read the distance. To measure a winding feature like a river or a road, use a thread laid along the curve or step along it with a pair of dividers set to a small opening, count the steps and multiply.

On a toposheet the true north is shown by a star-tipped arrow, the magnetic north by a half-arrow, and the grid north by the grid lines. The angle between true north and magnetic north is the magnetic declination, which is printed in the margin with the year of survey. Unless otherwise stated, the top of the sheet is north.

Directions between two points are given in two ways. In the compass method, the eight or sixteen points of the compass are used: north, north-east, east, south-east and so on. In the bearing method, the angle is measured clockwise from north, from 0° to 360°, using a protractor; a bearing of 90° is due east and 225° is south-west.

Area can be estimated by counting grid squares. On the 1:50,000 sheet each grid square is 2 cm × 2 cm on the map, which is 1 km × 1 km, or one square kilometre, on the ground. Count the full squares within the boundary of a forest or lake, add half of the partial squares, and the result is the area in square kilometres.

Gradient, the steepness of a slope, is calculated as vertical interval divided by horizontal equivalent. If two points 4 cm apart on the map (2 km on the ground) differ in height by 200 m, the gradient is 200 m in 2000 m, or 1 in 10. This figure tells a road builder or a farmer how steep the land really is.

A 1:50,000 toposheet is covered by a network of thin blue vertical and horizontal lines forming squares. This is the grid. The vertical lines are numbered along the top and bottom margins and are called eastings because their numbers increase towards the east. The horizontal lines are numbered along the left and right margins and are called northings because their numbers increase towards the north. Each line is 2 cm apart on the map, so the grid squares are 1 km × 1 km on the ground.

A four-figure grid reference identifies one whole grid square. The rule is: eastings first, northings second, always. To give the reference of a square, read the easting of its western (left) edge and then the northing of its southern (bottom) edge. If the square lies between easting 24 and 25 and between northing 61 and 62, its reference is 2461. A four-figure reference is used for features that fill a good part of a square, such as a village, a forest patch or a tank.

A six-figure grid reference fixes a point, such as a temple, a well, a spot height or a triangulation station, more exactly. Each grid square is imagined to be divided into ten equal parts along the easting and ten along the northing. Estimate how many tenths east of the western line the point lies, and how many tenths north of the southern line. A temple lying six-tenths east of easting 24 and three-tenths north of northing 61 has the reference 246613. The first three figures are the easting (24 plus 6 tenths) and the last three are the northing (61 plus 3 tenths). A six-figure reference locates a point to within 100 metres.

Common mistakes are writing northings before eastings, forgetting that the reference of a square is taken from the bottom-left corner, and reading tenths carelessly. A useful memory aid is: along the corridor, then up the stairs.

Grid references are used in the examination to ask a student to identify the feature found at a given reference, or to give the reference of a named feature. They are also the language of the army, of rescue teams and of surveyors, who can name any location on a sheet without describing it in words.

Relief, the shape and height of the land, is shown on a toposheet chiefly by contour lines. A contour line is an imaginary line joining all points that have the same height above mean sea level. On Indian toposheets contours are drawn in brown, and every fourth or fifth contour, called the index contour, is drawn thicker and labelled with its height in metres. The difference in height between two successive contours is called the contour interval or vertical interval, which is usually 20 metres on a 1:50,000 sheet.

Contours have some fixed properties. All points on one contour are at the same height. Contour lines never cross each other, because one point cannot have two heights; they may only meet or merge where there is a vertical cliff. Every contour closes upon itself, either within the sheet or beyond it. Closely spaced contours indicate a steep slope; widely spaced contours indicate a gentle slope. Evenly spaced contours show a uniform slope. Contours that are close at the top and wide at the bottom show a concave slope; contours wide at the top and close at the bottom show a convex slope.

Besides contours, other methods of showing relief also appear on the sheet. Spot heights are exact heights of particular points marked with a dot and a number, such as .347. Triangulation stations are surveyed points marked with a small triangle and height, used as reference points in the survey. Bench marks (BM) are heights marked on permanent objects such as bridges or buildings. Form lines are broken contours drawn where the survey was less exact. Hachures, short lines drawn down the slope, are used for cliffs and steep escarpments. Hill shading is sometimes added to give a three-dimensional look.

Reading contours allows a student to draw a cross-section or profile of the land. A line is drawn across the contours, the height at each contour crossing is transferred to a graph, and the points are joined to show the true shape of the slope. This exercise, along with calculation of gradient, forms a standard part of map-reading practice in Class 10.

Different landforms produce recognisable contour patterns, and the examination often asks the student to identify them. A conical hill appears as a set of roughly circular closed contours with the highest value in the centre, evenly spaced on all sides. A plateau or table-land shows closely spaced contours around the edges (the steep sides) but a wide gap in the middle where the top is flat; the highest contour encloses a large area. A ridge is a long narrow elevation and appears as elongated closed contours with the highest value along the centre line.

A valley is a low area between hills through which a stream usually flows. Its contours are V-shaped, and the important rule is that the V points upstream, towards the higher ground. A V-shaped valley has sharp Vs, common in the upper course of rivers; a U-shaped valley, formed by a glacier, has contours shaped like a U with a wide flat floor. A gorge shows contours running very close together on both sides of a stream.

A spur is a tongue of high land projecting from a hill into lower ground. Its contours are also V-shaped, but here the V points downhill, towards the lower ground, and no stream flows along it. Distinguishing a valley from a spur is a frequent test: in a valley the V points to higher values, in a spur the V points to lower values.

A cliff is a vertical or nearly vertical rock face and appears where several contours merge or run so close that they touch. An escarpment is a long steep slope on one side and a gentle slope on the other, so contours are crowded on one side and spread out on the other. A saddle or col is a dip between two hilltops and appears as two sets of closed contours side by side with lower contours passing between them like an hourglass. A waterfall occurs where a stream crosses closely packed contours; a special symbol is often added. A depression or hollow shows closed contours with values decreasing inward and is sometimes marked with small inward-pointing hachures.

Practising these patterns with a real toposheet helps a student describe the relief of a region: for example, dissected plateau with steep escarpments on the west, a broad river valley in the centre and low undulating plain in the east.

A toposheet cannot write the name of every feature, so it uses a standard set of conventional signs and symbols agreed upon by the Survey of India and printed in the legend at the bottom of the sheet. Signs are small drawings representing point features, such as a temple, a mosque, a church, a post office (PO), a police station (PS), a rest house (RH), a dak bungalow (DB), a well, a tube well or a mine. Symbols are used for line and area features, such as a metalled road (a continuous red double line), an unmetalled road (a broken red line), a cart track, a footpath (single dotted line), a broad-gauge railway (black line with cross-bars), a bridge, a canal, an embankment, and vegetation types.

Colours carry meaning of their own. Black is used for all cultural features drawn as lines and for names: railways, telegraph lines, boundaries, huts, and the grid numbers. Red shows roads, settlements (village and town blocks), and the grid lines on older sheets. Blue is for water features: perennial rivers, streams, canals, tanks, wells and the sea; a broken blue line means a seasonal stream. Brown is for contours, spot heights and sand features such as dunes. Green shows vegetation: dense forest in solid green, open scrub with scattered symbols, orchards with dotted patterns, and grass with tufts. Yellow is used for cultivated land and white for uncultivated or barren land. Purple or violet is used for additions made from aerial photographs or satellite data during revision.

Settlement symbols show the type of village: a group of solid red squares for a large village, a hollow square for a small hamlet, a single hut symbol for a scattered house. Town blocks are shaded red with main streets in white. Boundaries are shown by dot-and-dash lines of different patterns for international, state, district and tehsil or block limits.

Abbreviations are common: PO for post office, PTO for post and telegraph office, CH for circuit house, RS for railway station, Dt for distributary, R for river, N for nala or stream, Dt.B for district boundary. Learning these signs is essential because the map-reading question in the examination typically asks the student to identify features at given grid references or to describe the transport and settlement pattern of the area.

Once a student can read scale, contours and signs, the toposheet becomes a record of how nature and people share the land. Drainage is studied first. Perennial rivers appear as solid blue lines with the river name; seasonal streams appear as broken blue lines; wide sandy beds are shown with dotted brown patterns. The pattern formed by a main river and its tributaries is the drainage pattern: dendritic when it branches like a tree on uniform rock; trellis when tributaries join at right angles on folded rocks; radial when streams flow outward from a dome or volcanic hill; parallel on steep uniform slopes; rectangular on jointed rock; and centripetal when streams converge into a basin or lake. Tanks, dams, canals with their distributaries, and embankments show how water is managed for irrigation.

Vegetation is read from the green symbols. Dense forest on the hills, scattered scrub on dry uplands, and no vegetation symbol on the plains (which usually means cultivated land, sometimes tinted yellow) tell a story about rainfall, soil and slope. Reserved and protected forests are labelled RF and PF.

Settlement patterns reveal the economy. A nucleated or compact settlement, with houses clustered together, is typical of fertile plains with assured water, where villages sit close to a river or tank. A dispersed or scattered settlement, with huts spread over the area, is typical of hilly, forested or dry regions where farmland is patchy. A linear settlement stretches along a road, a canal or a river bank. The presence of a post office, police station, dispensary, school or market indicates a service centre or a small town.

Transport lines show how the area is connected. Metalled roads and a railway indicate a developed and accessible plain; only cart tracks and footpaths in the hills indicate remoteness. Bridges, ferries and causeways show where rivers are crossed. The student should describe the occupation of the people from these clues: cultivation near villages on the plain, forestry and grazing in the uplands, quarrying where mines or quarries are marked, and trade where a market (Bazar) or a railway station appears.

A model examination answer joins all these threads: it describes the relief, the drainage pattern, the vegetation, the settlement type and the transport, and then explains how each depends upon the others.

Remote sensing is the science of collecting information about an object, an area or a phenomenon from a distance, without coming into physical contact with it. Our eyes do remote sensing every day: they receive light reflected from objects and the brain interprets it. In geography the term is used for gathering data about the earth's surface with the help of cameras and sensors carried on aircraft or artificial satellites. The word 'remote' means at a distance, and 'sensing' means detecting energy.

The principle depends on the electromagnetic spectrum. The sun emits energy in a wide range of wavelengths, from very short gamma rays and X-rays, through ultraviolet, visible light (0.4 to 0.7 micrometres), near infrared, thermal infrared and microwaves, to long radio waves. When solar energy falls on the earth, every object absorbs some wavelengths and reflects others. Healthy green vegetation reflects very strongly in the near infrared and moderately in the green; clear water absorbs almost all infrared and reflects a little blue; dry soil and concrete reflect across the visible range. This unique pattern of reflection for each object is called its spectral signature. A sensor records the reflected or emitted energy in several separate wavelength bands, and by comparing the bands the interpreter recognises what lies on the ground.

Remote sensing is of two kinds. In passive remote sensing the sensor simply records natural energy, mainly reflected sunlight or the heat emitted by the earth; ordinary cameras and most optical satellite sensors are passive. In active remote sensing the sensor itself sends out energy, such as a radar pulse or a laser beam, and records what returns; RADAR and LiDAR are active systems and can work at night and through clouds.

The process has four stages: the energy source (the sun or the sensor), the interaction of energy with the atmosphere and the earth's surface, the recording of the returned energy by a sensor on a platform, and the transmission of the data to a ground station where it is processed into an image and interpreted. The platform is the vehicle that carries the sensor: a tripod or tower for ground-based work, a balloon, an aircraft or a drone for aerial work, and a satellite for space-based work. India's remote sensing programme is run by the Indian Space Research Organisation (ISRO), with data received and distributed by the National Remote Sensing Centre (NRSC) at Hyderabad.

An artificial satellite is a man-made object placed in orbit around the earth by a rocket, carrying instruments to observe the earth, relay communications or aid navigation. For remote sensing, two kinds of orbit matter.

A geostationary satellite orbits at about 36,000 km above the equator, moving from west to east at the same rate as the earth rotates, so it completes one orbit in 24 hours and appears to stand still over one point on the equator. It always sees the same third of the globe, which makes it ideal for continuous weather watching and communication. India's INSAT and GSAT series and the weather satellite Kalpana-1 are geostationary. Because it is so high, its images cover a huge area but show little detail.

A sun-synchronous or polar-orbiting satellite moves at a low height of 500 to 1,000 km in an orbit that passes nearly over the poles. As the earth rotates beneath it, the satellite scans a new strip of ground on every pass and crosses every place at the same local solar time, so the lighting is the same each time it images an area. It covers the whole globe in a fixed number of days, called the repeat cycle or revisit period. India's IRS (Indian Remote Sensing) satellites, such as IRS-1A (1988), Resourcesat, Cartosat and RISAT, are sun-synchronous. Landsat of the USA and SPOT of France belong to the same class.

The sensor is the instrument that measures the energy. A multispectral scanner records several bands of the spectrum at once, for example blue, green, red and near infrared, so that a false colour composite can be built later. A panchromatic sensor records a single wide band in black and white but at very fine detail. A thermal sensor records emitted heat, useful for sea-surface temperature and forest fires. A microwave sensor, as on RISAT, is active radar that images through cloud and darkness.

Each sensor has four kinds of resolution. Spatial resolution is the size of the smallest ground area that one picture element, or pixel, represents, such as 5.8 m for Resourcesat LISS-IV or about 1 m for Cartosat. Spectral resolution is the number and width of the bands recorded. Radiometric resolution is the number of brightness levels the sensor can distinguish. Temporal resolution is how often the same place is imaged, for example every 24 days. A higher spatial resolution shows smaller objects; a higher temporal resolution shows changes more often.

A satellite image is the picture produced from the digital data recorded by a satellite sensor. Unlike a photograph, it is built up from thousands of pixels, each carrying a number (the digital number) for the brightness recorded in each band. When the data of one band alone is displayed, the result is a black-and-white image. When three bands are combined and each is displayed in one of the primary colours red, green and blue, a colour image is produced.

If the red band is shown in red, the green band in green and the blue band in blue, the image looks like a natural photograph and is called a true colour composite. Such images are pleasant to look at but are not the most useful, because vegetation, water and soil are not sharply separated in visible light and haze in the blue band reduces clarity.

The standard image used by geographers is the false colour composite (FCC), in which the near-infrared band is displayed in red, the red band in green and the green band in blue. Because healthy vegetation reflects strongly in the near infrared, it appears in shades of red in an FCC: bright red for dense healthy crops and forests, pinkish or dull red for sparse or stressed vegetation. Clear deep water bodies absorb infrared and appear dark blue or black; shallow or silt-laden water appears lighter blue. Settlements, towns and roads made of concrete, asphalt and tin reflect fairly evenly and appear bluish grey to steel blue. Bare soil, sand and fallow land appear white, light grey or light brown. Clouds and snow appear brilliant white, and cloud shadows appear black. Wetlands and marshes show a mixture of red and blue tones.

Reading an FCC therefore requires the student to forget natural colours and remember the code: red is green vegetation, blue-black is water, blue-grey is built-up land, white is bare ground or cloud. Besides colour, the interpreter uses tone (lightness or darkness), texture (smooth or rough appearance), shape (a rectangular field, a winding river, a straight road), size, pattern (regular fields, radial streets) and association (a red patch beside a village is likely an orchard). These are the elements of visual image interpretation.

Images may also be processed by computer to classify every pixel automatically into land-use classes and to compare images of different dates to detect change, such as the growth of a city or the shrinking of a lake.

Satellite imagery has become the standard source of information about the earth because it is repetitive, covers huge areas, reaches inaccessible places and is available as digital data. Its uses touch almost every branch of geography and planning.

Weather and climate. Geostationary satellites such as INSAT provide cloud pictures every few minutes, allowing the India Meteorological Department to track the birth, path and landfall of cyclones such as Amphan (2020) and Yaas (2021) that struck West Bengal and Odisha, and to forecast the monsoon's advance.

Agriculture. Because vegetation shows so clearly in the infrared, crop areas can be mapped, crop health monitored, drought stress detected and yield estimated before harvest. India's FASAL programme uses this for forecasts of rice, wheat and other crops.

Forestry and land use. The Forest Survey of India prepares its two-yearly State of Forest Report from satellite data, measuring dense, open and scrub forest and detecting illegal clearing. Land-use and land-cover maps for every district are prepared from IRS imagery.

Water resources. Imagery maps surface water bodies, tracks the spread of floods in the Ganga and Brahmaputra plains, monitors the retreat of Himalayan glaciers, estimates snow cover for river-flow forecasts, and identifies potential groundwater zones from lineaments and landforms.

Disaster management. During floods, cyclones, earthquakes and forest fires, images taken before and after the event show the extent of damage and help rescue teams reach the worst-hit areas. Radar satellites work even when clouds hide the ground.

Urban and regional planning. High-resolution images from Cartosat show roads, buildings and encroachments, so city expansion can be measured and new infrastructure planned. Coastal erosion, changing river courses such as those of the Kosi and the Ganga near Malda, and the growth of river islands are all mapped from repeated images.

Geology and mining. Rock structures, faults and mineral-bearing zones are identified from their tone and texture, guiding exploration.

Ocean studies. Sea-surface temperature, chlorophyll content and potential fishing zones are mapped daily, and the information is broadcast to fishermen.

Satellite data also feeds the Geographic Information System (GIS), where images, toposheets and statistics are combined in layers on a computer for analysis, and the Global Positioning System (GPS), which locates points on the ground that are then matched with the image.

Both the toposheet and the satellite image represent the earth's surface, but they are prepared differently, look different and serve different purposes; a geographer uses them together.

Method of preparation. A toposheet is prepared from ground surveys and aerial photographs by trained surveyors, who select which features to show and draw them with conventional signs. A satellite image is a direct record of reflected or emitted energy captured by a sensor; it shows everything the sensor can detect, without selection.

Time. A toposheet may take years to survey, draw and print, and revision is slow; many Indian sheets carry survey dates several decades old, so new roads, dams or expanded towns are missing. A satellite image is obtained in a few days, and the same area can be imaged every few days, so it shows the present state of the land and allows change to be measured.

Content. A toposheet gives exact heights through contours, spot heights and bench marks, and names of villages, rivers and roads. A satellite image does not carry names or heights directly (unless processed into a digital elevation model), but it shows the actual pattern of crops, forests, water and buildings at the moment of imaging, including features a surveyor would leave out.

Scale and coverage. Toposheets are of fixed scale and fixed sheet size, roughly 750 sq km each. Images can be at coarse or fine resolution and can cover an entire state in one scene or a single city in another.

Reading. A toposheet is read through its legend, colours and grid; anyone with training can read it directly. An image is read through tone, texture, colour code of the FCC and knowledge of spectral signatures, and often needs computer processing.

Limitations. Toposheets are restricted in border areas for security, become outdated, and are expensive to revise. Satellite images can be hidden by cloud (except radar), need expensive equipment and expertise, and do not give place names or exact heights.

Using them together. The image is registered, or fitted, onto the toposheet's grid so that the new information seen in the image, such as a new canal or an expanded town, can be added to the map in purple. This is how the Survey of India now revises its sheets, and how GIS databases are built from both sources.