Chapter 2: Contours
The sheet is flat. The land is not. Spread a topographical sheet on a table and you have a piece of paper with length and breadth, and nothing else. Yet the country it stands for has hills that tire your legs, valleys that hold rivers, and slopes steep enough to make a road curl back on itself. Somewhere between the ground and the paper, the third dimension — height — has to be put back. Imagine standing at the edge of a village on such a sheet, deciding which of two tracks to take. Both reach the same place. One is a short straight line on the paper; the other loops twice as far around. The paper makes it look as though the short one wins. Then you notice the thin brown lines. Along the short track they are crowded almost on top of one another; along the long loop they lie wide apart. The short track climbs a wall of hillside. The long one walks around it. Nothing on that sheet had height in it. The brown lines carried the shape of the land anyway, and a walker who can read them knows the ground before reaching it. This chapter is about how they do it.
Diagram
Fig. 2.A1 Caption: Every contour line is the outline of one level slice through a hill, laid flat on the paper. Brief: Left, a hill cut by evenly spaced horizontal planes at 100 m intervals with each cut edge outlined; right, the same outlines projected onto a flat sheet as closed loops labelled 100 to 500; dotted vertical arrows point downward from each cut edge to its loop.
2.1 Representation Of Relief Features
Relief refers to the actual configuration of the land, which includes its altitude and its slope. Altitude is the height of the land above mean sea level, and slope is the rate at which that height changes as you move across the ground.
The relief features found on the earth’s surface include mountains, valleys, hills, plains and plateaus. All of these have three dimensions, namely length, breadth and height. The map on which they are shown has only two, length and breadth.
That is the difficulty the whole of map reading is built around. The height has to be put back onto a flat sheet by some agreed mark that the reader can decode. To solve this difficulty, a number of techniques involving a variety of methods have been evolved to show the earth’s relief features.
The Four Methods
The principal methods used are hachures, hill shading, form-lines and contours. Each of the first three gives an impression of the land, and only the last gives measured height.
Flowchart
Fig. 2.A2 Caption: Four ways of putting height back onto a flat sheet, and what each one actually records. Nodes: relief has three dimensions but the map has two → methods of showing relief → hachures (short disconnected lines drawn down the slope; records direction and steepness only) / hill shading (light and shadow effect; records steepness only) / form-lines (broken lines through places of equal height; records the general pattern only) / contours (the method used on survey sheets; records measured height)
Hachures
Hachures are short disconnected lines which follow the direction of the slope. In fact, hachures portray the direction in which water would flow down the slope over the surface, if it were allowed to flow freely.
The thickness and spacing of the lines carry the message. The lines are closely drawn and thick if the slopes are steep. If the mountain slope has a gentle gradient, that is, if it rises only a little over a long distance, then the lines are drawn thin and wide apart. Flat regions like a valley bottom and the top of a plateau are left blank, because there is no slope for the lines to follow.
Diagram
Fig. 2.1 Caption: Hachures Brief: Reproduce the book’s hachure drawing of a hill; short disconnected lines radiating outward down every slope, thick and crowded on the steep outer face, thin and spaced on the gentler ground, blank at the flat summit.
Hachures have serious drawbacks, and they are the reason the method is no longer used on survey sheets: (a) They give only an impression of the slope, and not the height of the land above sea level, so two hills of very different heights may be hachured almost identically. (b) They are difficult to draw and difficult to interpret, and they are time-consuming. (c) They obscure other topographical details in hilly regions, because the lines themselves fill the space in which roads, streams and settlements would have to be printed.
Hill Shading
Hill shading aims at producing the same effect as hachures in an easier and cheaper way. In the hill shading method, the relief of an area is shown by a ‘light and shadow effect’. It is based on the assumption that when a source of light like a big lamp is used to illuminate the relief model of the concerned area, shadows of the landforms are obtained.
The shades vary according to steepness. The slopes which face the source of light will have a lighter shade, while those turned away from the source of light will be in shadow. The shading is done by stippling, that is, by covering the area with fine dots rather than solid ink, and it is used on small scale maps, which cover a large area and can therefore carry only a general picture of the ground.
Diagram
Fig. 2.2 Caption: Hill Shading Brief: Reproduce the book’s stippled drawing of the same hill mass; dots dense on the slopes turned away from the assumed light source and sparse on the slopes facing it, valleys and plains left in a light tint.
Like hachures, hill shading gives no indication of the relative or the actual height, and it loses several distinctions a map reader needs: (a) No distinction can be made between an uphill and a downhill slope, because the shading records only how steep a face is, not which way it climbs. (b) A spur, which is a tongue of high land pushing out from a hillside, cannot be told apart from a valley cut into the same hillside. (c) A plain cannot be told apart from a plateau, since the hill slopes are shaded dark while the valleys and plains are both shown in a lighter tint. (d) Hill shading therefore sometimes obscures other features of the map.
Form-Lines
Form-lines are broken contour lines joining places with the same height above sea level. They represent only the general pattern of the ground and give no idea about definite heights.
This method is resorted to when the relief of an area is complicated and accurate data is not available, which is why the lines are printed broken rather than solid, as an honest signal that they have not been surveyed to a fixed value. The other principles of these form-lines, regarding direction and shape, are the same as those of contours, and they are therefore referred to as approximate contours.
Diagram
Fig. 2.3 Caption: Form-lines Brief: Reproduce the book’s drawing of a hill in broken lines only; concentric broken loops closing on themselves, no height figures printed anywhere on them.
Form-lines are used along with contour lines to show minor details of a mountainous or a hilly tract. The two are read together: the contour carries the measured height, and the form-line fills in the shape between one measured height and the next.
2.2 Contours
Contour lines are imaginary lines joining places which have the same height above mean sea level. Mean sea level is the average level of the sea, worked out over many years of tide readings, and every height on an Indian survey sheet is measured up from it.
Thus, a contour marked 50 means that all the places which are connected by it are located at a height of 50 metres, depending on the unit used. The line is imaginary in the sense that nothing is drawn on the ground; it is the surveyor’s way of tying together every point that happens to stand at one level.
On a topographical sheet the contour lines are shown in brown colour, and they are drawn in two ways: (a) Thick brown lines, which carry the rounder values and are printed heavier so that the eye can find a height quickly on a crowded sheet. (b) Thin brown lines, which fall between the thick ones and fill in the shape of the ground.
What A Contour Tells You
Contour lines show three things at once, and a student who can pull all three off the same set of lines can read relief: (a) Elevation of land. The height of a contour is indicated by figures inserted in a break in the line, so the value sits inside the line itself rather than beside it. (b) Steepness of its slope. When the contours are very close, they represent steep slopes. When they are farther apart, they represent gradually increasing slopes. The absence of contour lines indicates that the land is flat, that is, a low land. (c) Shape of land at various heights. The contour lines indicate the shape of land. For example, almost circular contours whose value decreases inside represent a lake.
2.2.1 Contour Interval
Contour interval means the difference of height between two consecutive contours. In Figure 2.4, one thick contour shows the height 300 m and the next thin contour depicts the height of 320 m. The contour interval is therefore 20 m.
Diagram
Fig. 2.4 Caption: Contour Interval Brief: Reproduce the book’s contour map of a hill, closed brown loops with the values as printed, the 300 m line drawn heavy and labelled Thick Contour, the next line drawn light and labelled Thin Contour, and the height figures set in breaks in the lines.
The interval is fixed for a whole sheet, and that is what makes contour reading possible: once you know the interval, you can count lines instead of reading every figure.
Vertical Interval (V.I.) is the vertical distance between any two contour lines. In most of the British ordnance maps the vertical interval is either 50 or 100 feet, feet being the unit used in Britain. In the 1 : 50,000 Survey sheets of India, the contour interval is 20 m.
Horizontal Equivalent (H.E.) is the actual distance between two points on two contour lines, measured along the ground rather than up the slope.
Diagram
Fig. 2.5 Caption: Vertical interval (V.I.) and Horizontal Equivalent (H.E.) Brief: Reproduce the book’s cross-section, with horizontal level lines drawn at 0 m to 600 m, the ground profile crossing them, and each pair of steps labelled V.I. for the vertical rise and H.E. for the horizontal distance, with C.I. marked at the foot of the slope.
Gradient
Gradient (G) refers to the slope of land, and it determines the steepness of a slope. It is the ratio between the vertical height and the horizontal distance measured along the ground, that is,
\[\text{Gradient} = \frac{\text{Vertical Interval}}{\text{Horizontal Equivalent}}\]
Here the vertical interval is the rise in height and the horizontal equivalent is the distance covered on the ground, and both must be expressed in the same unit before they are divided.
For example, a gradient of 1 : 5 means that for every five units of horizontal distance, there is a rise of one unit in the vertical distance. This is a rather steep slope. On the other hand, 1 : 50 indicates that the slope is rather gentle, because for every 1 unit of vertical rise, the horizontal distance travelled is 50 units.
2.2.2 Calculation Of Gradient Of A Landscape
The chapter works out one gradient in full, between the points A and B marked on Figure 2.6. Follow the three steps in order, because each one supplies a quantity the next one needs.
Diagram
Fig. 2.6 Caption: Gradient Brief: Reproduce the book’s contour map with its printed contour values, A marked outside on the lower contour and B on the higher contour inside, the straight line AB marked 3 cm, and the scale statement 2 cm = 1 km printed in the corner.
A gradient of 1 : 20 is a statement about walking, not about arithmetic. It says that for every twenty metres you cover across the ground between A and B, you rise one metre. Compared with the 1 : 5 slope described earlier, this is easy going, and it is the sort of slope a road can climb without doubling back on itself. Notice also what the calculation needed: a height taken from the contour values, a distance taken with a ruler, and the scale to turn map centimetres into ground metres. Take away the scale and the second step collapses, because 3 cm on paper means nothing until the sheet tells you what it stands for.
2.2.3 Principles Of Contours
Six principles govern how contour lines behave on every sheet, and together they are what allow a reader to trust the pattern: (a) Contour lines join places having the same height above mean sea level. (b) Contours are curved lines, whose curvature depends upon the configuration of the land, and they are never broken. (c) The spacing of the contour lines indicates the nature of the slope, so the gaps between the lines are read as carefully as the values on them. (d) Contours of different heights usually do not cross each other. This is because they represent a specific elevation at a given point. If they crossed, it would indicate that a point had two elevations at the same time, which is physically impossible. However, contours come very close to each other to show the steepness of the slope, or merge with each other as in a cliff. (e) Contour lines show stream direction with the V-shaped lines through which the streams and rivers run on a topo-sheet. Since water flows downhill, the V-shape always points in the opposite direction to that in which the water is flowing. (f) A contour line either closes on itself on the map, or it begins at one side of the map and ends at another. In the case of a ridge contour, the lines run parallel to each other, and in the case of a valley they point upstream.
2.3 Identification Of Landforms Marked By Contours
Contours give the general shape of the ground, but a survey sheet also has to record the height of particular points exactly. Four marks do that work, and each one is written differently so that the reader knows at a glance what kind of height is being quoted.
Triangulated Height
When the point has been accurately surveyed, and shown with a triangle, it is called a triangulation point, written on the sheet as △340. The height of such a point is called triangulated height, and it is determined trigonometrically, that is, by calculation from measured angles rather than by carrying a measuring rod up the slope. For example, the mountain peaks are indicated by triangulation points.
Spot Heights
A spot height refers to the height of a point on the ground above mean sea level. It is indicated on a map but not on the ground, so there is nothing to look for if you go to the place itself.
It is represented by a dot followed by a number on the map, written as •500. The number shows the height above mean sea level of that place. A spot height gives information about the height of a specific point only, and it gives no idea about relief features like mountains, plains and the rest, which is why a sheet needs contours as well as spot heights.
Read together, these four values do something a contour cannot. Contours on an Indian survey sheet are drawn at a fixed interval, so between one line and the next the sheet says nothing about height at all. The dotted, triangular and BM values are put in exactly to fill that silence, at the points where a reader most needs certainty, such as a summit, a road junction or a bridge approach. The 0·7 m difference between the two bench marks is also a reminder of how gentle the ground is there, since two marked points a short distance apart differ by less than a metre.
Relative Height And Relative Depth
In areas of steep slope, the difference between the maximum and the minimum is represented by the letter r, which indicates relative height. Thus, if one finds 3r• or •3r written on a topo-sheet, this means that the concerned place is higher by 3 metres than its natural lowest place.
For example, a dry tank with an embankment with 3r inside it means the top of the embankment is 3 m above the surrounding ground. The value is measured from the ground beside it, not from mean sea level, which is exactly why it is called relative.
Conversely, relative depth is shown in water bodies like wells or rivers, indicating the depth of water from the surface of the water body downwards. For example, 3r written near a river indicates that the river has a relative depth of 3 m.
Diagram
Fig. 2.A3 Caption: Relative height is measured from the ground beside a feature, not from mean sea level. Brief: Two panels; left, a dry tank with an embankment labelled 3r, with an arrow from the surrounding ground up to the embankment top marked 3 m; right, a river channel labelled 3r, with an arrow from the water surface down to the bed marked 3 m; a broken line low on both panels labelled mean sea level, well below the measured arrows.
Bench Marks
Bench marks are marks etched on a stone or shown on a building to indicate the accurate height determined by surveys. They serve as a reference for surveyors, who need a fixed starting height on the ground before any fresh levelling can begin.
A bench mark is written as BM followed by a number. The number indicates the height of the surface level at the marked point.
There is a difference between a spot height and a bench mark, and it is easy to lose. A spot height indicates only the height of the ground above mean sea level that is seen on a map, while a bench mark indicates the height of some point marked above the ground and includes the height of the object on which the bench mark is etched. Thus a bench mark is marked on both the map and the actual ground, whereas a spot height exists only on the map. What the two share is their reference: both are heights above mean sea level, and both are fixed by survey rather than estimated from the contour pattern.
2.4 Contour Diagrams
Every landform has a contour signature, and once you know the signature you can name the landform without ever seeing it. The diagrams that follow pair each pattern of lines with the cross-section of the ground that produced it, so that the plan view above and the side view below can be read together.
2.4.1 Slopes
Steep Slope
When the land suddenly slopes up or down, it is known as a steep slope. In a steep slope the contours are drawn close to one another, because a large rise is packed into a short horizontal distance.
Diagram
Fig. 2.7 Caption: A Steep Slope Brief: Reproduce the book’s two-part drawing; above, contours from 100 to 1100 crowded close together between A and B with the contour interval 100 m stated, and below, the cross-section along AB showing the ground falling sharply from 1100 m to 100 m, with vertical guide lines joining each contour to its point on the profile.
Gentle Slope
In a gentle slope, the change of elevation is gradual. Normally about a 1 : 25 gradient is called a gentle slope, and the contours in such a slope are placed wide apart.
Diagram
Fig. 2.8 Caption: A Gentle Slope Brief: Reproduce the book’s two-part drawing; above, contours 100 to 500 spaced widely between A and B, and below, the cross-section along AB showing a long, almost even fall from 500 m to the base, with vertical guide lines joining the two views.
The two slopes are best read against each other. Both drawings cover the same width of paper and both use the same contour interval, yet the steep slope carries eleven lines where the gentle one carries five. The spacing, not the values, is what your eye should go to first.
A Hill
A hill refers to a landform having a summit at a high elevation. A conical hill is like a cone which has a broad base, and its slope is almost the same on all sides.
Contours showing a conical hill are almost circular with the peak of the hill as their centre. The contours are drawn almost at regular distances, the value of which increases towards the centre, and that even spacing is the direct record of a slope that is the same all round.
A mountain is higher in elevation than a conical hill, and it has more contour lines representing it. The extra lines are not a matter of drawing style; at a fixed contour interval, greater height simply means more levels to be crossed on the way up.
Diagram
Fig. 2.9 Caption: A Conical Hill Brief: Reproduce the book’s two-part drawing; above, near-circular contours 100 to 600 evenly spaced with the highest value innermost, cut by the line XY, and below, the cross-section along XY showing a symmetrical cone rising to 600 m.
Valleys And The Water Divide
The same reasoning, run in reverse, gives the valley forms. Contours of valleys are the opposite of hills, with values decreasing towards the centre.
Diagram
Fig. 2.A4 Caption: V-Shaped Valley — rivers form these valleys along the upper course, and the contour values decrease towards the centre. Brief: Reproduce the book’s panel; above, sharply pointed contours 800 to 1200 opening towards the lower ground and cut by the line LM, and below, the cross-section along LM showing a narrow V-shaped notch between two high shoulders.
Diagram
Fig. 2.A5 Caption: Gorge or canyon — a deep and narrow valley having steep slopes, where the contours are closely spaced and the V formed by them is very narrow. Brief: Reproduce the book’s panel; above, tightly packed contours 700 to 1100 forming a very narrow V, cut by the line RS, and below, the cross-section along RS showing a deep, near-vertical-sided slot.
Diagram
Fig. 2.A6 Caption: U-shaped valley (U-V) — broad flat-floored valleys formed in the middle course of a river, or by the action of glaciers, with a hanging valley (H-V) where the tributary valley meets the main glacier. Brief: Reproduce the book’s panel; above, broadly rounded contours 900 to 1600 labelled U-V along the main valley floor and H-V where a tributary valley joins high on the side, and below, the cross-section showing a wide flat floor with steep walls and the tributary floor entering well above it.
Diagram
Fig. 2.A7 Caption: Water divide or watershed — a ridge which separates the tributaries draining towards one side from those draining towards the other. Brief: Reproduce the book’s panel; contours 800 to 1100 over an elongated highland with a broken line along the crest labelled Water Divide, streams in blue running away from the line on both sides with arrows showing the direction of flow, contours drawn close on the steeper flank and far apart on the gentler flank.
On a water divide the two sides slopes must be opposite and downwards. The side on which the ridge is steeper has its contours drawn closely together, whereas the side on which the ridge is gentle has its contours drawn far apart. That single line of high ground decides which river a raindrop ends up in.
Ridge
A ridge is a long and narrow highland sloping steeply downwards on its sides, and it has a number of summits. Contours showing a ridge are elongated and are closely spaced, the elongation recording the length and the crowding recording the steep flanks.
Sometimes there occur peaks and passes across a ridge. A pass is the low point where the crest dips, and it is the place a road or a track will take, because crossing anywhere else would mean climbing to a summit.
Diagram
Fig. 2.10 Caption: A Ridge Brief: Reproduce the book’s two-part drawing; above, elongated contours 300 to 700 running lengthways between P and Q with several 700 loops marking separate summits, and below, the section along PQ showing a long high mass with an uneven crest and steep sides.
Escarpment
An escarpment is an abrupt, comparatively long and regular steep face of a hill, a ridge or a plateau. An escarpment has a narrow top with a steep slope, called the scarp, on one side, and a gentle slope, called the dip, on the other.
The two faces can be told apart by their gradients as well as by their look. The steep slope has a gradient of 1 : 3 and close contour lines, whereas the gentle slope has a gradient of about 1 : 12.
A ridge with a steep scarp in the front and a dip slope on the back is called a cuesta. Cuestas are caused by the uneven erosional work of rocks on both the sides, since the two flanks are worn back at different rates. The highest part is called the crest, and the scarp and the dip always occur together.
Diagram
Fig. 2.11 Caption: Escarpment Brief: Reproduce the book’s two-part drawing; above, contours 100 to 700 crowded on one flank and widely spaced on the other between A and B, and below, the section along AB with the summit labelled Crest, the steep face labelled Scarp and the long gentle face labelled Dip.
Sometimes erosion by running water leads to the formation of V-shaped valleys on the dip side of an escarpment. Such an escarpment is called a dissected scarp, dissected meaning cut into by the streams that run down the long gentle back.
Drainage Systems
When it rains, the water slides down in the form of small streams that join other small streams, forming bigger main streams, and finally form a river. The course of a well-developed river can be divided into three stages, the upper, middle and lower course, each of which can be recognised with the help of contour lines.
Flowchart
Fig. 2.A8 Caption: Rain collects into ever larger channels, and the river it builds is read in three stages down the contour sheet. Nodes: rain falls → water slides downslope in small streams → small streams join other small streams → bigger main streams form → a river forms → upper course (steep slopes; valley formation) → middle course (gentle slopes; flood plains; meanders and oxbow lakes) → lower course (gentle slopes; deltas)
Diagram
Fig. 2.12 Caption: The Upper, Middle and Lower Courses of a River on a Topographical map Brief: Reproduce the book’s wide drawing; above, one continuous river drawn in blue across contours falling from 1900 to 500, with the section lines PQ, RS and TU marking the upper, middle and lower courses and a delta entering the sea at the right; below, the three cross profiles along PQ, RS and TU; and at the side the panel listing Upper Course — steep slopes, valley formation; Middle Course — gentle slopes, flood plains, meanders and oxbow lakes; Lower Course — gentle slopes, deltas.
Drainage Patterns
A drainage pattern refers to the pattern formed by the streams, rivers and lakes in a particular drainage area. These patterns are formed in response to the topography of the land, the nature of the rocks, tectonic activities, the supply of water and the gradient of the land, which is why the pattern on a sheet is evidence about the ground beneath it.
The most important drainage patterns are these: (a) The dendritic drainage pattern is a branching tree-like drainage pattern which is generally found on flat lands, and most of the rivers of the Indo-Gangetic Plains, the great level lowland of northern India, are of dendritic type. (b) The trellised drainage pattern is a rectangular drainage pattern with streams flowing parallel to each other and joining the main stream at right angles, and it is generally seen in areas with alternating hard and soft rocks or in regions of limestone rocks. (c) The radial drainage pattern is a drainage pattern with streams spreading out in all directions like the spokes of a wheel, and it can generally be seen where a conical hill or a volcano exists. (d) Disappearing streams occur when rivers or streams do not have enough water, so they dry up or disappear into the sand; they are indicated by broken lines, and this phenomenon is known as the underground drainage in the context of topographic maps.
Diagram
Fig. 2.13 Caption: Drainage Patterns Brief: Reproduce the book’s four panels in blue line work, labelled Dendritic Drainage, Trellised Drainage, Radial Drainage and Underground Drainage, with flow arrows on the channels, the radial panel labelled Highland at the centre, and the underground panel showing a river continuing as a dotted Dry Valley.
2.5 Conventional Signs And Symbols
Topographical maps use symbols to represent certain relief features and human activities. These are called conventional signs and symbols, and conventional here means agreed upon, so that the same mark carries the same meaning for every reader of every sheet.
They are used on a map because they give plenty of information in a small space, and maps can be drawn easily with them. They are simple to read. Many of the signs and symbols are standardised and used throughout the world, which is why a surveyor trained on one country’s sheets can make sense of another’s.
The key, or legend, of a map has the list of signs and symbols as well as their meanings. It is printed at the edge of the sheet, and nothing on a topographical map should be interpreted without it.
Reading A Survey Sheet
The sheet extract printed here shows what all of this looks like in one small piece of country. Contours, spot heights, bench marks, settlements, roads, railway lines and a powerline are all crowded into a few grid squares, each recorded by its own mark.
Map
Map 2.A1 Caption: Features on a topo-sheet. Brief: Reproduce the book’s annotated survey sheet extract covering eastings 39 to 45 and northings 85 to 91, with the grid lines and their numbering in red and every conventional symbol and colour tint legible; the labelled features are powerline with pole (unsurveyed), metalled road, spot height 317 m, ridge, gap, peak, hillock or knoll, broken ground, surveyed tree, national highway, broad gauge railway line, mosque, covered tank, post office, bench mark, cutting with tunnel, cart track, triangulated height, temple, highway with distance stone, chhatri and sand feature; the settlement names as printed on the sheet; show the full extent of India as officially defined only if any index map is included.
The chart of symbols given in this chapter sets out the full list, and a student should work through it in the way one works through an alphabet, a few marks at a time, until the shapes are read without stopping.
Diagram
Fig. 2.A9 Caption: Symbols and conventional signs. Brief: Reproduce the book’s full symbol chart with the name of each feature on the left and its sign drawn to the right in its correct colour, covering express highway, metalled and double carriageway roads, unmetalled roads with cart-track, pack-track and foot-path, streams and canals, dams and weirs, rivers, submerged rocks and swamps, wells and tanks, embankments and broken ground, broad gauge and other gauge railways, mineral line and kiln, contours with rocky slopes and cliffs, sand features, towns and villages, huts and towers, temple, chhatri, church, mosque, idgah, tomb and graves, lighthouse and buoys, mine, grass and scrub, palms and other trees, cultivated and wooded areas, international, state, district and forest boundaries, boundary pillars, triangulated heights, bench-marks, post office, rest house and police station, camping ground and reserved or protected forest, spaced names, hospital and dispensary, aerodrome and helipad, and power lines.
2.5.1 Hindi Terms Used In Survey Maps
Survey sheets of India print many local words directly on the map, and these are not translated. A reader who does not know them will miss a stream, a pond or an office of the revenue administration, so they must be learnt along with the symbols.
2.5.2 Colours In Contour Maps
Apart from conventional signs, different colours are used to show the distributional pattern of different land uses on the topographical sheets. Colour is read before shape, because the eye picks up a tint across a whole area faster than it picks up a small symbol.
Seven colours are used on an Indian topographical sheet, each carrying its own group of features: (a) Black is used for names, for the lines of latitude and longitude, for surveyed trees, heights and their numbering, for river banks, broken ground and dry streams, and for railway, telephone and telegraph lines. (b) Blue is used for water related features, for perennial rivers, which flow all through the year, and for perennial wells. However, minor non-perennial streams are shown in black, and irregular erratic streams are shown by dotted lines. (c) Green is used for forests in dark green, grasslands in light green, orchards, scattered trees and shrubs, and footpaths in forests. (d) White is left for rocky or badlands, that is, rough broken country that cannot be farmed, and for broken grounds and uncultivated lands. (e) Yellow is used for cultivable lands, so a broad yellow stretch on a sheet is a first sign of a farming countryside. (f) Brown is used for sand features like sand dunes and hills, for contour features like contour lines and their numbering, and for form-lines. (g) Red is used for grid lines, that is, the eastings numbered across the sheet and the northings numbered up it, along with their numbering, and for man-made features such as roads, cart tracks, buildings, huts, temples, chhatri, hospitals and aerodromes.
When A Topographical Map Is Not Enough
These days all the details of a place are easily available in the form of Global Positioning Systems, or GPS, on cellphones, and map reading is becoming a lost skill. But when one enters places such as the wilderness or the deep seas, cellphones lose connectivity, and map reading becomes a key survival skill.
We have studied that topographic maps show land features in detail, denoting roads, trails, notable landforms and other ground features that can help the user select a route. But in water bodies, especially near shorelines, a topo-map will show the general depth along the shoreline and nothing else. Here comes the use of nautical charts.
A nautical chart shows hydrographic data, which is detailed information on shoreline characteristics, water depth, tide predictions and other structures, as well as obstructions to navigation like reefs, shoals and sub-surface rocks. It is a navigational tool used by mariners to plot courses across open bodies of water or in high-traffic commercial waterways.
The nautical chart, in addition to basic land contours along the coastline, will reveal the location of rocks below the surface. This is useful for determining whether their alignment could create standing waves, depending on the timing of the tides. Locations of hazards or obstructions such as shoals, reefs and buoys make the chart a better reference when navigating along an unfamiliar coastline.
Diagram
Fig. 2.A10 Caption: Some common similarities and differences between topographic maps and nautical charts include: R-R1: relief i.e. detailed contour intervals, land features; B-B1: shoreline features i.e. shoals, sandbars, etc.; S-S1: Prominent landscape features, landmarks; D-D1: surface-subsurface features, hazards, information; I-I1: sea floor contours providing depth information. Brief: Reproduce the book’s paired image, a topographic map of a coastal stretch on the left and a nautical chart of the same stretch on the right, with the matched labels R and R1, B and B1, S and S1, D and D1, I and I1 placed on the corresponding features in each half.
2.6 Summary
- Relief refers to the actual configuration of the land, including its altitude and its slope, and it covers mountains, valleys, hills, plains and plateaus.
- Relief features have three dimensions, length, breadth and height, but a map has only two, so the height must be put back onto the sheet by an agreed mark.
- Four principal methods have been evolved to show relief.
- Hachures are short disconnected lines following the direction of the slope, thick and close on steep slopes, thin and wide apart on gentle ones, and blank on flat ground.
- Hill shading shows relief by a light and shadow effect, done by stippling, and is used on small scale maps.
- Form-lines are broken contour lines through places of the same height, giving the general pattern only, and are used along with contours to show minor details.
- Contours are the method used on survey sheets, and they alone carry measured height.
- Neither hachures nor hill shading gives the actual height, and hill shading cannot separate uphill from downhill, spur from valley, or plain from plateau.
- Contour lines are imaginary lines joining places which have the same height above mean sea level, printed in brown as thick and thin lines, with the value inserted in a break in the line.
- Contour lines show three things at once.
- The elevation of the land, from the figure printed on the line.
- The steepness of the slope, from the spacing, with close lines meaning steep and wide spacing meaning gentler ground.
- The shape of the land at various heights, so that almost circular contours whose value decreases inside represent a lake.
- The contour interval is the difference of height between two consecutive contours, and it is 20 m on the 1 : 50,000 Survey sheets of India.
- Three related quantities describe a slope.
- The vertical interval is the vertical distance between any two contour lines.
- The horizontal equivalent is the actual distance between two points on two contour lines.
- The gradient is the vertical interval divided by the horizontal equivalent, so 1 : 5 is a rather steep slope and 1 : 50 a rather gentle one.
- Contours obey fixed principles: they join places of equal height, they are never broken, their spacing shows the nature of the slope, they usually do not cross, they point upstream across a valley so that the V opens the way the water flows, and they either close on themselves or run from one edge of the map to another.
- Exact heights at particular points are printed with their own marks.
- A triangulated height is shown by a triangle and is fixed trigonometrically at a surveyed station.
- A spot height is shown by a dot and gives the height of one point on the map only.
- Relative height or depth, written with the letter r, is measured from the ground or water surface beside the feature, not from mean sea level.
- A bench mark, written BM, is etched on stone or a building and so exists on the ground as well as on the map.
- Each landform has its own contour signature.
- A steep slope has close contours and a gentle slope, of about 1 : 25 gradient, has widely spaced ones.
- A conical hill has almost circular contours at regular distances whose value increases towards the centre, and a mountain is higher with more contour lines.
- A V-shaped valley is cut in the upper course and a gorge or canyon is its deep, narrow form; a U-shaped valley is broad and flat-floored, and a hanging valley meets the main glacier from above.
- A ridge is long and narrow with elongated, closely spaced contours and several summits, and a water divide is the ridge separating tributaries draining to opposite sides.
- An escarpment has a steep scarp of 1 : 3 gradient on one side and a gentle dip of about 1 : 12 on the other, meeting at the crest; a cuesta has scarp in front and dip behind, and a dissected scarp is cut by V-shaped valleys on the dip side.
- A river’s course is read in three stages from the contours, the upper course with steep slopes and valley formation, the middle course with gentle slopes, flood plains, meanders and oxbow lakes, and the lower course with gentle slopes and deltas.
- Drainage patterns form in response to the topography, the rocks, tectonic activity, water supply and gradient, and the chief patterns are dendritic on flat lands, trellised where hard and soft rocks alternate, radial around a conical hill or volcano, and disappearing streams shown by broken lines.
- Conventional signs and symbols are standardised marks explained in the key, and Hindi terms such as nadi, nala, talab and dak bungalow are printed on Indian sheets untranslated.
- Colour carries land use: black for names and lines, blue for perennial water, green for forest and grass, white for rocky and uncultivated land, yellow for cultivable land, brown for contours and sand features, and red for grid lines and man-made features.
Flowchart
Fig. 2.A11 Caption: One sheet, read in order — colour, then value, then spacing, then shape, until the ground has a name. Nodes: ground has height but the sheet has only length and breadth → surveyor fixes heights above mean sea level → heights printed at points (triangle, dot, BM, r) and joined into contours at a fixed interval → read the spacing → close together? yes → steep slope; no → gentle slope → read the shape → almost circular with values rising inwards → hill or mountain; almost circular with values falling inwards → lake; V pointing upstream → valley, gorge or canyon; broad and flat-floored → U-shaped valley; elongated and closely spaced → ridge or water divide; close on one flank and wide on the other → escarpment with scarp and dip → back to the sheet for the next pattern
What This Chapter Cannot Tell You
Contours are drawn at a fixed interval, so between one line and the next the sheet says nothing at all; a small knoll or a gully shallower than the interval simply does not appear. The lines record height and shape, never the rock, the soil or the land use beneath them, which is why the colours and symbols have to be read alongside them. A survey sheet also fixes a date, not a present: everything on it was true when the survey was made, and a tank, a track or a stream may have changed since. ## 2.7 Check Your Concepts
- Look again at Fig. 2.4. State the contour interval shown there, and explain how the figures on the lines tell you that this ground is a hill and not a hollow.
- The contour interval on the 1 : 50,000 Survey sheets of India is 20 m. A path climbs from the 300 m contour to the 500 m contour. How many contour lines does it cross on the way up? Show your working.
- Using Fig. 2.12, trace how falling rain becomes a river, and then state one feature of the contour pattern by which each of the three courses — upper, middle and lower — can be recognised on a topographical sheet.
- An embankment 3 m high is marked 3r on a sheet whose contour interval is 20 m. Explain why no contour line on that sheet records the embankment, and say what this tells you about what contour lines can and cannot show.
- Explain how you would distinguish between the following on the basis of contours:
- Gentle slope and steep slope
- Scarp and dip
- Peak and depression
- What advantages do contours have over hill shading and hachuring?
- Give two points of difference between a bench mark and a spot height.
- Why do contours of different heights not cross each other?
- How do contour lines show the direction of the flow of rivers and streams?
- Draw the conventional sign or symbol for each of the following, using the correct colour where necessary:
- Cart-track and pack-track with pass
- Dam, masonry and dam, earthwork
- Dry river with water channel and river with island and rocks
- Lined well and unlined well
- Perennial well and dry well
- Permanent hut and temporary hut
- Temple and chhatri
- International boundary and state boundary
- Inspection bungalow and circuit house
- Hospital and dispensary