Chapter 5 Topographical Maps | Class 11th Practical Work in Geography revision notes

NCERT Class 11 Chapter 5 Practical Geography Topographical Maps

Topographical Maps

• These maps show important natural and cultural features such as relief, vegetation, water bodies, cultivated land, settlements, and transportation networks, etc. These maps are prepared and published by the National Mapping Organization of each country. For example, the Survey of India prepares the topographical maps in India for the entire country. The topographical maps are drawn in the form of series of maps at different scales. Hence, in the given series, all maps employ the same reference point, scale, projection, conventional signs, symbols, and colours.
• Topographic Map: A map of a small area drawn on a large-scale depicting detailed surface features both natural and manmade. Relief in this map is shown by contours.

Topographical Maps in India in 2 Series

• India and Adjacent Countries Series: the Survey of India prepared topographical maps under India and Adjacent Countries Series until the coming into existence of Delhi Survey Conference in 1937. Henceforth, the preparation of maps for the adjoining countries was abandoned and the Survey of India confined itself to prepare and publish the topographical maps for India as per the specifications laid down for the International Map Series of the World. However, the Survey of India for the topographical maps under the new series retained the numbering system and the layout plan of the abandoned India and Adjacent Countries Series. The topographical maps of India are prepared on 1: 10,00, 000,1: 250,000, 1: 1,25, 000,1: 50,000 and 1: 25,000 scale providing a latitudinal and longitudinal coverage of  and , respectively.
• International Map Series of the World: Topographical Maps under International Map Series of the World are designed to produce standardized maps for the entire World on a scale of 1: 10,00, 000, and 1: 250,000.
• The study of topographical maps is simple. It requires the reader to be acquainted with the legend, conventional sign, and the colours shown on the sheets.

• The Survey of India is India՚s central engineering agency in charge of mapping and surveying. Set up in 1767 to help consolidate the territories of the British East India Company It is one of the oldest Engineering Departments of the Government of India. The Survey of India՚s distinguished history includes the handling of the mammoth Great Trigonometric Survey under William Lambton and George Everest
• Among the many accomplishments of the Survey were the demarcation of the British territories in India and the measurement of the height of the Himalayan giants: Everest, K2, and Kanchenjunga. The Great Trigonometrical Survey of India started on 10 April 1802 with the measurement of a baseline near Madras. The East India Company thought that this project would take about 5 years but eventually it took more than 60 years, draining the profits of the Company, so much so it was brought under the Crown after 1857

• Toposheet number, state, district, latitude, longitude
• Legend sheet, index to sheet, map scale, district outline, legend
• National Map Policy: To provide, maintain and allow access and make available the National Topographic Database (NTDB) of the SOI conforming to national standards. To promote the use of geospatial knowledge and intelligence through partnerships and other mechanisms by all sections of the society and work towards a knowledge-based society.
• Two Series Of Maps – DSM Defense Series Maps (DSMs) – These will be the topographical maps (on Everest/WGS-84 Datum and Polyconic/UTM Projection) on various scales (with heights, contours, and full content without dilution of accuracy) . These will mainly cater for defense and national security requirements. This series of maps (in analogue or digital forms) for the entire country will be classified, as appropriate, and the Ministry of Defense will formulate the guidelines regarding their use.
• Two Series Of Maps – OSM Open Series Maps (OSMs) for supporting development activities in the country. OSMs shall bear different map sheet numbers and will be in UTM Projection on WGS-84 datum. Each of these OSMs (in both hard copy and digital form) will become “Unrestricted” after obtaining a one-time clearance of the Ministry of Defense. SOI will ensure that no civil and military Vulnerable Areas and Vulnerable Points (VA՚s/VP՚s) are shown on OSMs
• National Topographical Data Base (NTDB) NTDB in analogue and digital forms consisting of following data sets: National Spatial Reference Frame, National Digital Elevation Model, National Topographical Template, Administrative Boundaries, and Toponomy (place names) . Both the DSMs and OSMs will be derived from the NTDB

• Representative Fraction (RF) : It is the ratio between the distances on the map to its corresponding distance on actual ground. The RF on this map is 1: 50,000.
• Scale: Scale is the ratio between the distance of any two points on the map and the actual distance of the same points on the ground. The scale of the given map extract is 2 cm: 1 km or 1: 50,000.
• Contour: Contours are imaginary lines drawn on maps, joining all places with the same height above sea level.
• Contour Interval: The interval between two consecutive contours is called contour interval.
• Index Contour: Contour lines are thickened at regular intervals to make it easier to read contours. For example at every 100 mts, the contour line is made darker. The darker lines are called Index Contours.
• Triangulated Height: It is the height of a place which has been calculated using trigonometry, represented by a small triangle e. g. 
• Spot Height: The height of random places between contours shown with a dot. Eg – 0.425
• Bench Mark: Height of a place actually marked on a stone pillar, rock or shown on a building as a permanent reference. It is written as BM 200 m.
• Relative Height: Relative height is the height of a feature with reference to the height of the surrounding land and NOT to sea level. It is represented by the height with a small ‘r’ e. g. – 12r.
• Mixed Forest: A forest with more than two varieties of trees growing in close proximity to each other.
• Open Jungle: A forest where trees are widely scattered
• Dense Jungle: A forest where trees grow very close to each other
• OPEN SCRUB: Scrub is a vegetation found in regions with less than 100 cm of rainfall. Therefore, it indicates a dry region.
• Embankment: They are raised rock or soil filled constructions on which roads/railway tracks are built. Also made near tanks and rivers to prevent flooding
• Form Lines: Form lines are contour lines, but show only approximate heights above sea level as they are used to indicate the elevations of the area, which are not accessible for proper survey. Hence, they are drawn as broken lines and are called ‘form lines’ .
• Q. C. Q. D. , OC, OD, PQ, ETC: These are alphabetical codes used to represent the biggest grid sq. of 10,000 square kms

Methods of Relief Representation

• Methods of Relief Representation: The earth՚s surface is not uniform and it varies from mountains to hills to plateaus and plains. The elevation and depressions of the earth՚s surface are known as physical features or relief features of the earth. The map showing these features is called a relief map.
• A number of methods have been used to show the relief features of the Earth՚s surface on maps, over the years. These methods include hachure, hill shading, layer tints, benchmarks and spot heights and contours. However, contours and spot heights are predominantly used to depict the relief of an area on all topographical maps.
• Settlements, buildings, roads, and railways are important cultural features shown on topographical sheets through conventional signs, symbols, and colours.
• Four types of rural settlements may be identified on the map
  • Compact
  • Scattered
  • Linear
  • Circular
• Similarly, urban centres may also be distinguished as
  • Cross-road town
  • Nodal point
  • Market centre
  • Hill station
  • Coastal resort centre
  • Port
  • Manufacturing centre with suburban villages or satellite towns
  • Capital town
  • Religious centre
• Various factors determine the site of settlements like
  • Source of water
  • Provision of food
  • Nature of relief
  • Nature and character of occupation
  • Defence
• Density of settlement is directly related to food supply. Sometimes, village settlements form alignments, i.e.. they are spread along a river valley, road, embankment, and coastline, – and these are called linear settlements.
• In the case of an urban settlement, a crossroad town assumes a fan-shaped pattern, the houses being arranged along the roadside, and the crossing being at the heart of the town and the main market place. In a nodal town, the roads radiate in all directions

Contours

• Contours are imaginary lines joining places having the same elevation above mean sea level. A map showing the landform of an area by contours is called a contour map.
• Earlier, ground surveys and levelling methods were used to draw contours on topographical maps. However, the invention of photography and subsequent use of aerial photography have replaced the conventional methods of surveying, levelling and mapping. Henceforth, these photographs are used in topographical mapping.
• Contours are drawn at different vertical intervals (VI) , like 20,50, 100 metres above the mean sea level. It is known as contour interval. It is usually constant on a given map. It is generally expressed in metres. While the vertical interval between the two successive contour lines remains constant, the horizontal distance varies from place to place depending upon the nature of slope. The horizontal distance, also known as the horizontal equivalent (HE) , is large when the slope is gentler and decreases with increasing slope gradient.

• Contours: Imaginary lines joining all the points of equal elevation or altitude above mean sea level. They are also called “level lines.”
• Contour Interval: Interval between two successive contours. It is also known as vertical interval, usually written as V. I. Generally, it is constant for a given map.
• Cross-section: A side view of the ground cut vertically along a straight line. It is also known as a section or profile.
• Hachures: Small straight lines drawn on the map along the direction of maximum slope, running across the contours. They have given an idea about the differences in the slope of the ground.
• The slopes can broadly be classified into gentle, steep, concave, convex and irregular or undulating. The contours of different types of slopes show a distinct spacing pattern.
• Gentle Slope – When the degree or angle of slope of a feature is very low, the slope will be gentle. The contours representing this type of slope are far apart.
• Steep Slope – When the degree or angle of slope of a feature is high and the contours are closely spaced, they indicate steep slope.

Steps for Drawing a Cross-Section

The following steps may be followed to draw cross-sections of various relief features from their contours:

• Draw a straight line cutting across the contours on the map and mark it as AB.
• Take a strip of white paper or graph and place its edge along the AB line.
• Mark the position and value of every contour that cuts the line AB.
• Choose a suitable vertical scale, eg  cm = 100 metres, to draw horizontal lines parallel to each other and equal to the length of AB. The number of such lines should be equal or more than the total contour lines.
• Mark the appropriate values corresponding to the contour values along the vertical of the cross-section. The numbering may be started with the lowest value represented by the contours.
• Now place the edge of the marked paper along the horizontal line at the bottom line of the cross-section in such a way that AB of the paper corresponds to the AB of the map and mark the contour points.
• Draw perpendiculars from AB line, intersecting contour lines, to the corresponding line at the cross-section base.
• Smoothly join all the points marked on different lines at the cross section base.

• Concave Slope: A slope with a gentle gradient in the lower parts of a relief feature and steep in its upper parts is called the concave slope. Contours in this type of slope are widely spaced in the lower parts and are closely spaced in the upper parts.
• Convex Slope: Unlike concave slope, the convex slope is fairly gentle in the upper part and steep in the lower part. As a result, the contours are widely spaced in the upper parts and are closely spaced in the lower parts.

Landforms

• Conical Hill: It rises almost uniformly from the surrounding land. A conical hill with uniform slope and narrow top is represented by concentric contours spaced almost at regular intervals.
• Plateau: A widely stretched flat – topped high land, with relatively steeper slopes, rising above the adjoining plain or sea is called a plateau. The contour lines representing a plateau are normally close spaced at the margins with the innermost contour showing wide gap between its two sides.

• Valley: A geomorphic feature lying between two hills or ridges and formed because of the lateral erosion by a river or a glacier is called a valley.
• ‘V’ -shaped Valley: It resembles the letter V. A V-shaped valley occurs in mountainous areas. The lowermost part of the V – shaped valley is shown by the innermost contour line with very small gap between its two sides and the lowest value of the contour is assigned to it. The contour value increases with uniform intervals for all other contour lines outward.
• ‘U’ – shaped Valley: A U – shaped valley is formed by strong lateral erosion of glaciers at high altitudes. The flat wide bottom and steep sides makes it resemble the letter ‘U’ . The lowermost part of the U – shaped valley is shown by the innermost contour line with a wide gap between its two sides. The contour value increases with uniform intervals for all other contour lines outward.

• Gorge: In high altitudes, gorges form in the areas where the vertical erosion by river is more prominent than the lateral erosion. They are deep and narrow river valleys with very steep sides. A gorge is represented by very closely spaced contour lines on a map with the innermost contour showing small gap between its two sides.
• Spur: A tongue of land, projecting from higher ground into the lower is called a spur. It is also represented by Vshaped contours but in the reverse manner. The arms of the V point to the higher ground and the apex of ‘V’ to the lower ones.

• Cliff: It is a very steep or almost perpendicular face of landform. On a map, a cliff may be identified by the way the contours run very close to one another, ultimately merging into one.
• Waterfall and Rapids: A sudden and more or less perpendicular descent of water from a considerable height in the bed of a river is called a waterfall. Sometimes, a waterfall succeeds or precedes with a cascading stream forming rapids upstream or downstream of a waterfall. The contours representing a waterfall merge into one another while crossing a river stream and the rapids are shown by relatively distant contour lines on a map.

Interpretation

Marginal Information: It includes the topographical sheet number, its location, grid references, its extent in degrees and minutes, scale, the districts

• Relief – hill, plateau, plains, mountains
• Drainage – trellis, radial, ring
• Land use – vegetation, agriculture, services
• Transport and communication
• Settlement – rural and urban
• Occupation – lumbering, forestry, fishing

Interpretation Procedure