Chapter 5 Topographical Maps

Introduction To Topographical Maps

Maps are fundamental tools for geographers and are classified by scale and function. Among these, **topographical maps**, also known as general purpose maps, hold significant importance. They serve as base maps upon which other thematic maps can be created.

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Topographical maps are drawn at relatively **large scales**, which allows them to show detailed information about a small area. These maps depict both significant **natural features** (like relief, vegetation, water bodies) and **cultural features** (like cultivated land, settlements, transportation networks). They are prepared and published by the national mapping agency of each country, such as the **Survey of India** for India.

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Topographical maps are produced as a **series** of maps. This means that maps covering different areas within a country are produced at the same scales, using a consistent referencing system, projection, symbols, signs, and colors. This uniformity ensures that maps from the same series can be used together seamlessly and interpreted consistently.

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In India, topographical maps are prepared in two main series: the India and Adjacent Countries Series (historically) and the International Map Series of the World (presently). The **India and Adjacent Countries Series** was prepared by the Survey of India until 1937. After the Delhi Survey Conference in 1937, the Survey of India ceased mapping adjacent countries and focused on India. However, the numbering system and layout plan of the old series were retained for the new maps of India, aligning with specifications for the International Map Series.

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Topographical maps in India are available at various scales, each covering a specific latitudinal and longitudinal extent:

• 1 : 1,000,000 (4° $\times$ 4° coverage)
• 1 : 250,000 (1° $\times$ 1° coverage)
• 1 : 125,000 (30' $\times$ 30' coverage)
• 1 : 50,000 (15' $\times$ 15' coverage)
• 1 : 25,000 (5' $\times$ 7' 30" coverage)

The numbering system for these topographical maps is standardized, as shown in Figure 5.1, allowing users to identify the specific map sheet based on its coverage. (Figure 5.1 shows the numbering system for Survey of India topographical sheets).

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The **International Map Series of the World** aims to create standardized topographical maps for the entire globe at scales of 1:1,000,000 and 1:250,000.

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Reading a topographical map requires understanding the map's language, including the legend (key), conventional signs, symbols, and colors used. Figure 5.2 shows common conventional signs and symbols used on topographical sheets. (Figure 5.2 shows a table of conventional signs and symbols used on topographical maps).

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Glossary Terms:

Term	Definition
Contours	Imaginary lines on a map that connect points of equal elevation or altitude above a reference level (usually mean sea level).
Contour Interval (V. I.)	The constant vertical difference in elevation between two consecutive contour lines on a map.
Cross-section (Section or Profile)	A side view drawing that shows the variation in elevation of the ground surface along a specific line marked on the map, as if the land were cut vertically along that line.
Hachures	Short, fine lines drawn on a map along the direction of the steepest slope. They are denser and thicker on steeper slopes, giving a visual impression of relief and slope gradient.
Topographic Map	A detailed map of a small area, drawn on a large scale, depicting both natural features (like relief shown by contours) and man-made features.

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Methods Of Relief Representation

The Earth's surface is not flat; it has varied elevations and depressions, forming features like mountains, hills, plateaus, and plains. These are known as **physical features** or **relief features**. A map that shows these features is called a **relief map**.

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Historically, various methods have been used to represent relief on maps, including hachures, hill shading, layer tints (coloring areas between contours with different shades based on elevation), benchmarks (marked points with known exact elevation), spot heights (points with known exact elevation), and contours. On modern topographical maps, **contours** and **spot heights** are the most predominantly used methods for depicting the relief of an area.

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Contours

**Contours** are imaginary lines drawn on a map connecting all points that have the same elevation above mean sea level. A map that shows the landforms of an area using contour lines is called a **contour map**. The method of using contours to represent relief is versatile and provides a clear visual representation of the topography, including elevation, slope, and shape of landforms.

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Historically, contours were drawn on maps based on extensive ground surveys and levelling techniques to determine elevations. However, advancements in technology, particularly the use of **aerial photography** and photogrammetry, have largely replaced manual ground methods for topographical mapping and contouring.

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Contours are drawn at specific **vertical intervals (VI)**, which is the constant difference in elevation between two successive contour lines. Common contour intervals are 20, 50, or 100 meters above mean sea level, depending on the scale of the map and the ruggedness of the terrain. The contour interval is typically constant for a given map. While the vertical interval is fixed, the **horizontal distance** between contour lines varies depending on the slope. This horizontal distance is also called the **horizontal equivalent (HE)**. Where the slope is gentle, the contours are spaced far apart (large HE). Where the slope is steep, the contours are spaced closely together (small HE).

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Some basic properties of contour lines that help in interpreting relief:

• A contour line connects points of equal height above sea level.
• The shape and spacing of contour lines represent the landform's height, slope, and gradient.
• **Closely spaced contours** indicate a **steep slope**.
• **Widely spaced contours** indicate a **gentle slope**.
• When two or more contour lines merge or coincide, they represent a **vertical slope**, such as a cliff or waterfall.
• Contour lines of different elevations generally **do not cross** each other, except in the rare case of an overhanging cliff.
• Closed loops of contour lines generally represent hills or depressions. Closed loops with higher values inside indicate a hill or peak; closed loops with lower values inside (sometimes indicated by hachures pointing inwards) represent a depression or hollow.

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Drawing Of Contours And Their Cross Sections

Understanding contours involves being able to interpret different types of slopes and landforms from their contour patterns, as well as being able to draw a cross-section or profile of the land surface along a line on the map.

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Types of Slope represented by Contours:

• **Gentle Slope:** Contours are far apart.
• **Steep Slope:** Contours are closely spaced.
• **Concave Slope:** Slope is gentle at the bottom and steepens upwards. Contours are widely spaced in the lower parts (near the base) and become closer together in the upper parts (towards the summit).
• **Convex Slope:** Slope is steep at the bottom and becomes gentler upwards. Contours are closely spaced in the lower parts and become wider apart in the upper parts.
• **Uniform Slope:** Contours are evenly spaced.

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Types of Landform represented by Contours:

• **Conical Hill:** A hill that rises uniformly from the surrounding land with a narrow peak. Represented by concentric contour lines spaced almost at regular intervals, with the innermost contour being the smallest loop representing the peak area.
• **Plateau:** A flat-topped highland with relatively steep sides (scarps) rising above an adjoining plain or lower land. Represented by closely spaced contours at the margins (the scarp), with the innermost contour being widely spaced, indicating the flat or gently undulating plateau top.
• **V-shaped Valley:** A valley with steep sides, typically formed by river erosion in mountainous areas. Represented by V-shaped contour lines that point towards the higher ground (upstream). The innermost contour (lowest elevation) shows a narrow gap.
• **U-shaped Valley:** A valley with a wide, flat bottom and steep sides, typically formed by glacial erosion. Represented by U-shaped contour lines that point towards the higher ground, with the innermost contour (lowest elevation) showing a wide gap.
• **Gorge:** A deep, narrow river valley with very steep or nearly vertical sides, where vertical erosion is dominant. Represented by very closely-spaced contour lines that run nearly parallel, with the innermost contour showing a small gap where the river flows.
• **Spur:** A tongue of land projecting from higher ground into lower ground, essentially a ridge extending from a hill or mountain. Represented by V-shaped contour lines that point towards the lower ground (downslope). The apex of the V is towards the lower elevation.
• **Cliff:** A very steep or almost perpendicular rock face. Represented by contour lines running extremely close together, often merging into a single line.
• **Waterfall/Rapids:** A sudden drop in a river bed. Represented by contour lines that merge together where they cross the river at the location of the waterfall. Rapids (less abrupt drops) are shown by closely spaced contour lines crossing the river.

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Steps for Drawing a Cross-section (Profile) from Contours:

A cross-section provides a side view of the topography along a specific line. Here are the steps to draw a cross-section from a contour map:

1. Draw a straight line (e.g., label it AB) across the contour map along which you want to create the profile.
2. Take a strip of paper (or a graph paper strip) and place its edge precisely along the line AB drawn on the map.
3. Carefully mark on the edge of the paper strip the exact point where each contour line crosses the line AB. Write the elevation value of that contour next to the mark. Also, mark the start (A) and end (B) points of the line.
4. On a separate sheet of graph paper, draw a series of horizontal lines parallel to each other. These horizontal lines represent different elevations. Choose a suitable **vertical scale** (e.g., 1 cm = 100 meters) to determine the spacing between these horizontal elevation lines. Label these lines with appropriate elevation values, starting with an elevation slightly lower than the lowest contour value marked on your paper strip and ending with an elevation slightly higher than the highest value. The total length of the horizontal lines should be at least equal to the length of the line AB on the map.
5. Place the marked edge of the paper strip (from step 3) along the bottom-most horizontal line of your graph paper (the base of your cross-section). Ensure that the point A on the paper strip aligns with the starting point A on the graph paper.
6. From each mark on the paper strip representing a contour line, draw a perpendicular line upwards until it intersects the corresponding horizontal elevation line on the graph paper. For example, a mark for the 200m contour on the strip should be projected up to the 200m horizontal line on the graph paper.
7. Once all the contour points have been plotted, smoothly join the plotted points on the graph paper. The resulting line represents the topographic profile or cross-section of the land surface along the line AB.

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Identification Of Cultural Features From Topographical Sheets

Topographical sheets don't just show natural relief; they also depict significant **cultural features** – those created or modified by humans. These features, such as settlements, buildings, roads, and railways, are shown using standardized **conventional signs, symbols, and colors** as indicated in the map's legend (Figure 5.2). Identifying and understanding the distribution and patterns of these cultural features provides valuable insights into the human geography of the area depicted on the map.

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Distribution Of Settlements

Settlements, both rural and urban, are important cultural features. Their pattern of distribution on a topographical map reveals aspects about the area's human habitation. By examining the location, pattern, alignment, and density of settlements, along with comparing them with the natural features (relief, drainage, water sources) shown on the map, we can understand the factors influencing settlement sites.

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Based on their physical appearance and arrangement, rural settlements can often be identified on a topographical map as:

• **Compact settlements:** Houses are clustered closely together in a nucleated pattern, often around a central point like a water source or temple.
• **Scattered (Dispersed) settlements:** Houses are spread out over a wider area, often located on individual farms or small hamlets separated by fields.
• **Linear settlements:** Houses are arranged in a line along a road, railway line, river bank, canal, or coastline.
• **Circular settlements:** Houses are arranged in a circular pattern, often around a tank, lake, or village common.

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Urban centers can also be identified based on their size, density, and characteristic features shown on the map, and sometimes classified by their primary function or location (though detailed functional classification might require additional information beyond the map itself):

• Cross-road town (developed at a road junction)
• Nodal point (developed at a center from which routes radiate)
• Market center
• Hill station
• Coastal resort center
• Port town
• Manufacturing center (often with associated suburban or satellite settlements)
• Capital town
• Religious center

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The choice of location (site) for settlements is often determined by factors like availability of water (a primary necessity), provision of food (proximity to agricultural land), nature of relief (preferring flat or gently sloping areas), nature of occupation (e.g., proximity to resources), and historical factors like defense (choosing defensible locations). Examining the relationship between settlement locations and features like contour lines (relief) and drainage networks (water sources) helps understand the site factors. The density of settlements often reflects the carrying capacity of the land and the availability of food resources.

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The spatial arrangement or pattern of settlements (e.g., linear arrangement along a river or road) can be clearly observed on the map.

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Transport And Communication Pattern

Topographical maps depict the infrastructure facilitating movement and communication, such as roads, railway lines, tracks, footpaths, and major communication lines (like telegraph or telephone lines, though these are less common on modern maps). The type, density, and pattern of these networks are influenced by factors like the relief of the area (easier construction on plains), population density, and the level of resource development. Examining the transportation network provides insights into the connectivity and accessibility of different parts of the mapped area.

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Interpretation Of Topographical Maps

**Map interpretation** is the process of analyzing the information presented on a map to understand the geographical characteristics of the area, including the relationships between different natural and cultural features. It requires familiarity with map symbols and a sense of direction.

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To interpret a topographical sheet effectively, you should first orient the map and understand its basic parameters:

• Look for the **north line** (usually indicated by an arrow).
• Identify the **scale** of the map.
• Familiarize yourself with the **legend** or key, which explains the meaning of all the conventional signs, symbols, and colors used on the map. Conventional signs are internationally accepted, allowing anyone to read maps from different countries regardless of language.
• Note the **sheet number** and other marginal information.

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A systematic interpretation of a topographical sheet typically involves analyzing information under several key headings:

Marginal Information

This refers to the information provided around the edges (margins) of the map sheet. It includes crucial details like the sheet number, the name of the area covered, the geographical location in terms of degrees and minutes of latitude and longitude (grid references), the scale of the map, the contour interval, the names of the districts or administrative areas covered, the year of survey and publication, and the legend/key of symbols.

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Relief Of The Area

Analyze the contours and spot heights to understand the shape and elevation of the land. Identify major landforms like plains, plateaus, hills, and mountains. Note the location of peaks, ridges, and spurs. Determine the general direction of the slope across the area by observing how contour values change and river flow direction. Describe the type of slopes (gentle, steep, uniform, concave, convex) indicated by contour spacing. Identify features like valleys (V-shaped, U-shaped, gorges), cliffs, waterfalls, etc., based on their characteristic contour patterns.

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Drainage Of The Area

Study the river systems, streams, lakes, tanks, canals, and other water bodies shown on the map. Identify the main river and its important tributaries. Determine the **drainage pattern** (e.g., dendritic, radial, trellis, internal) by observing how the rivers and streams are arranged. Note the type and extent of valleys carved by the rivers.

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Land Use

Identify how the land is being used in different parts of the mapped area. Look for conventional signs and colors indicating:

• **Natural vegetation** (forest types like dense or open, scrubs) and its distribution. Distinguish between different categories of forests if shown (e.g., Reserved Forest, Protected Forest).
• **Agricultural land**, orchards, plantations.
• **Wastelands** or uncultivated areas.
• Urban areas, industrial areas.
• Locations of facilities and services like schools, colleges, hospitals, parks, airports, post offices, wells, temples, mosques, churches, electric substations, etc.

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Transport And Communication

Examine the network of roads (different types like metalled, unmetalled), railway lines (different gauges), cart tracks, camel tracks, footpaths, bridges, and major communication lines. Assess the density of the network and its relationship to the topography and settlements. Note the location of railway stations and other transport facilities.

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Settlement

Study the location, size, and pattern of human settlements, both rural and urban. For **rural settlements**, identify the pattern type (compact, dispersed, linear, circular). For **urban settlements**, note their location and potential function (e.g., administrative, religious, market town, port). Examine the relationship between settlement sites and factors like water sources (drainage) and relief (contours).

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Occupation

Infer the likely occupations of the people in the area based on the dominant land use and settlement types. For example, if agriculture is the primary land use in rural areas, the main occupation is likely farming. In areas with extensive forests, lumbering might be significant. In coastal areas, fishing could be important. In urban centers, services, business, and industry are likely major occupations.

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Map Interpretation Procedure

Effective map interpretation goes beyond simply identifying features; it involves analyzing the relationships between different features to understand the dynamics of the landscape and human activities. This means looking for causal connections, such as how relief influences drainage patterns, how the availability of water sources affects settlement locations, or how transportation networks relate to settlement distribution and economic activity.

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A systematic procedure for map interpretation:

1. **Orientation and Initial Assessment:** Use the map's index number to locate the area within India, which provides context about its general physiographic characteristics. Note the scale and contour interval to understand the map's coverage and the general ruggedness of the terrain. Identify the North line and orient the map. Study the legend thoroughly.
2. **Feature Mapping (Overlay Method):** Optionally, trace out specific features onto separate tracing sheets. For instance, create separate overlays for major landforms (contours), drainage networks, land use categories, and settlements/transportation.
3. **Individual Feature Description:** Describe the distribution pattern and characteristics of each major feature (relief, drainage, land use, settlements, transport) separately. Highlight the most significant aspects of each.
4. **Relationship Analysis (Superimposition):** Superimpose the overlays (or mentally compare the patterns of different features on the main map). Analyze the spatial relationships between pairs or multiple features. For example, superimpose the contour map and drainage map to see how relief influences river flow. Superimpose the land use map over the contour map to see if certain land uses are restricted to specific slopes or elevations. Superimpose the settlement map over the transport map to see how settlements are connected.
5. **Synthesis:** Combine the observations from individual feature descriptions and relationship analyses to provide a holistic interpretation of the geography of the area. Explain the interactions between the natural environment and human activities.

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Comparing topographical maps with aerial photographs or satellite images of the same area and scale can also help in updating information or gaining a different perspective on the landscape.

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Exercise

(Exercise questions are not included as per instructions.)