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Chapter 5 Topographical Maps
Maps are vital geographical tools used to represent the Earth's surface. Among various map types classified by scale and function, Topographical maps hold particular importance for geographers. They are often used as base maps upon which other thematic maps are created. Topographical maps are also referred to as general purpose maps because they show a wide array of both natural and cultural features.
These maps are drawn at relatively large scales to depict detailed surface features. Important natural features shown include relief (hills, valleys, plains), vegetation, and water bodies (rivers, lakes, wells). Cultural features (man-made) include cultivated land, settlements (villages, towns, cities), and transportation networks (roads, railways). Topographical maps are prepared and published by national mapping agencies, such as the Survey of India in India.
Topographical maps are typically produced as a series of map sheets covering an entire country or region. Within a series, all maps maintain consistency in their reference points, scale, projection system, conventional signs and symbols, and colours, ensuring uniformity and facilitating their use together.
In India, topographical maps are primarily prepared in two series:
- India and Adjacent Countries Series
- The International Map Series of the World
India And Adjacent Countries Series
This series of topographical maps was initially prepared by the Survey of India. After the Delhi Survey Conference in 1937, the preparation of maps for adjacent countries was discontinued. The Survey of India then focused on preparing topographical maps solely for India, following the international standards set for the International Map Series of the World. However, the Survey of India retained the established numbering system and layout plan from the abandoned India and Adjacent Countries Series for its new topographical map series covering India.
Topographical maps in India are available at various scales, each providing different levels of detail and covering different geographical areas:
- Scale 1:1,000,000 ($1:10^6$): Covers $4^\circ \times 4^\circ$ latitude/longitude (International Map Series standard).
- Scale 1:250,000 ($1:2.5 \times 10^5$): Covers $1^\circ \times 1^\circ$ latitude/longitude.
- Scale 1:125,000 ($1:1.25 \times 10^5$): Covers $30' \times 30'$ latitude/longitude.
- Scale 1:50,000 ($1:5 \times 10^4$): Covers $15' \times 15'$ latitude/longitude.
- Scale 1:25,000 ($1:2.5 \times 10^4$): Covers $5' \times 7' 30''$ latitude/longitude.
Figure 5.1 illustrates the numbering system used for these topographical map sheets in India.
Diagram illustrating the hierarchical numbering system used by the Survey of India for its series of topographical map sheets at different scales.
International Map Series Of The World
This is a project aimed at creating a standardized series of topographical maps covering the entire world. The standard scales for this series are 1:1,000,000 and 1:250,000. National mapping agencies of member countries contribute by producing map sheets for their respective territories according to the agreed-upon specifications.
Reading Of Topographical Maps
To effectively use topographical maps, one needs to understand the map language – the system of symbols, signs, and colours used to represent features. Each topographical sheet includes a legend or key that explains these symbols. Getting familiar with the legend is the first step in reading a map. Conventional signs and symbols (Figure 5.2 provides examples) are standardized, often internationally, allowing interpretation regardless of language.
Examples of standard conventional signs and symbols used on topographical maps to represent various natural and man-made features.
Glossary terms from this section:
- Contours: Imaginary lines connecting points of equal elevation above mean sea level. Also called level lines.
- Contour Interval: The vertical difference in elevation between two successive contour lines. It is usually constant for a given map and represents the Vertical Interval (V.I.).
- Cross-section: A vertical profile of the ground surface along a specific line, showing the side view of the relief. Also known as a section or profile.
- Hachures: Short lines drawn on maps along the direction of the steepest slope, giving a visual impression of the ground's gradient.
- Topographic Map: A large-scale map of a small area showing detailed natural and man-made features, with relief typically shown by contours.
Methods Of Relief Representation
The Earth's surface has varied relief features (elevations and depressions) like mountains, hills, plateaus, and plains. Maps depicting these variations are called relief maps. Several methods have been developed over time to represent relief on maps:
- Hachures: Small lines drawn in the direction of slope, denser and thicker on steeper slopes.
- Hill Shading: Applying shading to map features to simulate the effect of light and shadow, giving a visual impression of relief.
- Layer Tints: Using different colours or shades to represent different elevation ranges.
- Benchmarks and Spot Heights: Indicating the precise elevation of specific points (benchmarks for surveyed elevations, spot heights for individual points like peaks or road intersections).
- Contours: Imaginary lines connecting points of equal elevation. This is the most widely used method for representing relief accurately on modern topographical maps, often combined with spot heights.
Contours
Contours are imaginary lines that connect all points on a map that are at the same elevation above mean sea level. A map that uses contour lines to show the shape and elevation of the land is called a contour map. Contour lines are very useful for understanding the topography of an area.
The distance between consecutive contour lines in terms of elevation is called the contour interval (V.I.). This interval is chosen based on the scale of the map and the relief of the area, and it is usually constant throughout a single map sheet. While the vertical interval is constant, the horizontal distance between two consecutive contour lines varies depending on the slope of the land. This horizontal distance is called the horizontal equivalent (H.E.). H.E. is large where the slope is gentle (contours are far apart), and it is small where the slope is steep (contours are close together).
Contours were traditionally drawn based on ground surveys and leveling, but modern topographical mapping heavily relies on aerial photography and remote sensing data, which are processed to generate contour lines.
Basic properties of contour lines:
- A single contour line connects points of the same elevation.
- The shape and spacing of contour lines depict the height, slope, and gradient of landforms.
- Closely spaced contours indicate a steep slope.
- Widely spaced contours indicate a gentle slope.
- Contour lines merge or run very close together to represent vertical features like cliffs or waterfalls.
- Contour lines of different elevations usually do not cross each other, except in the case of an overhanging cliff.
Drawing Of Contours And Their Cross Sections
Understanding how contour spacing relates to different types of slopes is crucial for interpreting topographical maps. Slopes can be gentle, steep, concave, convex, or irregular. The spacing of contours reflects these different slope types:
- Gentle Slope: Contours are far apart, indicating a low gradient.
- Steep Slope: Contours are closely spaced, indicating a high gradient.
The arrangement of contours also represents specific landforms (examples shown below). A cross-section or profile shows a side view of the relief along a specific line drawn across the contours on the map. Drawing cross-sections helps visualize the terrain represented by the contours. The process involves transferring contour points and their elevations along a chosen line onto a vertical profile, using appropriate horizontal and vertical scales.
Types of slope based on contour spacing changes:
- Concave Slope: Gentle at the bottom (widely spaced contours) and steep at the top (closely spaced contours).
- Convex Slope: Steep at the bottom (closely spaced contours) and gentle at the top (widely spaced contours).
Gentle Slope
Characterized by widely spaced contour lines on the map. The horizontal distance (H.E.) between successive contours is large, indicating a low angle of inclination of the land surface.
Steep Slope
Characterized by closely spaced contour lines on the map. The horizontal distance (H.E.) between successive contours is small, indicating a high angle of inclination of the land surface.
Concave Slope
Represented by contours that are widely spaced in the lower elevations and progressively become closer together at higher elevations, indicating a slope that gets steeper as you move uphill.
Convex Slope
Represented by contours that are closely spaced in the lower elevations and progressively become farther apart at higher elevations, indicating a slope that is steeper at the base and becomes gentler as you move uphill.
Conical Hill
A hill that rises uniformly from its surroundings and has a narrow summit. On a contour map, a conical hill with uniform slopes is represented by concentric contour lines (circles or ellipses) that are spaced almost at regular intervals, with the innermost contour representing the highest elevation near the top.
Plateau
A large, flat-topped highland area with relatively steep sides rising above the surrounding terrain (plain or sea). On a contour map, a plateau is shown by contour lines that are closely spaced along the edges (representing the steep sides) and then widely spaced with the innermost contour(s) having a large gap, indicating the flat top area.
Valley
An elongated depression situated between hills or ridges, often formed by river or glacial erosion. On a contour map, valleys are typically represented by V-shaped or U-shaped contour lines. The apex (point) of the V or U shape points towards the higher elevation or upstream direction.
- V-shaped Valley: Characteristic of river valleys, especially in mountainous areas where downcutting is dominant. Contours form sharp V-shapes pointing uphill, with the innermost contour representing the lowest elevation in the valley bottom.
- U-shaped Valley: Characteristic of valleys carved by glaciers. Contours form broader, U-shapes pointing uphill, representing the flat valley floor and steep sides.
Gorge
A deep, narrow river valley with very steep or almost vertical sides, formed by intense vertical erosion by a river, particularly in resistant rocks or during rapid uplift. On a map, a gorge is shown by very closely spaced contour lines, with the innermost contour (representing the river bed) showing a small gap between its sides.
Spur
A ridge or tongue of land that projects outwards and downwards from a higher elevated area (like a mountain or plateau) into lower ground, often between two valleys or streams. On a contour map, a spur is represented by V-shaped contour lines where the apex (point) of the V points towards the lower elevation or downstream direction, and the arms of the V point towards the higher ground.
Cliff
A very steep or nearly vertical rock face or slope. On a map, a cliff is shown by contour lines that run extremely close together and may even merge into a single line where the slope is perfectly vertical or overhanging.
Waterfall And Rapids
A waterfall is a sudden, near-vertical drop of water in a river channel from a considerable height. Rapids are sections of a river where the gradient increases and the flow becomes turbulent, often preceding or succeeding a waterfall. On a contour map, a waterfall is indicated where contour lines merge or come extremely close together as they cross a river channel. Rapids are represented by relatively closely spaced contours crossing a river, indicating a steeper section than normal, but not as steep as a waterfall.
Steps for Drawing a Cross-section (Profile) from a Contour Map:
- Draw a straight line (e.g., line AB) across the area on the contour map for which you want to draw the profile.
- Take a strip of paper (or graph paper) and align its edge along the line AB on the map. Mark the endpoints A and B on the paper strip.
- Along the edge of the paper strip, mark the position and write down the elevation value of every contour line that crosses the line AB on the map. Also mark the points where the line AB intersects any rivers or other relevant features.
- On a separate sheet, draw a horizontal line that is equal in length to the line AB on the map. Above this line, draw parallel horizontal lines at regular vertical intervals, corresponding to the contour interval on the map, using a suitable vertical scale (e.g., 1 cm represents 100 meters). Label these lines with the corresponding elevation values. Start the vertical scale from an elevation slightly below the lowest contour line marked on your paper strip.
- Place the marked paper strip along the horizontal base line (the lowest elevation line) of your cross-section grid, aligning the A and B marks.
- From each mark on the paper strip representing a contour line, draw a vertical line upwards until it intersects the corresponding elevation line on your cross-section grid.
- Smoothly connect all the plotted points to create a profile line that represents the shape of the ground surface along the line AB on the map.
Identification Of Cultural Features From Topographical Sheets
Topographical maps show significant man-made or cultural features using specific conventional signs, symbols, and colours (refer back to Figure 5.2). These features provide crucial information about human activity and development in the area depicted on the map. Important cultural features include settlements, buildings, roads, and railways. Studying their location, patterns, and density helps understand the human geography of the region.
Distribution Of Settlements
Settlements (villages, towns, cities) are represented on topographical maps. Their distribution can be analyzed based on site (location relative to resources like water, food), location pattern (how buildings are arranged), alignment (if they follow a linear feature), and density (number of buildings/settlements per area). Comparing settlement patterns with physical features (contours, drainage) helps understand factors influencing settlement location. Basic factors influencing the site of settlements include water source, food availability, relief (suitable terrain), type of occupation, and defense needs.
Different patterns of rural settlements that can be identified include:
- Compact: Houses built close together.
- Scattered (Dispersed): Houses spread far apart over an area.
- Linear: Settlements stretched out in a line along a road, river, coastline, or embankment.
- Circular: Settlements arranged in a circular form around a central point (e.g., a well, a fort, a temple).
Urban settlements can be identified and sometimes characterized by their primary function or location (e.g., cross-road town, market center, hill station, port town, industrial center, capital city, religious center).
Transport And Communication Pattern
Topographical maps show transportation and communication networks (roads, railway lines, footpaths, communication lines, post offices) using conventional symbols. The pattern and density of these networks are influenced by factors like relief (terrain), population density, and resource development in the area. Analyzing the transport network provides insights into connectivity and accessibility within the region.
Interpretation Of Topographical Maps
Map interpretation involves more than just reading symbols; it requires understanding the relationships between the various features shown on the map. Knowledge of map language (symbols, colours, scale, orientation) is essential. Interpretation involves studying the physical and cultural features to understand the area's geography, environment, and human activities.
A systematic approach to interpreting a topographical sheet typically involves examining different categories of information:
Marginal Information
Information printed in the margins of the map sheet, such as the sheet number, location coordinates (latitude and longitude extent), scale, contour interval, projection system, the states or districts covered, edition date, and the legend/key explaining symbols. This information is fundamental for correctly referencing and understanding the map.
Relief Of The Area
Study the contour lines, spot heights, and benchmarks to understand the general topography – whether it is a plain, plateau, hills, or mountains. Identify prominent features like peaks, ridges, valleys, spurs, and cliffs. Determine the overall direction of the slope based on how contour values change across the map. Describe the type of slope (gentle, steep, concave, convex) using contour spacing.
- Identify specific features like hills, plateaus, plains, and mountains and describe their characteristics (shape, elevation, dissection).
Drainage Of The Area
Identify the major rivers and their tributaries. Observe their courses, presence of features like gorges, waterfalls, meanders, ox-bow lakes, or braided channels. Determine the drainage pattern (dendritic, radial, trellis, etc.) formed by the river network. Note lakes, tanks, wells, canals, or other water bodies.
Land Use
Analyze how the land is being used based on vegetation symbols, cultivation patterns, and presence of non-natural features. Identify areas under natural vegetation/forest (dense, open, type of forest), cultivated land (types of crops if indicated, irrigated/unirrigated), fallow land, wasteland, industrial areas, residential areas, and areas dedicated to facilities and services (schools, hospitals, parks, airports, power stations).
Transport And Communication
Examine the types and density of transportation networks (roads - metalled/unmetalled, tracks, paths, railways, waterways). Note major communication lines and the location of communication services like post offices.
Settlement
Study the distribution, type, and pattern of human settlements. Distinguish between rural and urban settlements based on symbols and size. For rural settlements, identify patterns (compact, dispersed, linear, circular) and their relationship to physical or cultural features. For urban settlements, try to infer their type or function (e.g., administrative, commercial, industrial, transport node, religious center).
Occupation
Infer the likely primary occupations of the people in the area based on the dominant land use and settlement type. For example, extensive cultivated land and rural settlements suggest agriculture is the main occupation. Forests might indicate lumbering or forest-based activities. Coastal areas might suggest fishing. Cities and towns imply occupations primarily in services, business, or industry.
Map Interpretation Procedure
Interpreting a topographical map involves analyzing different features and understanding the causal relationships between them. For example, why are settlements located where they are? How does relief influence drainage or land use? This requires a systematic approach:
- Start with the marginal information to understand the map's location, scale, and basic coverage. The scale and contour interval provide initial clues about the terrain and level of detail.
- Use tracing sheets to isolate and study individual features or categories (e.g., one sheet for contours/landforms, one for drainage, one for land use, one for settlements/transport).
- Describe the spatial distribution patterns of each feature separately, noting key characteristics (e.g., where are forests located? Where are settlements clustered?).
- Superimpose pairs of these tracing sheets (e.g., drainage over contours, settlements over land use) to identify relationships between different features. For instance, a pattern might emerge showing settlements clustered along roads in flat areas near water sources.
Comparing topographical maps with aerial photographs or satellite imagery of the same area can help update information and provide a more comprehensive understanding.
Exercise
Content for Exercise is excluded as per your instructions.