Principles of Geologic Mapping | Geology Optional for UPSC PDF Download

Introduction

• Topographical maps, also known as topo-sheets or topo-maps, are detailed two-dimensional representations of the Earth's three-dimensional surface. They are used to depict both natural and man-made features with precision, making them essential tools for various fields such as urban planning, natural resource management, and environmental studies.
• These maps incorporate contours, which are imaginary lines connecting points of equal elevation above mean sea level. Contours allow for the representation of the Earth's relief features, including mountains, valleys, and depths below the ocean's surface. By using contours, topographical maps convey information about the height of peaks, the steepness of slopes, and the depths of bodies of water.
• Topographical maps are prepared at a relatively large scale, capturing a symbolic picture of the intricate details of small areas. They are published by national mapping organizations, such as the Survey of India (SOI), which has been producing these maps since its establishment in 1767. The SOI uses modified polyconic map projection to create maps of India, ensuring accurate representation of the country's diverse terrain.
• In addition to relief features, topographical maps also display surface water bodies like rivers and lakes, as well as cultural features such as roads, railways, canals, and settlements. This combination of physical and cultural information makes topographical maps invaluable for a wide range of applications, from city planning to geological research.

Definition of Topographical Map

• A topographical map is a detailed, two-dimensional representation of the Earth's three-dimensional surface, showing both natural and man-made features with precision.
• It uses contours to depict elevation changes, making it possible to understand the relief, steepness, and depth of various landforms.

Importance of Conventional Signs and Symbols

• Conventional signs and symbols are crucial for accurately representing and understanding the diverse range of features depicted on topographical maps.
• They provide a standardized way to convey information about natural elements like mountains, rivers, and forests, as well as man-made structures such as roads, buildings, and boundaries.
• By using these symbols, map readers can quickly and easily interpret the map without confusion, ensuring that the information is conveyed effectively and efficiently.

Features of Topographical Maps

Natural Features:

• Contours: Used to represent elevation and the steepness of slopes.
• Rivers and Lakes: Shown with specific symbols to indicate water bodies.
• Forests and Vegetation: Represented by symbols indicating dense greenery.
• Mountains and Hills: Shown with contour lines indicating height.
• Valleys: Indicated by contour lines that form a "V" shape.

Man-made Features:

• Roads and Highways: Represented by specific line styles indicating the type of road.
• Railways: Shown with distinct symbols indicating railway lines.
• Buildings and Towns: Indicated by symbols representing urban areas.
• Boundaries: Shown with specific line styles indicating administrative boundaries.
• Industrial Areas: Represented by symbols indicating factories and industrial zones.

The relationship between natural and man-made features is crucial in topographical maps, as they often interact and influence each other. For example, roads may follow the contours of the land, and towns may be located near rivers for water supply. Understanding this relationship is key to interpreting the maps accurately.

International Map of the World

International Map of the World (IMW) is a series of world maps with a scale of 1:10,00,000, representing 4° of latitude and 6° of longitude. Each map is known as a millionth map due to its scale. The IMW project was initiated in 1913 but faced delays due to world wars, economic depressions, and funding issues in poorer countries, leading to its abandonment by the mid-1980s.

The IMW uses a regular grid system for numbering maps, with 4° intervals of latitude from the equator and 6° intervals of longitude from Greenwich. Each map bears an international number indicating its hemisphere, latitude, and longitude zone. For example, the map number NF44 indicates the Northern Hemisphere, latitudinal zone F, and longitudinal zone 44.

International Numbering System of IMW for Northern Hemisphere

National Mapping Agencies

• The IMW series was meant to be published by national mapping agencies following detailed specifications.
• Around 850 maps were completed out of the planned 2500 before the project was abandoned.

Global Publishers of National Topographical Maps

Topographical Maps of India

Introduction

Topographical maps in India are created by the Survey of India (SOI) and are essential for understanding the country's terrain, features, and for various planning purposes. These maps are produced in different series and scales, depending on the level of detail required and the specific use cases.

1. International Map Series of the World

• This series was initiated following an international agreement in 1909 and involves the publication of maps at a scale of 1:10,00,000.
• The maps cover areas of 4° of latitude and 6° of longitude.

2. India and Adjacent Countries (IAC) Series

• This series includes topographical maps of India and its adjoining countries, such as Afghanistan and China, at the same scale of 1:10,00,000.
• Maps are prepared for every 4° × 4° of latitude and longitude.
• After the Delhi Survey Conference in 1937, SOI focused only on India, but retained the layout and numbering system of the IAC series.

Types of Topographical Maps Prepared by SOI

(i) Million Map (Scale: 1:1,000,000)

• Covers 4° × 4° of latitude and longitude.
• Examples of serial numbers include 39, 40, 41, etc.

(ii) Degree Maps (Scale: 1:250,000)

• Each million map is divided into 16 parts, labeled A to P.
• Covers 1° × 1° of latitude and longitude.
• Example serial numbers include 58A, 58B, 58C, etc.

(iii) Quarter Degree Maps (Scale: 1:50,000)

• Each degree map is divided into 16 parts, each covering 15' × 15' of latitude and longitude.
• Example serial numbers include 58A/1, 58A/2, ... 58A/16.

(iv) Topographical Maps (Scale: 1:25,000)

• Each quarter degree map is divided into four parts, each covering 7' 30" × 7' 30" of latitude and longitude.
• Example serial numbers include 58A/1/NW, 58A/1/NE, 58A/1/SE, and 58A/1/SW.

Current Status and Future Plans

• By the 1980s, India was fully mapped at a scale of 1:50,000.
• The 1:250,000 scale maps are now primarily compiled from existing data.
• Mapping at 1:25,000 scale continues, with about 40% of the country covered to date.

New National Map Policy 2005

• Under this policy, there are now two series of topographical maps:
• Defence Series Maps: These maps are created for defence and national security purposes, using the Everest or WGS 84 Datum and various projections.
• Open Series Maps: These maps facilitate developmental activities and are made available for unrestricted use after clearance from the Ministry of Defence.

Purchasing Topographic Maps

• Specific topographic maps can be purchased from SOI offices and map sales counters.
• A list of outlets is available on the Survey of India website.

Numbering System of Indian Topographical Maps

The Survey of India (SOI) prepares topographical maps covering most of India. To identify and name a specific map, the SOI uses a numbering system. These maps are made at a scale of 1:10,00,000, covering the area from 4°N to 40°N latitude and 44°E to 124°E longitude. Each map covers an area of 4° latitude by 4° longitude and is numbered with a two-digit integer, like 58.

Indexing of Maps

• The numbering for these maps starts from 39 and goes up to 88. Each 4° × 4° map is divided into 16 smaller parts, each measuring 1° latitude by 1° longitude. These parts are named with letters from A to P, creating designations like 58A, 58B, and so on.
• Further Division
• Each degree map is further divided into 16 parts, each measuring 15' latitude by 15' longitude. The numbering of these parts starts from 1 in the northwest corner and goes up to 16, resulting in designations like 58P/1, 58P/2, and so on.
• Subdivision of Quarter Degree Maps
• Each quarter-degree map (15' × 15') is also divided into four parts, each covering 7' 30" by 7' 30". These parts are identified by their cardinal directions: Northwest (NW), Northeast (NE), Southwest (SW), and Southeast (SE). For example, maps are labeled as 58P/4/NW, 58P/4/NE, 58P/4/SW, and 58P/4/SE.

Reading of Topographical Maps

Topographical maps offer a detailed representation of an area, displaying both natural and man-made features. However, it's impractical to include every observed detail on the map. To address this, a standard system of symbols, conventional signs, letters, and colours is used to illustrate the features on topographical maps.

Before studying a topographical map, it's essential to familiarize yourself with the following:

• Conventional Signs: These are the standard symbols used to represent various features on the map.
• Legend: The legend explains the meaning of the symbols and signs used on the map.
• Marginal Information: This includes additional information provided on the margins of the map, which can be helpful in understanding the map better.
• Map Scale: The scale indicates the relationship between distances on the map and actual distances on the ground.
• Map Orientation: This refers to the direction in which the map is oriented, usually indicated by a compass rose.

Symbols and Colors on Topographical Maps

Topographical maps use a standard system of symbols and colors to represent different features, making them more informative and easier to read. Here are some common symbols and colors used:

• Yellow: Indicates areas sown with crops.
• Red: Represents roads, human settlements, and fence lines.
• Blue: Shows water bodies such as marshes, lakes, reservoirs, rivers, and bays.
• Vegetation: Includes woodland, scrubland, grasslands, orchards, and vineyards.
• Black: Used for railway lines, power lines, and boundaries.
• Brown: Represents topographic contours.

Marginal Information on SOI's Topographical Maps

The margins of topographical maps published by the Survey of India (SOI) contain a wealth of useful information. Here’s a detailed breakdown of the information typically found in the margins:

• Top Margin:
  • State Name: The name of the state is printed at the top of the map.
  • District Name and Year of Publication: These details are printed on the top left side of the map.
  • Map Number: The top right side includes the topographical map number.
  • Direction of Magnetic North: Indicated on the top right side.
  • Edition and Scale: The edition of publication and the scale of the map are also provided in the top right corner.
• Bottom Margin:
  • Graphic Scale:. graphic scale is displayed in the bottom margin.
  • Contour Interval: The contour interval is indicated below the graphic scale.
• Index of Conventional Symbols: This index is found on the left and right margins at the bottom of the map, explaining the symbols used.
• Administrative Index: Located on the bottom right side, this index shows the administrative boundaries represented on the map.
• Publishing Agency: The name of the publishing agency is printed on the bottom margin of the map.

Map Scale

• A map scale shows how a distance on the map relates to a distance on the ground.
• Topographic maps usually have a graphic scale at the center of the bottom margin.

Map Orientation

• In a Survey of India (SOI) topographical map, north is at the top, south is at the bottom, east is on the right, and west is on the left.
• The black lines on the map's edges represent the longitudes and latitudes of that specific area.

Identification of Features

Topographical maps provide a detailed representation of both natural and man-made features in a specific area. To interpret these maps accurately, one must be able to identify, locate, and describe various physical features.

The Earth's surface is diverse, comprising various physical features such as mountains, hills, plateaus, plains, rivers, and lakes. On topographical maps, these relief features are depicted using contours and spot heights. The spacing and arrangement of contour lines help in determining the type of relief present in the area.

For instance, a plain area is indicated by the presence of few or no contour lines, while a mountainous region is characterized by closely packed contour lines.

• Different arrangements of contour lines represent various relief features.
• Contours help in understanding the slopes and landforms of an area.

a) Contours Representation for Different Types of Slopes

• Convex Slope: Contour lines are closer together at the bottoms of the slopes than at the tops.
• Concave Slope: Contour lines are closer together at the tops of the slopes than at the bottoms.
• Uniform Slope: Contour lines are evenly spaced.
• Irregular Slope: Contour lines are unevenly spaced.
• Steep Slope: Contour lines are closely spaced.
• Gentle Slope: Contour lines are widely spaced.

Hachures: Hachures are small straight lines drawn on the map in the direction of maximum slope, running across the contours. They indicate differences in the slope of the ground.

b) Contours Representation for Different Types of Landforms

• Ridge: Contour lines form an elliptical pattern, with values decreasing outward.
• Depression:. depression is indicated by closed hachured contour lines.
• Spot Heights: Spot heights, indicating elevation above sea level, are marked by numbers on topographical maps. They provide supplementary information and are displayed at specific locations such as surveyor’s stations, mountain tops, road crossings, and lake surfaces.
• Conical Hill: Represented by concentric contours spaced at regular intervals, with contour lines closing near the top.
• Plateau: Contour lines are closely spaced at the margins, with the innermost contour line showing a wide gap between its sides.
• V-shaped Valley: Depicted by V-shaped contours, where the innermost contour has the lowest value, increasing uniformly outward.
• Gorge: Represented by very closely spaced contour lines, with the innermost contour showing a small gap between its sides.
• Waterfall: Contour lines merge while crossing a river.
• Cliff: Indicated by contour lines running very close to each other and merging.

Vegetation

In SOI topographical maps, natural vegetation is depicted in green. This vegetation may be protected or reserved, and other types may be identified using conventional symbols.

Drainage

The drainage system of an area is influenced by the slope, nature, and attitude of the topography. Before examining the drainage system visible on a topographical map, it is important to describe the slope gradient using contour lines. The slope of an area determines the direction of drainage. Drainage is analyzed based on its pattern, texture, and density. There are six basic types of drainage patterns:

• Dendritic
• Trellis
• Parallel
• Radial
• Angular
• Rectangular

More detailed information about these patterns can be found at www.physicalgeography.net  . The major rivers, their tributaries, and the extent of valleys formed by them can also be determined.

Cultural Features

Topographical maps offer a wealth of information about cultural features, which include various human-made and organized aspects of the landscape. These features are represented using conventional signs, symbols, and colors on the maps. Here are the important cultural features depicted in topographical maps:

• Land Use: The maps indicate different types of land use such as: Natural Vegetation and Forests: Reserved, protected, dense, or open forests. Cultivable Land: Agricultural land and orchards. Wasteland: Areas of barren or unusable land. Industrial Land: Zones designated for industrial activities. Other Uses: Land used for various purposes like schools, colleges, parks, hospitals, airports, canals, wells, reservoirs, and tanks. The maps may also reflect general occupations of the people, including agriculture, forestry, and fishing.
• Means of Transport and Communication: Topographical maps provide insights into the means of transport and communication available in an area, along with their density. This includes: Railway Lines: Types of railway lines such as broad gauge and meter gauge. Roads: National and state roads, categorized as metalled (paved) or unmetalled (unpaved). Mineral Lines: Tracks used for transporting minerals. Transways and Ropeways: Alternative transport routes. Telegraph and Electrical Lines: Communication and power transmission lines. Footpaths and Waterways: Pedestrian paths and navigable water routes. Post Offices: Locations of postal service facilities.
• Patterns of Settlement: The maps also depict types of settlements (urban or rural) and their patterns, such as: Settlement Patterns: Compact, scattered, circular, or linear settlements. Sites and Size: Information about the location and size of settlements. Studying settlement patterns provides valuable insights into population density, urban and rural centers, relief features, water sources, food availability, and the occupations of residents in a particular area.

Steps for Interpreting a Topographical Map

To interpret a topographical map accurately, one must be familiar with the conventional signs, symbols, and legends used on such maps. These signs and symbols are consistent worldwide, allowing anyone with a good understanding of them to read maps from different countries, regardless of the language. Here are the main steps involved in interpreting a topographical map:

1. Marginal Information:

• Topographical map number
• Name of the place
• Year of publication
• Longitude and latitude extent
• Scale
• Districts covered
• Contour interval
• Grid reference

Physiographic Information:

a. Relief:

• Identification of general topography, including mountains, hills, plateaus, plains, and valleys.
• Details about their height, depth, and slope nature.

b. Drainage:

• Identification of major rivers and their tributaries.
• Types of drainage patterns present in the area.

c. Vegetation:

• Areas covered by natural vegetation.
• Types and categories of forests.

3. Cultural Information:

a. Land Use:

• Identification of arable land, wasteland, industrial areas, and public land.
• Information about occupations prevalent in the area.

b. Transport and Communication:

• National, state, and district boundaries.
• Metalled and unmettled roads.
• Airports, railway lines, post offices, and other communication lines.

c. Settlements:

• Types and patterns of settlements.
• Urban centers and their characteristics.

Check Your Progress II

1) Physical features shown in topographical maps include:

• Mountains
• Valleys
• Plains
• Plateaus
• Rivers
• Lakes
• Forests
• Glaciers
• Hills
• Coastal areas

2) In a topographical map: Green colour typically represents vegetation, such as forests, grasslands, and agricultural land. Blue colour is used to depict water bodies, including rivers, lakes, and ponds.

Example of Topographical Map Interpretation

• In this section, we will focus on how to interpret a topographical map using a sample map at a 1:50,000 scale, which covers a part of topographical map No. 64F/1. This map represents a portion of the Mandla district and a small part of the Shahdol district in the state of Madhya Pradesh, India.
• The map was surveyed in 1975-76 and has a scale of 2 cm to 1 km, with a contour interval of 20 meters. It is situated between latitudes 22°45' N and 23°00' N, and longitudes 81°00' E and 81°15' E, covering an area of approximately 700 square kilometers.
• For our interpretation, we will focus on a specific part of the map, roughly between 22°55' N to 23°00' N and 81°00' E to 81°15' E. To enhance your understanding, you can also use the Google Earth website to visualize the same area.

Identification of Marginal Information

• The map 64F/1 represents a large part of the Mandla district and a small part of the Shahdol district in Madhya Pradesh, India.
• The area was surveyed in 1975-76, and the map has a scale of 2 cm to 1 km, with a contour interval of 20 meters.
• The region lies between latitudes 22°45' N to 23°00' N and longitudes 81°00' E to 81°15' E, covering an area of about 700 square kilometers.
• For better understanding, a specific part of the map will be interpreted, and the Google Earth website can be used to visualize the same area.

Interpreting Physical Features

The topographical map depicts a plateau region characterized by the east-west flowing Narmada River, which divides the area into northern and southern sections.

• River Narmada:
  • Bends and Direction:The river exhibits three significant bends:
    • North of Dindori: The river flows in a north-northwest direction.
    • South of Bicharpur Village: It bends westward.
    • Gangpur Village: The river flows westward, almost parallel to the latitude, before bending sharply south.
    • West of Dhaural Village: The river bends again and continues westward, maintaining its latitude.
• Northern Plateau:
  • Terrain: The northern plateau is marked by uneven terrain, with an elongated hill trending northeast-southwest in the northwestern part.
  • Hill Characteristics: The maximum height of these hills is approximately 880 meters, and the contour intervals suggest that many hills have flat tops.
  • Additional Peaks: Several other hill peaks are present in the region.
• Natural Vegetation:
  • Green Patches: The map indicates areas of natural vegetation, with a large green patch (Khami Reserve Forest) in the hilly region north of Khami village.
  • Forest Description: The Khami Reserve Forest is characterized by a fairly dense mixed jungle that is reserved and protected.

Interpreting Cultural Features

Human Settlements

• The red dots and symbols on the map indicate areas of human settlement.
• Rural settlements in this region are mostly scattered and often appear in a linear pattern in many places.
• Dindori is the largest village in this area, located in the eastern part of the region.
• Dindori village is well-connected by roads, with main metal roads leading to Jabalpur and Mandla. The other roads are unmetaled but are generally motorable during the dry season.
• The area also features various cart tracks and footpaths that connect villages to the roads.

Agriculture

• The absence of yellow colour on the map suggests that agriculture is not the predominant activity in this region.
• However, the relatively wide spaces between the contours near the Narmada River and its tributaries may indicate the presence of some agricultural land.
• Given that agriculture is not the main occupation, it can be inferred that forestry, animal husbandry, and cattle rearing could be the primary occupations of the local population.

Summary of Topographical Maps

In this unit, we have covered the following key points about topographical maps:

• Definition:. topographical map is a three-dimensional representation of the Earth's surface features, showing their elevation and layout.
• Contour Lines: These lines are crucial in topographical maps as they indicate the height or depth of land above or below the mean sea level.
• Scale and Classification: Topographic maps can be prepared on large or small scales and are classified into various types based on their function and scale.
• Survey of India: In India, the Survey of India is responsible for preparing and publishing topographical maps at different scales.
• Scales Used: Topographical maps in India are prepared at scales of 1:1,000,000 (million map), 1:250,000 (degree map), 1:50,000 (quarter map), and 1:25,000 (special map), covering specific latitudinal and longitudinal areas.
• Interpretation: Understanding a topographical map requires familiarity with the conventional signs, symbols, colors, and key provided on the map.
• Interpretation Steps: The major steps in interpreting a topographical map include studying the marginal information and identifying physical and cultural features.
• Relationship Between Features: Topographical maps depict the causal relationship between physical and cultural features, making them useful for various purposes.