Topographical map
The Survey of India (SOI), which operates under the Department of Science & Technology of the Indian government, stands as the country's oldest scientific department. Established in 1767 and headquartered in Dehradun, Uttaranchal, it has cultivated a rich heritage over time. As the leading cartographic agency of the nation, it bears a distinct responsibility to comprehensively explore and map India's geographical expanse. Its purpose extends to providing foundational maps for expeditions and integrated development, ensuring optimal utilisation of resources for India's current and future progress, prosperity, and security.
Survey of India (SOI) Topographical Map: India and adjacent Areas
The intricate map of the Indian terrain owes its completion to the meticulous efforts of a notable lineage of British surveyors, spearheaded by Mr. Lambton and Sir George Everest. These British surveyors meticulously mapped the geographical area spanning from coordinates (44°E, 4°N) to (104°E, 40°N), encompassing the entirety of the then Indian subcontinent as well as a substantial portion of Asia (referred to as Table-1, 2 & 3).
Old System
Updated system
To achieve this, the designated area was initially divided into 106 standardised rectangles, each measuring (4° × 4°). These rectangles were assigned numerical designations from 1 to 106 (as shown in Figure 1). These were then depicted on a scale of 1 inch to 16 miles (1:1,000,000) and referred to as million sheets or 1M Sheets. Each of these 1M sheets contains sixteen degree sheets measuring (1° × 1°) on a scale of 1 inch to 4 miles (1:250,000), labelled with alphabets A to P. Each degree sheet is further subdivided in two ways. Firstly, each sheet is divided into four quadrant sheets measuring (30' × 30') on a scale of 1 inch to 2 miles, identified as NW, NE, SW, and SE. Secondly, each degree sheet is divided into sixteen inch sheets measuring (15' × 15') on a scale of 1 inch to a mile, labelled with numerals 1 to 16.
Reference system of Old Series of Defence Series Maps (DSMs)
In the late 1970s, topographical maps were revised using metric scales, integrating updated data from ground and aerial surveys. More detailed maps were created at scales of 1:25,000, referenced in two formats. Some 1:50,000 sheets were reproduced, each containing six 1:25,000 sheets measuring (7'30" × 5'), labelled numerically from 1 to 6. Additionally, each 1:50,000 sheet featured four 1:25,000 sheets measuring (7'30" × 7'30") labelled as NW, NE, SW, and SE.
Over time, scientific surveying principles were advanced by modern technology, including Remote Sensing (RS) and Geographic Information Systems (GIS), to meet diverse needs like national security and sustainable development. The SOI assumed a pioneering role in furnishing user-centric, cost-effective, reliable, and high-quality geospatial data and intelligence. This was accomplished through the introduction of a new National Map Policy (NMP) on May 19, 2005, which radically transformed mapping practices, referencing schemes, scales, dimensions, map products, and accessibility.
The SOI facilitates the global exchange of information, ideas, and technological innovations among data producers and users. These stakeholders gain access to high-resolution data in real-time at an affordable cost. The SOI produces two map series: Defense Series Maps (DSMs) for defence and national security purposes, and Open Series Maps (OSMs) primarily intended to support developmental activities within the country. The DSMs are topographical maps created with precision (on Everest/WGS-84 Datum and Polyconic/UTM Projection) in various scales, including elevations, contours, and comprehensive content.
Conversely, the OSMs are designed to bolster development endeavours across India. These are depicted using the Universal Transverse Mercator (UTM) Projection on the WGS-84 datum. Each of these Open Series Maps (OSMs), available both in physical and digital formats, along with a comprehensive topographical database, will become "unrestricted" after securing a one-time clearance from the Ministry of Defence. The Survey of India (SOI) ensures that no sensitive civil or military areas and vulnerable points (VAs/VPs) are displayed on these OSMs.
New Series or Open Series Maps (OSM)
Reference System of OSMs
In this mapping configuration, India is encompassed by 32 UTM zones, each spanning (6° × 4°) in size (designated as B46, C42, C43, C44, C46, D42, D43, D44, D46, E43, E44, E45, F42, F43, F44, F45, F46, G42, G43, G44, G45, G46, G47, H42, H43, H44, H45, H46, H47, 143, 144, and J43). These zones are drawn at a scale of 1:1,000,000 and are referred to as million sheets.
Each million sheet comprises 24 degree sheets drawn at a scale of 1:250,000 and are labelled with alphabets from A to X. Each degree sheet includes 16 15' sheets measuring (15' × 15') at a scale of 1:50,000, designated by numerals 01 to 16. A 15' sheet is further divided in two ways: firstly, it contains 4 quadrant sheets measuring (7'30" × 7'30") at a scale of 1:25,000 labelled as NW, NE, SE, and SW; secondly, each sheet includes 25 sheets measuring (3' x 3') at a scale of 1:10,000, again designated with alphabets from A to Y. Each 3'-sheet is then divided into 25 sheets measuring (36" × 36") at a scale of 1:2,000.
Some basic Information on Topographical Map: India and Adjacent Series
Topographical Maps, referred to as Topomaps or Toposheets, bear similarity to the Ordnance Maps of Great Britain and Europe, as well as the USGS maps in the United States. The British Surveyors created a total of 106 Million Sheets, covering the then Indian Subcontinent and neighbouring areas including Iran, Afghanistan, Tibet, parts of China, and Southeast Asia.
Million Sheets (1:1,000,000) are constructed using Lambert's Conical Orthomorphic Projection with the Everest Datum. For instance, Toposheet No. 1 at a scale of 1:1,000,000 corresponds to Tabriz in Iran. India is covered by 39 Million Sheets: 39-49, 51-58, 61-66, 72-74, 77-79, 82-84, 86-88, 91, and 92.
The Inch Sheets (1:63,360) or the 15' Sheets (1:50,000) are drawn using Modified Polyconic or International Projections.
With advancements in science and technology, Indian topographical sheets have been periodically updated and redrawn with greater detail on various larger scales. The open series maps (OSM) in digital formats, with a new layout, are now available across various scales, featuring fully digitised data layers, following the introduction of the new National Map Policy (NMP) on May 19, 2005.
Topographical Map: International Series
In international series, topographical map has been prepared on a 1:100,000 scale across the globe in following manner:
The estimated total of Million Sheets covering the entire globe is 2222, calculated as follows:
a) Between 60°N and 60°S of (4° × 6°) dimension = 2 × (60° × 4°) × (360° ÷ 6°) = 1800
b) Between 60°N/S and 88°N/S of (4° × 12°) dimension = 2 × (28°/49) × (360°/12°) = 420
c) Between 88°N/S and 90°N/S, covering polar regions = 2 × 1 = 2
Each million sheet has been divided into 16 degree sheets with scale 1: 250,000.
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