Topographical maps and the map series

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Detail

Topographical map (also called a general purpose map): a map drawn at a fairly large scale from careful, precise surveys of the ground.
Large scale means a small area is shown in great detail. For example, 1 cm on the map may stand for only 0.5 km on the ground.
It shows two kinds of features:

Natural features	Cultural (man-made) features
Relief (the ups and downs of the land: hills, valleys, plains)	Settlements (villages, towns)
Vegetation (forests, scrub, grassland)	Transport (roads, railways, tracks)
Water bodies (rivers, lakes, tanks, wells)	Cultivated land (farmland)

Base map: the topographical map is the base map. All other maps are drawn from it, including thematic maps of soils, population or land use.
Scales used (NCERT, Class 11, Introduction to Maps): topographical maps are large-scale series maps at 1:250,000, 1:50,000 and 1:25,000.

In every country, the national mapping organisation makes topographical maps.
In India this is the Survey of India (SoI), under the Department of Science & Technology.
SoI was set up in 1767. It is the oldest scientific department in India and one of the oldest survey organisations in the world [3].
The Great Trigonometric Survey began in 1802 [3].
The National Map Policy 2005 makes SoI responsible for producing, maintaining and sharing the topographic map database of the whole country [5].

Map series: a set of map sheets that together cover a large area. Every sheet in the series uses the same:
reference point
scale
projection (the method used to show the round Earth on a flat sheet)
signs, symbols and colours
Result: you can place sheets side by side, and they match. A symbol means the same thing on every sheet.

Definition: a series of topographical sheets that the Survey of India prepared for India and its neighbouring countries.
Turning point: the Delhi Survey Conference, 1937
Before 1937, SoI mapped India and its neighbours together in this series.
After 1937, SoI stopped mapping neighbouring countries.
Indian sheets kept the old numbering and layout, but from then on they followed the specifications of the International Map Series of the World.

Definition: a plan to make standard maps of the whole world at two scales:
1:1,000,000 (the "millionth map")
1:250,000
Since 1937, Indian sheets have followed its specifications (NCERT).
Origin:
The International Geographical Congress of 1891 proposed that member countries work together to produce a 1:1,000,000 map of the world [4].
Albrecht Penck, a German geographer, came up with the 1:1,000,000 world map and promoted it. It is known as the International Map of the World [4].
The specifications were agreed fairly soon, but production in the early years was slow [4].

Why the change: the old sheets used the Polyconic projection and the Everest datum, and many sheets were restricted for security reasons. The National Map Policy 2005 asked SoI to give users more access to map data without putting national security at risk [5][2].
Datum: the reference surface or model of the Earth's shape from which positions and heights are measured.

Feature	Defence Series Maps (DSMs)	Open Series Maps (OSMs)
Purpose	Defence and national security needs [5]	Mainly to support development activities and use by the general public [5][2]
Datum / projection	Everest or WGS-84 datum; Polyconic or UTM projection [5]	WGS-84 datum; UTM projection only [5][2]
Content	Full content: heights, contours, no loss of accuracy, on various scales [5]	Topographic content showing natural and man-made features and terrain [3]
Access	Classified as appropriate. The Ministry of Defence sets the rules for their use [5]	"Unrestricted" in paper and digital form after a one-time clearance by the Ministry of Defence [5]
Sheet numbers	Old numbering continued	Different map sheet numbers from DSMs [5]
Publisher	Survey of India	Brought out only by SoI [5]

Key terms:
UTM (Universal Transverse Mercator): a world grid system that splits the Earth into narrow north–south zones, so that each zone can be mapped accurately.
WGS-84 (World Geodetic System 1984): the global datum that GPS uses.
Access today:
On 11 April 2017 (250th year of SoI), the "nakshe" portal was launched. OSMs can be downloaded free as PDF files at 1:50,000 after Aadhaar-based login [3].
The release says this follows the National Map Policy 2005 and the Digital India programme [3].
Maps can also be bought from authorised sale agents, from Regional Geo-spatial Data Centres, or online at onlinemaps.surveyofindia.gov.in [2].
Other SoI products include Guide Maps at 1:20,000 (plains) and 1:10,000 (hills), and Tourist Maps at 1:50,000 for selected cities [2].

Topographical map = general purpose map, large-scale, made from precise survey, and it is the base map for all other maps.
National mapping agency of India = Survey of India, set up in 1767, the oldest scientific department of the Government of India [3].
Delhi Survey Conference, 1937: India stopped mapping neighbouring countries but kept the old numbering and layout.
International Map Series of the World scales = 1:1,000,000 and 1:250,000. Trap: 1:50,000 is not one of them.
International Map of the World was proposed at the International Geographical Congress, 1891, by Albrecht Penck [4].
National Map Policy 2005 created two series: DSM (Everest/WGS-84, Polyconic/UTM, classified) and OSM (WGS-84 datum, UTM projection, unrestricted) [5].
An OSM becomes unrestricted after a one-time Ministry of Defence clearance. Trap: the clearance is not given by SoI or ISRO [5].
nakshe portal (2017): free OSMs, 1:50,000, PDF, Aadhaar login [3].
Large-scale topographical series in India = 1:250,000, 1:50,000, 1:25,000.

Topographical maps are the foundation of planning: they are the base map for thematic, land-use, watershed, disaster-risk and infrastructure maps. Errors or gaps in them carry into every map built from them.
Open access versus security: the National Map Policy 2005 split maps into classified DSMs and unrestricted OSMs. This shows how a country balances development needs (planning, research, business) against national security [5].
Global standards help comparison: India adopted International Map Series specifications after 1937 and moved OSMs to the WGS-84/UTM system [2]. Both steps make Indian maps compatible with GPS, GIS and world datasets. This helps disaster response, cross-border river studies and digital governance.
Making data easy to get: free OSM downloads on nakshe, linked to Digital India, widen access for students, panchayats and planners [3].