Topographical map

Also called: Toposheet, General purpose map · Topic: Topographical Maps: Relief Representation and Interpretation · NCERT: Class 11, Ch 1 "Introduction to Maps"; Class 11, Ch 5 "Topographical Maps"

Meaning

A topographical map (also called a toposheet or general purpose map) is a fairly large-scale map made from careful, precise surveys of the ground. It shows both natural features (relief, drainage, vegetation) and cultural or man-made features (settlements, roads, railways, farmland). It is the base map from which all other maps, such as soil, population or land-use maps, are drawn. In India, the Survey of India prepares these maps in series.

Explanation

What it shows and why it is the "base map"

• Large scale: a small area is shown in great detail.

• Example: 1 cm on the map may stand for only 0.5 km on the ground.

• Natural features

• Relief (the ups and downs of the land: hills, valleys, plains)
• Vegetation (forests, scrub, grassland)

• Water bodies (rivers, lakes, tanks, wells)

• Cultural (man-made) features

• Settlements (villages, towns)
• Transport (roads, railways, tracks)

• Cultivated land (farmland)

• Why it is the base map

• It records the real ground accurately and in detail.
• Thematic maps (maps on one theme, such as soils, population or land use) are built on top of it.
• So a mistake on the topographical map carries into every map drawn from it.

Map series and scales

• Map series: a set of map sheets that together cover a large area. Every sheet in a 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: sheets can be laid side by side and they match. A symbol means the same thing on every sheet.

• Scales (NCERT, Class 11, Introduction to Maps): topographical maps are large-scale series maps at 1:250,000, 1:50,000 and 1:25,000.

How India's series developed

• India and Adjacent Countries Series: the Survey of India first mapped India and its neighbouring countries together in one series.
• Delhi Survey Conference, 1937: this was the turning point.
• After 1937, SoI stopped mapping neighbouring countries.
• Indian sheets kept the old numbering and layout.

• From then on, they followed the specifications of the International Map Series of the World.

• International Map Series of the World: a plan to make standard maps of the whole world at 1:1,000,000 (the "millionth map") and 1:250,000.

• The International Geographical Congress of 1891 proposed that member countries work together on a 1:1,000,000 world map [4].
• Albrecht Penck, a German geographer, came up with this 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].

Two series today: DSM and OSM (National Map Policy 2005)

• Why the change came
• The old sheets used the Polyconic projection and the Everest datum. A datum is the reference model of the Earth's shape from which positions and heights are measured.
• 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].

• Defence Series Maps (DSMs)

• Made for defence and national security needs [5].
• Use the Everest or WGS-84 datum and the Polyconic or UTM projection [5].
• Show full content (heights, contours) with no loss of accuracy, on various scales [5].

• Classified as needed. The Ministry of Defence sets the rules for their use [5].

• Open Series Maps (OSMs)

• Made mainly to support development activities and for use by the general public [5][2].
• Use only the WGS-84 datum and the UTM projection [5][2].
• Become "unrestricted" in paper and digital form after a one-time clearance by the Ministry of Defence [5].

• Have different sheet numbers from DSMs and are 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 each zone can be mapped accurately.
• WGS-84 (World Geodetic System 1984): the global datum that GPS uses.

Where it occurs

• World: in every country, the national mapping organisation makes topographical maps. World-wide standard sheets follow the International Map of the World plan at 1:1,000,000 and 1:250,000 [4].
• India: who makes them
• 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].

• Under the National Map Policy 2005, SoI produces, maintains and shares the topographic map database of the whole country [5].

• India: how users get them

• nakshe portal: launched on 11 April 2017, in SoI's 250th year. OSMs can be downloaded free as PDF files at 1:50,000 after an Aadhaar-based login [3].
• The portal 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].

• Related SoI products: Guide Maps at 1:20,000 (plains) and 1:10,000 (hills), and Tourist Maps at 1:50,000 for selected cities [2].

• Where they are used: as the base for thematic, land-use, watershed, disaster-risk and infrastructure maps, and for planning by students, panchayats and planners.

Don't confuse with

• Thematic map: shows one theme only (such as soils, population or land use) and is drawn from the topographical map. A topographical map is a general purpose map that shows many features together.
• Small-scale map (atlas or wall map): shows a large area in little detail. A topographical map is large-scale, showing a small area in great detail.
• International Map Series of the World scales: these are 1:1,000,000 and 1:250,000 only. The common Indian toposheet scale of 1:50,000 is not one of them.
• Defence Series Map vs Open Series Map: a DSM is classified and may use the Everest datum and Polyconic projection. An OSM uses WGS-84 and UTM only and is unrestricted after Ministry of Defence clearance [5].

Prelims Hooks

• A topographical map is a general purpose map: large-scale, made from precise surveys, and the base map for all other maps.
• India's large-scale topographical series: 1:250,000, 1:50,000 and 1:25,000.
• Delhi Survey Conference, 1937: India stopped mapping neighbouring countries but kept the old numbering and layout, and adopted International Map Series specifications.
• The International Map of the World (1:1,000,000) was proposed at the International Geographical Congress, 1891, and promoted by Albrecht Penck [4].
• National Map Policy 2005: DSM (Everest/WGS-84, Polyconic/UTM, classified) and OSM (WGS-84, UTM, unrestricted). Trap: an OSM is cleared once by the Ministry of Defence, not by SoI or ISRO [5].
• nakshe portal (2017): free OSMs at 1:50,000, as PDFs, with Aadhaar login. Survey of India was founded in 1767 [3].

Mains Points

• Topographical maps are the foundation of planning (GS-I/GS-III)
• 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 on them, so accurate and up-to-date toposheets support good planning.

• Balancing open access and security

• The National Map Policy 2005 split maps into classified DSMs and unrestricted OSMs [5].
• This lets planners, researchers and businesses use map data while defence-sensitive content stays protected.

• Free OSM downloads on nakshe, linked to Digital India, widen access for students, panchayats and planners [3].

• Global standards help comparison

• India adopted International Map Series specifications after 1937 and moved OSMs to the WGS-84/UTM system [2].
• This makes Indian maps work with GPS, GIS and world datasets, which helps disaster response, studies of rivers that cross borders, and digital governance.

Related concepts

• India and Adjacent Countries Series
• International Map Series of the World

Read more

• Topographical maps and the map series