Choosing a projection: world maps and the mapping of India

Detail

1. The basic rule for choosing a projection

• Map projection means a method of drawing the curved surface of the earth (the globe's grid of latitudes and longitudes) onto a flat sheet. No flat map can keep everything correct. Some area, shape, distance or direction is always lost.
• So the choice depends on three things:
• Purpose of the map: what it must show correctly (area, shape or direction).
• Size of the region: the whole world, a continent, a country or one small sheet.

• Shape and position of the region: whether it stretches east–west or north–south, and whether it lies near the equator, in the middle latitudes or near a pole.

• The property you need decides the family of projection:

• Equal-area (homolographic): every region keeps its true size compared with others. Best for distribution maps (population, crops, forests, rainfall).
• Conformal (orthomorphic): keeps the true shape of small areas and true angles. Best for navigation.
• Compromise: neither equal-area nor conformal, but keeps total distortion low. Best for general-purpose world maps.

2. World maps

Goode's homolosine projection

• Made by J. Paul Goode (USA) in 1923. It is equal-area.
• "Homolosine" means a join of two equal-area projections:
• The sinusoidal projection is used for low latitudes (near the equator).

• The homolographic (Mollweide) projection is used for high latitudes.

• It is interrupted: the map is cut open over the oceans.

• Cutting the map lets each continent have its own central meridian (a straight north–south line through its middle).
• Shapes near that line stay good, so continents look compact and are less stretched.

• The result is a set of lobes that look like a peeled orange. This is why it is also called an "orange-peel map" [6]. The Goode projection splits the globe into six unequal lobes [7].

• Use: atlases use it for world distribution maps, where true area matters more than unbroken oceans.

• Limitation: the oceans are broken, so it is poor for ocean currents or shipping routes.

Robinson projection

• Made by Arthur H. Robinson (USA) in 1963. It is a compromise projection.
• It is not equal-area and not conformal.

• It balances the errors so the world "looks right", and land at high latitudes is stretched much less than on Mercator [6].

• National Geographic Society (NGS):

• The NGS used the Van der Grinten projection (a world drawn inside a circle) from 1922 to 1988 [6].
• In 1988 the NGS dropped Van der Grinten and adopted Robinson, to show the areas of land masses more truly [6].

• The NGS then switched to the Winkel Tripel (NCERT: in 1998). The NGS says the Winkel Tripel replaced Robinson on its signature world maps and was first printed as a National Geographic Magazine map supplement in April 1995 [6].

• Winkel Tripel: "Tripel" is German for "triple". The projection tries to keep three properties low in error together: area, distance and direction [6].

Projection	Year / author	Property	Main use
Mercator	1569, Gerardus Mercator [6]	Conformal (true shape and direction locally)	Sea and air navigation charts
Goode's homolosine	1923, J. Paul Goode	Equal-area, interrupted	World distribution maps in atlases
Robinson	1963, Arthur Robinson	Compromise	General world maps (NGS 1988 onward)
Winkel Tripel	Oswald Winkel; NGS from the 1990s	Compromise (area, distance, direction)	NGS world maps today [6]

3. Other standard matches (purpose → projection)

• Navigation charts → Mercator.
• Any straight line on Mercator is a rhumb line (loxodrome: a line that cuts all meridians at the same angle).
• A navigator can follow it on a fixed compass bearing.

• Drawback: areas grow larger towards the poles. Greenland looks about as big as Africa.

• Polar regions → zenithal (azimuthal) projection.

• The plane touches the globe at the pole.

• Distortion is low near the pole and grows outwards. This fits the round shape of polar areas.

• Mid-latitude countries → conical projection.

• A cone touches the globe along a mid-latitude standard parallel, where the scale is true.
• It suits countries that stretch east–west in the middle latitudes, such as the USA and Europe.
• Bonne's or a two-standard-parallel conic works well for such regions.

4. India, old system: polyconic on the Everest spheroid

• The Survey of India (SoI), India's national mapping agency under the Department of Science & Technology, made its older topographical sheets (toposheets: detailed maps of relief, drainage and settlements) on the polyconic projection.
• Datum: these sheets used the Everest spheroid. A spheroid (or datum) is the model of the earth's size and shape used for mapping. The Everest spheroid was fitted to the Indian region by George Everest's survey in 1830.
• How the polyconic works, step by step:
• Many cones are used, not one. Each parallel gets its own cone that touches the globe exactly along that parallel (a tangent cone).
• Each parallel is drawn as an arc of its own cone. The arcs are not concentric (they do not share one centre).
• The central meridian is a straight line and is true to scale. The other meridians are curves.

• Result: the map is accurate near the central meridian of each sheet. Error grows towards the east and west edges.

• Why it suited India: each toposheet covers a small area (for example 15′ × 15′ for a 1:50,000 sheet). So every sheet stays close to its own central meridian and the error is tiny.

• Limitation: the projection is neither equal-area nor conformal. Next-door sheets do not fit perfectly into one large map.
• Today: the SoI Defence Series Maps (DSMs) still use the Everest/WGS-84 datum with the polyconic/UTM projection. They carry full heights, contours and content [2][3].

5. India, new system: UTM on WGS-84 (National Map Policy 2005)

• National Map Policy (NMP) 2005: announced by the Central Government on 19 May 2005 [2][3].
• It makes the Survey of India responsible for producing, maintaining and sharing the topographic map database of the whole country [2].

• It created two map series [2][3]:

  • Defence Series Maps (DSMs): for the defence and national security needs of the country.
  • Open Series Maps (OSMs): made only by the SoI, mainly to support development work.

• Open Series Maps:

• They use the UTM projection on the WGS-84 datum and carry sheet numbers that differ from the old sheets [2].
• They are published at 1:250,000, 1:50,000 and 1:25,000 scales for the general public [4].

• They do not show the grid or classified information, so they are in the unrestricted category [4].

• WGS-84 (World Geodetic System 1984): one global model of the earth. GPS uses it. Moving from the local Everest spheroid to WGS-84 lets Indian maps match satellite positions directly.

• Universal Transverse Mercator (UTM): a world grid system built from many transverse Mercator projections.
• Transverse Mercator means Mercator turned on its side. The cylinder touches the globe along a meridian, not the equator. So a narrow north–south strip has very little error.
• Coverage: 80°S to 84°N. The polar areas use a separate polar grid [5].
• Zones: 60 zones, each 6° of longitude wide [5]. Zone 1 starts at 180° and the numbers rise eastward.
• Scale factor: 0.9996 on each zone's central meridian and about 1.0010 at the zone's edge [5]. Scale is exactly true (1) along two lines about 180 km east and west of the central meridian [5]. This spreads the error evenly across the zone.

• Metric grid: positions are given as eastings and northings in metres inside each zone.

• India in UTM: India lies in zones 42 to 47.

• Zone 42 covers 66°E–72°E (Gujarat and the western edge of India).

• Zone 47 covers 96°E–102°E (the eastern tip of Arunachal Pradesh).

• Why UTM matters: GPS units and GIS layers (Geographic Information System: computer maps built from stacked data layers) use UTM's metric grid. Distances and areas are easy to work out in metres, and survey, satellite and map data fit together. (Satellite data and GIS are covered in the remote-sensing-gis note; sheet numbering is covered in the topographical-maps note.)

Feature	Old SoI toposheets	Open Series Maps (NMP 2005)
Projection	Polyconic	Universal Transverse Mercator
Datum	Everest spheroid (local)	WGS-84 (global, GPS-compatible)
Accurate along	Central meridian of each sheet	Central meridian of each 6° zone (scale 0.9996)
Access	Restricted in many areas	Unrestricted, for the public [4]
Scales	1:250,000; 1:50,000; and others	1:250,000; 1:50,000; 1:25,000 [4]

Prelims Hooks

• Goode's homolosine (1923) is equal-area and interrupted over the oceans. It joins the sinusoidal (low latitudes) and Mollweide (high latitudes) projections, and it is the "orange-peel" map used for world distribution maps.
• Robinson (1963) is a compromise projection, neither equal-area nor conformal. Trap: it is often wrongly called "equal-area".
• NGS world-map order: Van der Grinten (1922–1988) → Robinson (1988) → Winkel Tripel (first printed April 1995 [6]; NCERT: 1998).
• Matches: navigation → Mercator; poles → zenithal; mid-latitude east–west countries → conic; world distributions → equal-area (Goode's).
• Polyconic: each parallel is an arc of its own tangent cone, and only the central meridian is straight and true to scale. It was used for old SoI toposheets on the Everest spheroid.
• National Map Policy was announced on 19 May 2005. Its two series are Defence Series Maps and Open Series Maps. OSMs use UTM + WGS-84.
• UTM: 60 zones × 6°, from 80°S to 84°N, with a central-meridian scale factor of 0.9996. India lies in zones 42–47.
• Trap: UTM is built on the transverse Mercator (cylinder touching a meridian), not the normal equatorial Mercator.

Mains Points

• Purpose decides the projection. Equal-area maps (Goode's) keep distribution maps of population, poverty or forest cover honest. Mercator's huge high-latitude areas can make northern countries look bigger and more important than they are, so a projection is a choice about how the world is shown.
• Why India moved from polyconic/Everest to UTM/WGS-84: the old system was a local datum, its sheets did not join cleanly, and it did not match GPS. UTM on WGS-84 gives a single metric grid that works with GPS, satellite images (ISRO/NRSC Bhuvan) and GIS. This supports planning for roads, cities, watersheds and disaster mapping.
• NMP 2005 balanced security and development: Defence Series Maps keep full detail for national security, while unrestricted Open Series Maps [2][4] give planners, researchers and citizens access to maps. This laid the base for later liberalisation of geospatial data in India.
• No single best world map: compromise projections (Robinson, Winkel Tripel) suit general reading and school atlases. Specialist maps need specialist projections (navigation needs conformal, distribution needs equal-area, polar maps need zenithal).