The earth's shape and the geographical grid

Detail

1. The earth is nearly a sphere, but not a perfect one

• The earth is close to a ball (sphere), but it is not a perfect one.
• Why it is not perfect (cause → process → result):
• Cause: the earth spins (rotates) on its axis.
• Process: the spin pushes material outward most strongly at the equator, because that is where the surface moves fastest.

• Result: the earth bulges at the equator and is slightly flattened at the poles. So the equatorial radius (centre to equator) is longer than the polar radius (centre to pole).

• Oblate spheroid means a sphere that is squashed a little at the top and bottom and is wider around the middle. This is the simple shape the earth looks like.

• Size of the bulge:
• Equatorial radius is about 6,378.1 km. Polar radius is about 6,356.8 km [2][3] (NCERT-level rounding: about 6,378 km and 6,357 km).
• The difference is only about 21 km. The earth is flattened by about one part in 300 [3]. So from space it still looks round.

Feature	Equatorial radius	Polar radius
Measured from centre to	Equator (0°)	North / South Pole (90°)
Length (approx.)	~6,378 km	~6,357 km
Why	Bulge caused by rotation	Flattening at the poles

2. Geoid: the earth's true shape

• Geoid means "earth-shaped". It is the earth's true shape: slightly flattened at the poles and bulging at the equator.
• More precise meaning (beyond NCERT):
• The geoid is the surface that mean sea level would form if it were extended under the continents as an imaginary sea-level surface [5].
• At every point it is at right angles to the pull of gravity [2].

• It is not smooth. Rock under the surface is not spread evenly. Some places have heavy, buried masses. This makes gravity a little stronger or weaker in places (gravity anomalies: small differences from normal gravity), so the geoid has gentle bumps and dips [2].

• Geoid vs reference ellipsoid:

• Mapmakers cannot calculate easily on a bumpy surface. So they use a smooth, regular, mathematical shape called a reference ellipsoid [3][5].
• An ellipsoid is set by two numbers: the semi-major axis (equatorial radius) and the semi-minor axis (polar radius) [3].

Shape	What it is	Smooth?	Used for
Sphere	Perfect ball	Yes	Simple globes, basic teaching
Oblate spheroid / ellipsoid	Ball flattened at the poles	Yes	Mapping and GPS calculations
Geoid	Mean-sea-level surface shaped by gravity	No (gently bumpy)	Heights above sea level, the "true" shape

3. India's reference ellipsoid: from Everest to WGS-84

• Great Trigonometrical Survey: began in 1802. George Everest worked out India's reference ellipsoid from its measurements in 1830. It was named the Everest Ellipsoid [4].
• It was revised in 1888 and again in 1956, when it was renamed the Modified Everest ellipsoid [4].
• Origin point: Kalyanpur, Madhya Pradesh, roughly in the centre of India. Here the geoid and the ellipsoid were taken to touch, with no gap between them. Its position, found by observing the stars, is 24°07′05.48″N, 77°39′14.75″E [4].
• Everest vs WGS-84:
• The Everest ellipsoid is non-geocentric: its centre is not at the earth's centre. It was fitted to India only [4].
• GPS uses WGS-84 (World Geodetic System 1984), which is geocentric: its centre is at the earth's centre [4].

• India decided to move to WGS-84 in the early 2000s. Seven conversion values were worked out with the Bursa-Wolf model. These were used to change all old topographical maps from the Everest datum to WGS-84, with 3 to 5 m accuracy [4].

• Datum means the reference surface (ellipsoid plus origin point) that all latitudes, longitudes and heights on a map are measured from.

4. Why we need the poles as starting points

• A sphere has no natural starting point. Every point on a perfect ball looks the same as every other.
• Rotation solves this:
• The earth spins around an imaginary line called the axis.
• The axis meets the surface at two points that stay fixed: the North Pole and the South Pole.
• These two poles are the fixed reference points used to draw all the other lines.

5. The geographical grid

• Geographical grid means a network of imaginary lines drawn on the globe that cross each other, built from the two poles. It lets us fix the position of any place.
• It has two sets of lines:

Line	Direction it runs	What it measures	Shape
Parallels of latitude	East-west	How far a place is north or south of the equator	Full circles, parallel to the equator
Meridians of longitude	North-south, from pole to pole	How far a place is east or west of the Prime Meridian	Half-circles meeting at the poles

• The two sets cross at right angles. Each crossing point is one exact location.

6. Geographical coordinates

• Geographical coordinates are a place's latitude and longitude written together, for example 28°37′N, 77°13′E.
• They are angles, not distances. They are measured from the centre of the earth.
• Units:
• Degree (°): the main unit.
• Minute (′): 1° = 60′.
• Second (″): 1′ = 60″.

• So 1° = 3,600″.

• What coordinates give us: the location of a place, the distance between two places and the direction from one place to another.

7. Understanding coordinates through everyday examples (Class 6)

• Market example: a shop described as the "7th shop in the 5th row". Two numbers, row and position, fix one shop exactly. Latitude and longitude work the same way on the globe.
• Chess example: moves are written as "d2 to d4". A letter (column) and a number (row) fix each square. This is a simple grid with coordinates.
• Lesson: any position on a flat or curved surface needs two values that cross.

8. Applying coordinates: Delhi and India

• Class 6 places Delhi at about 29°N, 77°E. These are rounded figures. Delhi is really near 28°37′N.
• India's extent: about 8°N to 37°N in latitude and 68°E to 97°E in longitude (more detail in the India-location topic).

• India lies fully in the Northern and Eastern Hemispheres.

• Kalyanpur (Madhya Pradesh, about 24°N, 77°40′E), India's old survey origin, lies near the middle of this latitude range [4].

Prelims Hooks

• The earth is an oblate spheroid: it bulges at the equator and is flattened at the poles because of rotation.
• Equatorial radius (~6,378 km) > polar radius (~6,357 km). The difference is about 21 km, and the flattening is about 1/300 [2][3].
• Geoid = the earth's true shape = the mean-sea-level surface at right angles to gravity. It is irregular, not a smooth ellipsoid [2][5].
• Parallels of latitude run east-west and meridians of longitude run north-south. A common trap is to swap the line's direction with the direction it measures.
• Geographical coordinates are angles, not distances: 1° = 60′ and 1′ = 60″.
• The Everest Ellipsoid (1830) came from the Great Trigonometrical Survey (began 1802). It was renamed the Modified Everest ellipsoid in 1956 [4].
• India's old geodetic origin is Kalyanpur, Madhya Pradesh [4].
• Everest ellipsoid = non-geocentric. WGS-84 (used by GPS) = geocentric. India moved to WGS-84 in the early 2000s [4].
• Delhi is about 28°37′N, 77°E. India spans about 8°N–37°N, 68°E–97°E.

Mains Points

• Rotation shapes the earth:
• Spin → outward push is strongest at the equator → equatorial bulge and polar flattening.

• This shape affects the length of a degree of latitude, satellite orbits and how accurate maps are.

• Geoid vs ellipsoid in mapping: heights are measured from the geoid (mean sea level), while GPS positions come from the ellipsoid. Mixing them up can put a point metres out of place. This matters for flood mapping, building canals and dams, and drawing boundaries.

• India's datum shift (Everest → WGS-84): it brought old Survey of India maps in line with GPS and satellite data, with 3–5 m accuracy [4]. This supports GIS-based planning, disaster management and the National Map Policy. It is a good example of geospatial infrastructure for GS-I/GS-III answers.
• The grid as a universal language: two crossing coordinates fix any place on earth without ambiguity. This is the base for navigation, time zones (longitude) and climate zones (latitude).