Geoid
Topic: Latitude, Longitude and Time · NCERT: Class 11, Ch 1 "Introduction to Maps"; Class 11, Ch 3 "Latitude, Longitude and Time"; Class 11, Ch 4 "Map Projections"
Meaning
Geoid means the earth's true shape. The earth is nearly a sphere, but not a perfect one. As it rotates, it bulges at the equator and is slightly flattened at the poles. So the equatorial radius is larger than the polar radius. A shape like this is called an oblate spheroid, which means a ball squashed a little at the top and bottom. Map-makers need the true shape to measure distances and draw maps correctly.
Example
The earth's equatorial radius is about 6,378 km and its polar radius is about 6,357 km. A place on the equator, such as Quito in Ecuador, is therefore farther from the earth's centre than the poles are.
Don't confuse with
- Sphere: in a perfect sphere every radius is equal. The geoid is wider at the equator.