Reduced earth
Topic: Map Projections · NCERT: Class 11, Ch 4 "Map Projections"
Meaning
The reduced earth is a model of the earth shrunk down to the scale of the map. The graticule is drawn on it first, before being projected onto paper. The graticule is the net of parallels and meridians. Its radius is the earth's radius (taken as 640,000,000 cm) divided by the denominator of the representative fraction (RF). The RF is the scale written as a ratio, like 1:250,000,000. Every measurement in drawing a projection starts from this radius.
Example
For a map at 1:250,000,000, the radius of the reduced earth is 640,000,000 ÷ 250,000,000 = 2.56 cm. NCERT uses this value to draw a simple conical projection for the area from 10°N to 70°N and 10°E to 130°E. At 1:300,000,000, the radius works out to about 2.1 cm.
Don't confuse with
- Scale of the map: the scale is the ratio between map distance and ground distance. The reduced earth is the actual small globe that you build using that scale.