Global properties
Topic: Map Projections · NCERT: Class 11, Ch 4 "Map Projections"
Meaning
Global properties are the four qualities that a globe shows correctly and that a map projection tries to keep: correct distance, correct shape, correct area and correct direction. A globe keeps all four because it is round like the earth. Once the round surface is moved onto flat paper, some of them get spoiled. No single projection keeps all four at once. Over large regions, true area and true shape cannot exist together on one map. A mapmaker therefore picks the property the map's purpose needs most.
Example
Mercator's projection (1569) keeps the shapes of small areas true, but it gives up correct area. On it, Greenland (about 2.2 million sq km) looks as big as Africa (about 30 million sq km). An equal area map of the same world keeps the sizes right but bends the shapes.
Don't confuse with
- Tissot's indicatrix: this is a tool for seeing how much a projection breaks these properties. It is a set of small circles drawn across a map, and it is not a property itself.