Perspective projection
Topic: Map Projections · NCERT: Class 11, Ch 4 "Map Projections"
Meaning
A perspective projection is drawn by casting the shadow of the globe's graticule from a light source onto a developable surface. A developable surface is one that can be laid flat, such as a sheet of paper, a cone or a cylinder. The position of the light decides the type:
- at the centre of the globe, it is gnomonic
- on the surface opposite the touching point, it is stereographic
- at infinity, with parallel rays, it is orthographic.
Example
Imagine a glass globe with wire parallels and meridians and a bulb inside it. If a sheet of paper is placed to touch the globe at the North Pole, the shadow of the wires on the paper gives a gnomonic map of the Arctic. On this map, the meridians spread out from the pole like the spokes of a wheel.
Don't confuse with
- Conventional projection: this is built only from mathematical formulae, with no light or shadow. Mercator's projection is a conventional projection, not a perspective one.