Gnomonic projection
Topic: Map Projections · NCERT: Class 11, Ch 4 "Map Projections"
Meaning
A gnomonic projection is a perspective projection with the light source at the centre of the globe. Light from the centre throws the shadow of the graticule onto a sheet of paper touching the globe. On this map, every great circle appears as a straight line. A great circle is any circle that cuts the globe into two equal halves, such as the equator, and an arc of it is the shortest route between two points on the earth. So a straight line on a gnomonic chart shows the shortest route.
Example
Navigators planning a long ocean voyage first draw a straight line between the two ports on a gnomonic chart. This line is the great-circle route. They then copy it onto a Mercator chart as a chain of short rhumb lines, because each rhumb line can be steered on a single compass bearing.
Don't confuse with
- Stereographic projection: here the light is on the globe's surface, opposite the touching point, not at the centre.
- Mercator's projection: on Mercator, straight lines are rhumb lines with a constant bearing, not great circles.