Understanding Map Projections

GNOMONIC
PROPERTIES
Shape
Increasingly distorted from the center; moderate
distortion within 30 degrees of the center point.
Area
Distortion increases with distance from the center;
moderate distortion within a 30 degree radius of the
center.
Direction
Accurate from the center.
Distance
No line has an accurate scale, and the amount of
distortion increases with distance from the center.
The central meridian is 0°, and the latitude of origin is 90° S.
DESCRIPTION
This azimuthal projection uses the center of the earth
as its perspective point. All great circles are straight
lines, regardless of the aspect. This is a useful
projection for navigation because great circles
highlight routes with the shortest distance.
PROJECTION METHOD
This is a planar perspective projection viewed from
the center of the globe. The projection can be any
aspect.
POINT OF TANGENCY
A single point anywhere on the globe.
Polar aspect—North Pole or South Pole.
Equatorial aspect—Any point along the equator.
Oblique aspect—Any other point.
LINEAR GRATICULES
All meridians and the equator.
Scalar Distortion for Polar Aspect
Degrees from Center (°) 15.0 30.0 45.0 60.0
Meridian Distortion (%) 7.2 33.3 100.0 300.0
Latitude Distortion (%) 3.5 15.5 41.4 100.0
LIMITATIONS
This projection is limited by its perspective point and
cannot project a line that is 90 degrees or more from
the center point; this means that the equatorial
aspect cannot project the poles and the polar aspects
cannot project the equator.
A radius of 30 degrees produces moderate distortion,
as indicated in the table above. This projection
should not be used more than about 60 degrees
from the center.
USES AND APPLICATIONS
All aspects—Routes of navigation for sea and air.
Polar aspect—Navigational maps of polar regions.
Equatorial aspect—Navigational maps of Africa and
the tropical region of South America.
60 • Understanding Map Projections