Understanding Map Projections

Glossary
angular units
The unit of measurement on a sphere or a spheroid,
usually in degrees. Map projection parameters such as the
central meridian and standard parallel are defined in
angular units.
aspect
The conceptual center of a projection system. See also
equatorial, oblique, and polar aspect.
azimuth
An angle measured from north. Used to define an oblique
aspect of a cylindrical projection or the angle of a
geodesic between two points.
azimuthal projection
A form of projection where the earth is projected onto a
conceptual tangent or secant plane. See planar projection.
central meridian
The line of longitude that defines the center and often the
x origin of a projected coordinate system.
circle
A geometric shape for which the distance from the center
to any point on the edge is equal.
conformal projection
A projection on which all angles at each point are
preserved. Also called an orthomorphic projection (Snyder
and Voxland, 1989).
conic projection
A projection resulting from the conceptual projection of
the earth onto a tangent or secant cone. The cone is then
cut along a line extending between the apex and base of
the cone and laid flat.
cylindrical projection
A projection resulting from the conceptual projection of
the earth onto a tangent or secant cylinder, which is then
cut from base to base and laid flat (Snyder and Voxland,
1989).
datum
1. A reference frame defined by a spheroid and the
spheroid’s position relative to the center of the earth.
2. A set of control points and a spheroid that define a
reference surface.
datum transformation
See geographic transformation.
eccentricity
A measurement of how much an ellipse deviates from a
true circle. Measured as the square root of the quantity
1.0 minus the square of the ratio of the semiminor axis to
the semimajor axis. The square of the eccentricity, ‘e 2 ’, is
commonly used with the semimajor axis, ‘a’, to define a
spheroid in map projection equations.
ellipse
A geometric shape equivalent to a circle that is viewed
obliquely; a flattened circle.
ellipsoid
When used to represent the earth, the three-dimensional
shape obtained by rotating an ellipse about its minor axis.
This is an oblate ellipsoid of revolution, also called a
spheroid.
ellipticity
The degree to which an ellipse deviates from a true circle.
The degree of flattening of an ellipse, measured as 1.0
minus the ratio of the semiminor axis to the semimajor
axis. See also flattening.
equal-area projection
A projection on which the areas of all regions are shown
in the same proportion to their true areas. Shapes may be
greatly distorted (Snyder and Voxland, 1989). Also known
as an equivalent projection.
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