Understanding Map Projections

planar projection
A form of projection where the earth is projected onto a
conceptual tangent or secant plane. Usually, a planar
projection is the same as an azimuthal projection (Snyder
and Voxland, 1989).
polar aspect
A planar projection with its central point located at either
the North or South Pole.
prime meridian
A meridian of reference that defines the origin of the
longitude values, 0° east or west.
projected coordinate system
A reference system that defines the locations of points on
a planar surface.
radius
The distance from the center to the outer edge of a circle.
reference ellipsoid
See ellipsoid.
rhumb line
A complex curve on the earth’s surface that crosses every
meridian at the same oblique angle; a straight line on the
Mercator projection. Also called a loxodrome (Snyder and
Voxland, 1989).
scale factor
A value (usually less than one) that converts a tangent
projection to a secant projection. Represented by ‘k 0
’ or
‘k’. If a projected coordinate system doesn’t have a scale
factor, the standard point or lines of the projection have a
scale of 1.0. Other points on the map have scales greater
or lesser than 1.0. If a projected coordinate system has a
scale factor, the standard point or lines no longer have a
scale of 1.0.
secant projection
A form of map projection where the conceptual surface
of the projection (cone, cylinder, or plane) cuts through
the earth’s surface.
semimajor axis
The equatorial radius of a spheroid. Often known as ‘a’.
semiminor axis
The polar radius of a spheroid. Often known as ‘b’.
sphere
A three-dimensional shape obtained by revolving a circle
around its diameter.
spherical coordinate system
A system using positions of longitude and latitude to
define the locations of points on the surface of a sphere
or spheroid.
spheroid
When representing the earth, the three-dimensional shape
obtained by rotating an ellipse about its minor axis. This is
an oblate ellipsoid of revolution, also called an ellipsoid.
standard line
A line on a sphere or spheroid that has no length
compression or expansion after being projected.
Commonly, a standard parallel or central meridian.
standard parallel
The line of latitude where the projection surface touches
the surface. A tangent conic or cylindrical projection has
one standard parallel, while a secant conic or cylindrical
projection has two. A standard parallel has no distortion.
State Plane Coordinate System
A projected coordinate system used in the United States
that divides each state into one or more zones to minimize
distortion caused by the map projection. Also known as
SPCS and SPC.
tangent projection
A form of map projection where the conceptual surface
of the projection (cone, cylinder, or plane) just touches
the earth’s surface.
true-direction projection
A form of projection that shows lines with correct
azimuths from one or two points.
unit of measure
See angular units or linear units.
Universal Transverse Mercator
A projected coordinate system that divides the world into
60 north and south zones, six degrees wide.
104 • Understanding Map Projections