Understanding Map Projections

High Accuracy Reference Network
A resurvey of NAD 1983 control points using GPS
techniques. The resurvey date is often included as part of
the datum name—NAD 1983 (1991) or NAD91.
High Precision Geodetic (or GPS) Network
A resurvey of NAD 1983 control points using GPS
techniques. The resurvey date is often included as part of
the datum name—NAD 1983 (1991) or NAD91.
HPGN
See High Precision Geodetic (or GPS) Network.
Interrupted projection
Discontinuities and gaps are added to a map to decrease
the overall distortion. The world is divided, usually along
certain meridians, into sections, or gores. Each section has
its own projection origin.
latitude
The angular distance (usually measured in degrees) north
or south of the equator. Lines of latitude are also called as
parallels. See geodetic latitude and geocentric latitude.
latitude of center
The latitude value that defines the center (and sometimes
origin) of a projection.
latitude of origin
The latitude value that defines the origin of the
y-coordinate values for a projection.
linear units
The unit of measurement, often meters or feet, on a plane
or a projected coordinate system. Map projection
parameters such as the false easting and false northing are
defined in linear units.
longitude
The angular distance (usually measured in degrees) east or
west of a prime meridian.
longitude of center
The longitude value that defines the center (and
sometimes origin) of a projection.
longitude of origin
The longitude value that defines the origin of the
x-coordinate values for a projection.
major axis
The longer axis of an ellipse or spheroid.
map projection
A systematic conversion of locations from angular to
planar coordinates.
map scale
The ratio of a length on a map to its length on the ground.
meridian
The reference line on the earth’s surface formed by the
intersection of the surface with a plane passing through
both poles. This line is identified by its longitude.
Meridians run north–south between the poles.
minor axis
The shorter axis of an ellipse or spheroid.
NAD 1927
North American Datum of 1927. A local datum and
geographic coordinate system used in North America.
Replaced by NAD 1983. Also known as NAD27.
NAD 1983
North American Datum of 1983. A geocentric datum and
geographic coordinate system used in North America. Also
known as NAD83.
oblate ellipsoid
An ellipsoid created by rotating an ellipse around its minor
axis.
oblique aspect
A planar or cylindrical projection with its central point
located at some point not on the equator or at the poles.
parallel
A reference line on the earth’s surface that runs east–west
around a sphere or spheroid and is parallel to the equator.
Latitude lines are parallel circles.
parameters
Values that define a specific instance of a map projection.
Parameters differ for each projection and can include
central meridian, standard parallel, scale factor, or latitude
of origin.
Glossary • 103