Glossary and Summary of Projection Properties
There
are three main classes of projection according to their type:
Cylindric, Conic, Azimuthal
Then
there are four main classes of distortion properties:
Equal
Area, Conformal, Equidistant, Azimuthal
See
the Album
of Map Projections (USGS)
Aspect: Location of the tangency; polar,
equatorial or elsewhere (ie., oblique).
Conformal: At all points the scale is the
same in all directions. Preserves shape. Distorts area.
Coordinate system: Cartesian coordinates (x,y) on a plane, or geographic
coordinates (lat, long). Have a point of origin.
Datum: A reference point or surface
from which positional measurements are be made, based on a spheroid. (More on datums, Mike Price ESRI)
Developable Surface: A flattenable
surface, eg, cone, cylinder or plane (azimuth).
Equal Area: Projection which preserves areas
as they are on earth's surface. Distorts shape.
Equidistant: Where true distances occur only
from center of map, or along special lines. Distorts area.
Flattening: how much the earth's sphere is
flattened, eg., 1 in 300
Geoid: Approximation of earth's shape
using equal gravitational attraction
Graticule: The system of latitude and
longitude; the spherical coordinate system.
Great Circle: Intersection of a plane slicing
sphere in half. Shortest distance between two points on a
sphere. Constantly changing direction unlike rhumb line.
Projection: a transformation of the three
dimensional curved earth to a two dimensional map
Pseudocylindrical: Parallels are parallel,
meridians are curved. Eg., Robinson.
Rhumb Line or Loxodrome:
Line of constant and true direction. Not shortest distance (see Great Circle).
Secant: The intersection of a conic, azimuthal or cylindrical flattenable
surface along two separate lines.
Spheroid: a sphere-like shape, the earth
for example. Based on a model of the elliptical shape, called an ellipsoid
Tangent: The intersection of a conic or
cylindrical flattenable surface and the globe along a
single line.
Tissot's Indicatrix: Measurement of projection error
by circles or ellipses corresponding to circles on earth's surface. Captures areal and angular distortions.
Named Projections to Know (Minimum)
Name |
Type |
Properties |
Area
Suitable For |
General
Uses |
Mercator Robinson Lambert Albers |
Cylindric Pseudocylindric Azimuthal Conic |
Conformal Compromise Equal Area Equal Area |
Region/Sea/World World Hemisphere/Continent/Region Continent/Region/Medium
Scale |
Topographic/Navigation Thematic/Presentational Thematic/Presentational Thematic/Presentational |
Sources: USGS poster; An Album of Map Projections
(USGS #1453).