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).