## Geocart Projections

What is a projection?

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orthographic

analemma

Classifications

Azimuthal
Perspective
Neither conformal nor equal area

Graticule

Polar aspect:
Meridians: Equally spaced straight lines intersecting at the central pole. Angles between them are the true angles.
Parallels: Unequally spaced circles centered at the pole, which is a point. Spacing decreases away from the pole.
Equatorial aspect:
Meridians: Central meridian is a straight line. Outer meridians 90° away are circles. Other meridians are unequally spaced elliptical arcs intersecting at each pole and concave toward the central meridian. Spacing decreases away from the central meridian.
Parallels: Unequally spaced straight parallel lines perpendicular to the central meridian. Spacing decreases away from the Equator. Parallels intersect the outer meridian, however, at equal intervals.
Symmetry: About the central meridian or the Equator
Oblique aspect:
Meridians: Central meridian is a straight line. Other meridians are semiellipses intersecting at each pole. Spacing decreases away from the central meridian.
Parallels: Complete or partial ellipses, all of the same shape (or eccentricity); their minor axes lie along the central meridian. Spacing decreases away from the center of projection.

Range

No more than one hemisphere at a time

Scale

True at the center and along any circle having its center at the projection center but only in the direction of the circumference of the circle. Scale decreases radially with distance from the center.

Distortion

Only the center is free of distortion, which increases rapidly away from the center. Extreme distortion near the edge of the hemisphere.

Special features

Perspective projection of the globe onto a tangent plane from an infinite distance (that is, orthogonally); thus, the map has the look of a globe. All great or small circles (including all meridians and parallels) are shown as elliptical arcs or straight lines.

Usage

Pictorial views of the Earth, resembling those seen from space

Origin

Apparently developed by Egyptians and Greeks by the 2d century B.C.

Similar projections

General Vertical Perspective, when projected from more than a few thousand kilometers above the Earth

Description adapted from J.P. Snyder and P.M. Voxland, An Album of Map Projections, U.S. Geological Survey Professional Paper 1453. United States Government Printing Office: 1989.