Map Projection Properties

Map Projection Properties Definition: Alterations of area, shape, distance, and direction on map projections. Why? All maps contain error because of the 3D -> 2D transformation process. How? Rendering a spherical surface on a plane causes tearing, shearing, or compression of the surface.

Four Map Projection Properties Area Shape Distance Direction

Four Projection Properties Major Properties Minor Properties Area Shape Distance Direction Cannot exist everywhere on map Mutually Exclusive

Map Distortion Distortions are unavoidable when making flat maps of a globe Distortion may take different forms in different parts of the map Few points where distortions are zero Distortion is usually less near the points or lines of intersection where the map surface intersects the globe

Map Distortion A map can show one or more – but never all – of the following at the same time: True directions True distances True areas True shapes

Equal Area Map Projection A.K.A. Equivalent Map Projection Goal: Preserve area relationships of all parts of globe Identifying Marks: Meridians and parallels are not at right angles. Distance distortion is often present. Shape is often skewed.

Equal Area Map Projection Useful for… General quantitative thematic maps. When it is desirable to retain area properties.

Cylindrical Equal-Area

Area Hammer-Aitoff

Conformal Map Projection Shape Conformal Map Projection A.K.A. Orthomorphic Map Projection Goal: Preserve angles around points and shape of small areas Same scale in all directions from/to a point. Identifying Marks: Meridians intersect parallels at right angles. Areas distorted significantly at small scales. Shapes of large regions may be severely distorted.

Conformal Map Projection Shape Conformal Map Projection Useful for… Large-scale mapping. Phenomena with circular radial patterns (e.x. radio broadcasts, average wind directions)

Shape Mercator Projection

Equidistant Map Projection Distance Equidistant Map Projection Goal: Preserve great circle distances. Distance can be held true from one to all other points, or from a few points to others, but not from all points to all other points. Scale is uniform along lines of true distance. Identifying Marks: Neither conformal or equal area, and look less distorted.

Equidistant Map Projection Distance Equidistant Map Projection Useful For… General Purpose Maps Atlas Maps

Equidistant Cylindrical Distance Equidistant Cylindrical

Azimuthal Map Projection Direction Azimuthal Map Projection A.K.A. True Direction Goal: Preserve true direction from one point to all other points. Direction not true between non-central points. Useful for… Preserving direction from one point.

Azimuthal Equidistant Map Projection Direction

Combination of Projections on a Single Projection Equal Area Conformal Equidistant Azimuthal -- No Yes Yes denotes they can be combined No denotes they cannot be combined

Minimum Error Map Projection A.K.A. Compromise Map Projection Goal: Simultaneously minimize all four map projection properties Useful for… General geographic cartography

Robinson Map Projection Minimum Error Robinson Map Projection

Organization of Cartographers for Social Equality – West Wing Clip http://www.youtube.com/watch?v=n8zBC2dvERM