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May 19 2011

The World is Flat

Yes, the world is still seen as flat in one very common everyday application that nearly every single American uses: a map. If you’re using a GPS system for your car, a CADD or GIS program at work, Mapquest, Bing, or Google Maps, then you’re viewing a flat earth. We are moving closer and closer to a 3D realm but we’re not quite there yet. A traditional 2D map in all its simplicity and ease of use, be it digital or print, is still a powerful tool. That means, if you’re looking at a 2D map, you’re viewing a map based on a projection.

A projection is essentially a method of taking a 3D surface (the Earth) and viewing it on a 2D flat plane for the purpose of mapmaking (cartography). Because of the nature of going from 3D to 2D, projections always distort at least one aspect between shape, area, distance, or direction.

Conic

Conic projections are best demonstrated by placing a cone over a round ball. Conic projections (Albers Equal Area, Lambert Conformal Conic) are good for East-West land areas. The secant conic projection (see figure below) is used most often.

image

Cylindrical

Cylindrical  projections (such as Transverse Mercator) are good for North-South land areas. The most common is transverse mercator. Universal Transverse Mercator (UTM) is just a series of these cylinders creating strips of UTM zones.

image

Azimuthal

The Azimuthal is like looking from space. Azimuthal (Lambert Azimuthal Equal Area) is good for global views, as opposed to continental views.

image

Other projections include Texas’ state wide projection, Space Oblique Mercator, and unprojected latitude/longitude systems.

Preserving Properties

Most projections can preserve just 1 property well and some can preserve 2. Albers Equal Area preserves the correct earth surface area. In the Lambert Conformal Conic projection, the local angles are shown correctly (preserves shape). In the Lambert Azimuthal Equal Area, all directions are shown correctly relative to the center.

The zones for the State Plane coordinate system (probably the most common system used by engineers and surveyors) were developed (in the 1930s!) to keep distortion of these properties to only 1 in 10,000. To do this, each state uses different projections that work best for the shape of the zones within that state.

Sources:

Map Projection Overview (by Peter Dana)

Map Projection (from the GIS Lounge)

Other Resources:

Map Projections: A Working Manual (by John Snyder)

The Map Room

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