Map Projections

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Only a globe can accurately portray locations, directions, and distances of the Earth’s surface. Although not actually a sphere, but a spheroid, the Earth only approximates a true sphere due to the force of rotation that expands the Earth at the equator and flattens it at the poles. This elliptical nature of the Earth is a concern to cartographers; for most practical purposes of navigation, the Earth can be considered a sphere. On the Earth, meridians are straight and meet at the poles; parallels are straight and parallel. Meridian spacing is widest at the equator and zero at the poles; parallels are equally spaced. Scale is true for every location. Because a globe is not possible for practical aeronautical charts, mathematicians and cartographers have devised numerous systems, known as projections, to describe features on the Earth in the form of a plane or flat surface. A map projection is a system used to portray the sphere of the Earth, or a part thereof, on a plane or flat surface.

Locations on the Earth are described by a system of latitude and longitude coordinates. By convention, latitude is named first, then longitude. The reference point of latitude is the equator, with latitude measured in degrees north and south of the equator. Longitude, as discussed in Chap. 1, is measured east and west of the prime meridian (Greenwich meridian).

Any point on the Earth can be described using the system of latitude and longitude in degrees, minutes, and seconds. With the sophistication of today’s navigational systems, aeronautical charts and publications express latitude and longitude in degrees, minutes, and hundredths of minutes. A degree is an arc that is 1/360 of a circle; therefore, a point with latitude 47°N longitude 122°W would be the intersection of the parallel 47° north of the equator and the meridian 122° west of the Greenwich meridian. Degrees can be further subdivided into minutes (′), which represent 1/60 of a degree, and seconds (″), which represent 1 60 of a minute. For example, the Seattle-Tacoma International Airport is located N47°26.28′ W122°18.67′ (47 degrees 26.28 minutes north and 122 degrees 18.67 minutes west).

Each degree of latitude equals 60 nautical miles (nm). Because meridians meet at the poles, a degree of longitude decreases in length with distance north or south of the equator; therefore, only for the special case of the equator does a degree of longitude equal 60 nautical miles.

The goal of the map projection is to accurately portray true areas, shapes, distances, and directions. This includes the condition that lines of latitude are parallel and meridians of longitude pass through the Earth’s poles and intersect all parallels at right angles.

Areas. Any area on the Earth’s surface should be represented by the same area at the scale of the map. These projections are termed equal-area or equivalent.

Distances. The distance between two points on the Earth should be correctly represented on the map. These projections are termed equidistant.

Directions. The direction, or azimuth, from one point to other points on the Earth should be correct on the map. These projections are termed azimuthal or zenithal.

Shapes. The shape of any feature should be correctly represented. The scale around any point must be uniform. These projections are termed conformal.

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