## Latitude, Longitude and the Prime Meridian

The latitude-longitude system is the system most commonly used to locate features on topographic and other maps. Lines of latitude form the top and bottom margins of the map and lines of longitude from the left and right margins. Latitude is the angular distance measured with respect to a central point along a plane passed through the earth at the position of the earth's largest circumference. This plane is designated as a line of zero degrees ( 0° ) and is referred to as the equator of the earth (see Figure 7.1). Latitude then varies between 0° and 90° north and south of the equator.

Longitude is the angular distance measured east or west from a plane that passes through the north and south poles at the position of Greenwich, England. This line of longitude is referred to as the prime meridian and is assigned a value of 0° longitude. The position of the prime meridian was chosen arbitrarily. Because the earth is essentially a sphere, lines of longitude range from 0° to 180° east and west of the prime meridian with 0° east and west longitude beginning at the prime meridian and 180° east and west longitude being at a point on the other side of the earth directly opposite the prime meridian. The line defining the 180° east and west longitude is referred to as the International Date Line. International time is measured with respect to the prime meridian, while the International Date Line serves to mark the change in days. The plane defined by the prime meridian and the International Date Line serves to divide the earth into eastern and western hemispheres.

Latitude and longitude provide a very accurate method for locating points on the surface of the earth or for defining a specific area such as the area covered by a topographic map. This accuracy is the result of the way in which latitude and longitude are measured where 1° of latitude or longitude is divided into 60 equal interval or 60 minutes ( 60' ), and each minute is subdivided into 60 equal intervals or 60 seconds ( 60" ). The apostrophe symbol ( ' ) is used to denote the minutes of a degree ( ° ), and the quotation symbol ( " ) denotes the seconds of one minute. Thus 1° = 60' = 3600".

Topographic maps developed by the United States Geological Survey (U.S.G.S.) are commonly available in three sizes: 30', 15' and 7½' (7' 30") quadrangles. A 30' quadrangle is 30' (½ of a degree of latitude or longitude) on a side and represents one-quarter of the area defined by 1° of latitude or longitude [8063 to 11,547 sq. km (3,150 to 4,510 sq. miles)](see Figure 7.2). A 15' quadrangle is one-quarter of the area of a 30' quadrangle and a 7½' quadrangle is one-quarter of a 15' quadrangle (Figure 7.2). The range in area that quadrangles represent is controlled by the convergence of line of longitude as they approach the north and south poles.

Because of the difference in areas covered by these topographic maps, a 7½' quadrangle would be chosen if great detail was desired for a small area, a 15' quadrangle for medium detail of a larger area, and a 30' quadrangle for general detail of a rather large area. Topographic maps which cover even larger areas are also available, but because they cover one or more degrees of latitude and longitude the convention is to refer to these maps by thier scale such as "one to two hundred and fifty thousand (1:250,000)", "one to five hundred thousand (1:500,000)", and "one to one million (1:1,000,000)." The largest of these, 1:1,000,000, covers an area of 188,760 to 263,063 sq. km (73,734 to 102,758 sq. mi).

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