The earth's surface is arbitrarily divided into a system of reference coordinates called latitude and longitude. This coordinate system consists of imaginary lines on the earth’s surface called parallels and meridians (fig. 2.1). Both of these are best described by assuming the earth to be represented by a globe with an axis of rotation passing through the North and South poles. Meridians are circles drawn on this globe that pass through the two poles. Meridians are labeled according to their positions, in degrees, from the zero meridian, which by international agreement passes through Greenwich near London, England. The zero meridian is commonly referred to as the Greenwich meridian. If meridional lines are drawn for each 15° in an easterly direction from Greenwich (toward Asia) and in a westerly direction from Greenwich (toward North America), a family of great circles will be created. Each one of the great circles is labeled according to the number of degrees it lies east or west of the Greenwich or zero meridian. The 180° west meridian and the 180° east meridian are one and the same great circle, and constitute the International Date Line.

     A great circle represents the intersection of a plane that connects two points on the surface of a sphere and passes through the center of the sphere, in this case, the earth. The intersection of the plane with the surface divides the earth into two equal halves—hemispheres—and the arc of the great circle is the shortest distance between two points on the spherical earth.

     Another great circle passing around the earth midway between the two poles is the equator. It divides the earth into the Northern and Southern Hemispheres. A family of lines drawn on the globe parallel to the equator constitute the second set of reference lines needed to locate a point on the earth accurately. These lines form circles that are called parallels of latitude, labeled according to their distances in degrees north or south of the equator. The parallel that lies halfway between the equator and the North Pole is latitude 45° North, and the North Pole itself lies at latitude 90° North.

     This system of meridians and parallels thus provides a means of accurately designating the location of any point on the globe. Santa Monica, California, for example, lies at about longitude 118° 29’ West and latitude 34° 01’ North. For increased accuracy in locating a point, degrees may be subdivided into 60 subdivisions known as minutes, indicated by the notation’. Minutes may be subdivided into 60 subdivisions known as seconds, indicated by the notation”. Thus, a position description might read 64° 32’ 32” East, 44° 16’ 18” South.

     Meridional lines converge toward the North or South Pole from the equator, and the length of a degree of longitude varies from 69.17 statute miles at the equator to zero at the poles. Latitudinal lines, on the other hand, are always parallel to each other. However, because the earth is not a perfect sphere but is slightly bulged at the equator, a degree of latitude varies from 68.7 statute miles at the equator to 69.4 statute miles at the poles. Thus, the area bounded by parallels and meridians is not a true rectangle. U.S.G.S. quadrangle maps are also bounded by meridians and parallels, but on the scale at which they are drawn, the convergence of the meridional lines is so slight that the maps appear to be true rectangles. The U.S.G.S. standard quadrangle maps embrace an area bounded by 7½ minutes of longitude and 7½ minutes of latitude. These quadrangle maps are called 7½-minute quadrangles. Other maps published by the U.S.G.S. are 15-minute quadrangles, and a few of the older ones are 30-minute quadrangles.

     Meridians always lie in a true north-south direction, and parallels always lie in a true east-west direction. Magnetic north, however, is the direction toward which the north-seeking end of a magnetic compass needle will point. Because the magnetic poles are not coincident with the north and south ends of the earth’s rotational axis, magnetic north is different from true north except on the meridian that passes through the magnetic North Pole. The angle between true north and magnetic north is called the magnetic declination, and is normally shown on the lower margin of most U.S.G.S. maps for the benefit of those who use a compass in the field to plot geological or other data on a base map (e.g., a U.S.G.S. standard quadrangle map).