Cartography

Distance and direction are used to describe the location of an object in space. In conversation, terms like right and left, up and down, or here and there are used to indicate direction and distance. These terms are useful only if you know the location of the speaker; in other words, they are relative. Cartographers, however, need objective terms for describing location, because maps are intended for use by many individuals in many different situations. The system of latitude and longitude, a geographical coordinate system developed by the Greeks, is used by cartographers for describing location.

Earth is an oblate, rotating around an axis tilted approximately 23.5 degrees. The two points where the axis intersects Earth's surface are called the poles. The equator is an imaginary circle drawn around the center of Earth, equidistant from both poles. A plane that sliced through the earth at the equator would intersect the axis of Earth at a right angle. Lines drawn around the earth to the north and south of the equator and at right angles to Earth's axis are called parallels. Any point on Earth's surface is located on a parallel.

An arc is established when an angle is drawn from the equator to the axis and then north or south to a parallel. Latitude is the measurement of this arc in degrees. There are 90 degrees from the equator to each pole, and sixty minutes in each degree. Latitude is used to determine distance and direction north and south of the equator.

Meridians are lines running from the north pole to the south pole, dividing Earth's surface into sections, like those of an orange. Meridians intersect parallels at right angles, creating a grid. Just as the equator acts as the line from which to measure north or south, a particular meridian, called the prime meridian, acts as the line from which to measure east or west. There is no meridian that has a natural basis for being considered the prime meridian. The prime meridian is established by international agreement and currently runs through the Royal Observatory in Greenwich, England. Longitude is the measurement in degrees of the arc created by an angle drawn from the prime meridian Earth's axis and then east or west to a meridian. There are 180 degrees west of the prime meridian and 180 degrees east of it. The international dateline lies approximately where the 180th meridian passes through the Pacific Ocean.

Using the geographical coordinate system of latitude and longitude, any point on the earth's surface can be located with precision. For example, Buenos Aires, the capital of Argentina, is located 34 degrees 35 minutes south of the equator and 58 degrees 22 minutes west of the prime meridian. Anchorage, the capital Alaska, is located 61 degrees 10 minutes north of the equator and 149 degrees 45 minutes west of the prime meridian.