How Do Map Coordinates Work?

Map coordinates are usually based on the system of latitude and longitude, which was first recorded by the astronomer Ptolemy in 150 CE. Another coordinate system that is widely used is the Universal Transverse Mercator system, or UTM. Part of reading a map or even a GPS device is knowing which coordinate system is being used.

  1. Latitude

    • Latitude lines are parallel lines that measure distance from the equator, an imaginary line running around the exact middle of the earth. The equator is 0 degrees latitude, while the South Pole is 90 degrees south latitude and the North Pole is 90 degrees north latitude. Latitude coordinates are divided into degrees and minutes. One degree equals 69 miles or 111 km on land, or 60 nautical miles. One minute is 1.15 miles or 1.85 km on land, or 1 nautical mile. So, a point on earth at 45 degrees 3 minutes north latitude would be 3,108.45 miles north of the equator.

    Longitude

    • Longitude lines, also known as meridians, are lines that measure the distance east or west from the prime meridian, which is the line that passes through Greenwich, England. The prime meridian is 0 degrees, and all of the other lines are a certain number of degrees east or west. Each longitude line runs from the North Pole to the South Pole. Unlike the latitude lines, which are an exactly equal distance apart from each other at all points, the longitude lines come together at the Poles. This means that the degrees and minutes are not a constant measurement as they are with latitude. The closer you are to either pole, the smaller the distance represented by a degree of longitude.

    The UTM Coordinate System

    • The Universal Transverse Mercator, or UTM system, is another commonly used coordinate system for maps. In the UTM system, every grid on the map is eight degrees in height and six in width. Each grid is labeled with an alphanumeric combination, in which the letter represents the north-south coordinate and the number indicates the east-west coordinate. So, if the grid is marked as 11S, the 11 refers to which sector is indicated in east-west terms, and the S in north-south terms. The locations on the map grid are measured in meters from the equator along the north-south axis, and in meters from a point to the west of the map. UTM maps are also marked with 1,000-meter points to further specify exact location.

    Maps and GPS Devices

    • Most GPS or Global Positioning System devices come with a standard setting for latitude and longitude. If you're navigating with the aid of a map, the map may also use latitude and longitude or it may use the Universal Transverse Mercator system or even some other less common coordinate system. GPS devices can usually be reset to UTM if needed, so it's important to make sure that your map and your GPS device are using the same coordinate system, especially if you're going off-road.