How are Latitude and Longitude Determined?

To put the answer to our question in the most basic terms, points of latitude and longitude tell us where we are on the planet Earth. The equator is the starting point for determining latitude. The Prime Meridian is the starting point for determining longitude.

The position of these lines is determined by the distance from the equator or the Prime Meridian, with the position stated in degrees. Latitude and longitude are indicated by lines that appear on maps of the Earth. These lines form complete circles around the Earth, though the lines of longitude converge at the North Pole and South Pole.

Equator, Prime Meridian

To give an example let’s use the equator, an imaginary line that runs around the middle of the Earth, sort of like a belt at the mid-section. The latitude of this line is 0 degrees. The line for one degree north latitude would circle the Earth 60 nautical miles north of the equator. The line for one degree south latitude would circle the Earth 60 nautical miles south of the equator. Continuing in either direction we would eventually be at one of the poles, which are located at 90 degrees north and 90 degrees south. Lines of latitude are always parallel.

An example of longitude would start with the Prime Meridian that runs north and south through Greenwich, England. The location of this imaginary line representing 0 degrees longitude was determined after discussion and dispute about the proper starting point. Longitude, or distance from the Prime Meridian, is stated in degrees as well. But the measurement is associated with the time difference between the Prime Meridian and the point you are trying to determine – east or west of the Prime Meridian.

A person could tell someone else exactly where he or she is standing by stating latitude and longitude in degrees, then be more precise by breaking this down into minutes and seconds – 60 minutes in each degree, 60 seconds in each minute. A major city in the United States might be located at 38 degrees, 53 minutes, 23 seconds north of the equator – 38° 53’ 23”N – and 77 degrees, 0 minutes, 27 seconds west of the Prime Meridian – 77° 00’ 27” W. This is, in fact, the location of Washington D.C.

Find Me on the Map

One of the interesting things about latitude and longitude is this: two people can be at 45 degrees north latitude and be on opposite sides of the Earth. This means they are the same distance north of the equator. But without a measurement of longitude – east or west of the Prime Meridian – we would have difficulty finding them. If this person is also known to be at 45 degrees west we can find them by following a line from the North Pole down to the equator at 45 degrees west of the Prime Meridian. The point at which this line crosses the line of latitude at 45 degrees north is their location.