How to Calculate Degrees, Minutes and Seconds

Mercator's work still leads sailors around the world.

When Flemish cartographer Gerardus Mercator laid out his map of the world in 1569, he introduced the world to maps with parallel lines of longitude and an effective grid system, using coastwise landmarks in lieu of lines of latitude. Today the Universal Transverse Mercator coordinate system uses degrees, minutes and seconds to pinpoint the location of any point on the planet, whether you navigate by a satellite-based system or by the stars.

Instructions

1. Divide a circle into 360 equal slices and, in the width of each slice, you have the equivalent of 1 degree. On a nautical chart, each degree is 60 nautical miles wide. Since the agreed-upon prime meridian of the Earth is located at the Royal Observatory in Greenwich, England, this means that if you take an imaginary voyage to New Orleans, located at 90 degrees west longitude, or 90 degrees west of the prime meridian, you are about 5,400 miles west of Greenwich.

2. Slice the degree into smaller parts---minutes---by dividing by 60 again. Since there are 60 minutes of arc in each degree and each degree is 60 nautical miles, each minute equals 1 nautical mile. Since you're imaginary journey to New Orleans will require you to travel up the Mississippi River, you might find yourself at 90 degrees 03 minutes west longitude as you approach Algiers Point. Three additional minutes means 3 nautical miles are added to the total distance from Greenwich: You're now 5,403 miles west of the prime meridian.

3. Chop each minute into 60 seconds, just as you would if you were reckoning time, and each second equals 1/60th of a nautical mile, or 101.3 feet. If you make landfall on the western end of the Governor Nicholls Street Wharf in New Orleans, you're full location is 90 degrees, 03 minutes, 28 seconds west of the prime meridian. Multiplying 101.3 feet by 28 and, by adding the result to the previous distance from Greenwich, you discover you are 5,403.45 miles---5,403 miles and 2,735.1 feet---west of the prime meridian of the Earth.

References

• Library of Congress: Atlases
• "The American Practical Navigator"; National Geospatial-Intelligence Agency; 2010