Latitude and longitude (lat/long)is used to express location on earth. These imaginary lines, which run both north and south, and east and west form a grid, much as streets and avenues might form a city-wide grid.
Your location on earth can be identified by the intersection of a specific latitude and longitude line. Such an intersection is called a "coordinate."
To my way of thinking, lat/long is important stuff to know for anyone who ventures off the beaten path outdoors. What if, for instance, you need to communicate your specific whereabouts in a distress situation? It's much better to be able to state your specific lat/long coordinate than something like, "Well, we turned off Highway 41 and went down some dirt road for a while, then we walked off into the swamp a ways." This goes double for big-water boaters making a mayday call.
If lat/long has been a mystery to you, the good news is that it's relatively easy to learn. This page and this accompanying video is meant to be a good introduction to the subject.
Understanding Longitude Lines
As you may have seen on the video, longitude lines run north and south, and converge at each pole. Another name for a longitude line is "meridian." You'll sometimes hear it called a "meridian of longitude."
Each longitude line is assigned a numerical value. The "prime meridian" is the longitude line that runs through Greenwich, England, and is assigned a value of 0 degrees. Going east from Greenwich, the lines are assigned values up to 180 degrees East, which is that line on the opposite side of the earth from Greenwich. Going west from Greenwich, the lines are assigned values up to 180 degrees West. Actually, the 180 degrees West line, and the 180 degrees East line are one and the same. This is the international date line.
Understanding Latitude Lines
Latitude lines run east and west, but they never converge. Instead, they run parallel to one another. In fact, they're also called "parallels of latitude." The main latitude line is the Equator, which runs around the earth's center. The Equator is assigned a value of 0 degrees. Going north from the Equator, latitude lines are assigned values up to 90 degrees North, which the the latitude of the North Pole. Going south from the Equator, latitude lines are assigned values up to 90 degrees South, the latitude at the South Pole.
Since latitude and longitude lines are imaginary, they're not limited to those lines with full-degree values. Actually, there can be an infinite number of these grid lines.
To increase precision, a degree is divided into 60 minutes, and a minute is divided into 60 seconds.
A degree of latitude is 60 nautical miles. If degrees are divide into 60 minutes, then obviously each minute of latitude is equal to one nautical mile. Since a nautical mile is 6,000 feet, each second of latitude equals 100 feet.
IMPORTANT NOTE: You only measure distance in degrees of latitude, because they are consistent since lat lines are parallel. Never measure distance in degrees of longitude, because these lines eventually converge at the poles, so there's no consistent distance between them.
Latitude and Longitude coordinates can be expressed in one of three ways:
1. Degrees, minutes, and seconds 2. Degrees, minutes, and decimals of minutes 3. Degrees, and decimals of degrees
Always state the latitude first, then the longitude. Since there are only 90 degrees of maximum latitude, latitude values are given in two digits. Because there are 180 degrees of maximum longitude, longitude values are expressed in three digit numbers, with a leading zero or leading zeroes when the longitude value is under 100.
You should also state the hemisphere, either east or west for longitude, and north or south for latitude. The hemisphere statement can come either first or last. It doesn't matter.
Many freshwater marine charts use "degrees, minutes, and seconds," while most marine charts use "degrees, minutes, and decimals of minutes."
Examples of Lat/long Coordinates
20 degrees, 32 minutes, 10 seconds North 080 degrees, 15 minutes, 27 seconds West
20 degrees, 32.17 minutes North 080 degrees, 15.45 minutes West