Reading GPS Coordinates – Latitude and Longitude
The latitude and longitude based coordinate system is an angular coordinate system (which makes sense because the earth is a sphere). Thus positions (angular positions) are always calculated relative to two global references.
The reference for latitudes is the equator. Places located North or South of the equator are given a relative angular position based on the following.
1 degree = 69 miles; a degree is further sub-divided into 60 minutes.
1 minute = 6072 feet; a minute in turn is sub-divided into 60 seconds.
1 second = 101.2 feet.
Similarly, the reference for longitudes is the Prime Meridian that passes through Greenwich in London, England. Places East or West are given a relative angular position based on the following.
1 degree = 42 miles; a degree is further sub-divided into 60 minutes.
1 minute = 3696 feet; a minute in turn is sub-divided into 60 seconds.
1 second = 61.6 feet.
When this system is used there can be three ways of representing the given latitude, longitude pair. Sometimes the system as described above is used and is the degrees/minutes/seconds system. Alternatively, as with most GPS units a degrees/minutes system is used. In this case we would have the minutes divided according to a decimal system. This system is called the degrees/decimal minutes system. Obviously in this case a minute still represents the values noted above, but we no longer use seconds but rather fractions of the minute to represent fractions of distance smaller than the minute (i.e. smaller than 6072 feet and 3696 feet, respectively). The third system uses only degrees and uses decimal notation to represent all intermediate distances. Thus a degree of longitude is 69 miles = 364320 feet, and 0.1 degree is 36432 feet, and so on. Take a look at the table in the article "Understanding GPS coordinates" to understand this concept better. Hopefully, more and more commercial GPS units will use this degree decimal system in the future. This is also the system used on Google Maps.
For each of the three systems there can be a separate convention for representing North/South and East/West. Either a letter is used to represent whether the given value is N/S or E/W, or if not present, then positive values indicate North and East, while negative values indicate South and West. Again, Google Maps uses a degree-decimal system with the +/- indicating N/S and E/W. Addresses in North America will therefore have a positive latitude (North) and a negative longitude (West). Use this tool at Jeep Reviews to convert between the three systems.