How to Read GPS Coordinates - Understanding Longitude and Latitude
Coordinates define a position on a grid system. They indicate distances of a point, horizontal(x) and vertical(y), from know references or axes. Typically thus the two values shown as coordinates specify the two distance measures from the reference axes. Same is true of the GPS coordinates read from GPS navigators/receivers. The two values read from the instrument specify the latitude and longitude of the point at which you have the GPS receiver positioned at that time. Continue on to the next section, for the more detailed discussion starting with what the latitude and longitude of a point on the ground mean.
Part of the confusion surrounding how to read GPS coordinates stems from the fact that coordinates are often written in a number of different ways. However, the important thing to remember is that they are all based on the same system and the same references. So understanding the meaning of GPS coordinates will allow you to not only understand how to read the coordinates but also how to interpret the different systems and move back and forth between them. This is true in particular, of all latitude, longitude systems, where what really changes is the way in which the coordinate information is represented.
There are however a few other coordinate systems in use which can make the task a little bit more difficult such as the UTM system. They are not discussed here to confuse the issues anymore. What is discussed is the different ways the latitude/ longitude details are presented. A number of web-based utilities help make the conversion from one system to another if you need one. Earth Point is an example of such a tool. Also, take a look at Boulter.com. In fact, even for the conversions between different latitude, longitude representations you could use a web-based utility, such as the converter from the FCC.
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 latitude 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.