LATITUDE & LONGITUDE COORDINATES

A key geographical question throughout the human experience has been, "Where am I?" In classical Greece and China, attempts were made to create logical grid systems of the world to answer this question. The ancient Greek geographer Ptolemy created a grid system and listed the coordinates for places throughout the known world in his book Geography. But it wasn't until the middle ages that the latitude and longitude system was developed and implemented. This system is written in degrees, using the symbol

Latitude

When looking at a map, latitude lines run horizontally. Latitude lines are also known as parallels since they are parallel and are an equal distant from each other. Each degree of latitude is approximately 69 miles (111 km) apart; there is a variation due to the fact that the earth is not a perfect sphere but an oblate ellipsoid (slightly egg-shaped). To remember latitude, imagine them as the horizontal rungs of a ladder ("ladder-tude"). Degrees latitude are numbered from 0° to 90° north and south. Zero degrees is the equator, the imaginary line which divides our planet into the northern and southern hemispheres. 90° north is the North Pole and 90° south is the South Pole.

Longitude

The vertical longitude lines are also known as meridians. They converge at the poles and are widest at the equator (about 69 miles or 111 km apart). Zero degrees longitude is located at Greenwich, England (0°). The degrees continue 180° east and 180° west where they meet and form the International Date Line in the Pacific Ocean. Greenwich, the site of the British Royal Greenwich Observatory, was established as the site of the prime meridian by an international conference in 1884.

GPS COORDINATES

Most of us never need to use numerical GPS coordinates to take advantage of the many location-based services that are available to us. We simply input an address, or click through from an Internet search, or automatically geotag photos, and our electronic devices take care of the rest. But dedicated outdoors-people, geocachers, pilots, sailors, and more do often need to use and understand numerical GPS coordinates. And some of us technophiles are interested in the workings of the GPS systems just out of curiosity. Here then is your guide to GPS coordinates.

The global GPS system actually doesn't have a coordinates system of its own. It uses "geographic coordinates" systems that already existed before GPS, including: Latitude and Longitude

GPS coordinates are most commonly expressed as latitude and longitude. This system divides the earth into latitude lines, which indicate how far north or south of the equator a location is, and longitude lines, which indicate how far east or west of the prime meridian a location is.

The example of reading the Latitute and Longitude is shown above:

1. In this system, the equator is at 0 degrees latitude, with the poles being at 90 degrees north and south. The prime meridian is at 0 degrees longitude, extending east and west. 
2.  Under this system, an exact location on the earth's surface can be expressed as a set of numbers. The latitude and longitude of the Empire State Building, for example is expressed as N40° 44.9064', W073° 59.0735'. The location may also be expressed in a numbers-only format, per: 40.748440, -73.984559. 
3. With the first number indicating latitude, and the second number representing longitude (the minus sign indicates "west"). Being numeric-only, the second means of notation is the most commonly used for inputting positions into GPS devices.

BASIC CONCEPT OF GPS

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include:

• the time the message was transmitted and,
• satellite position at time of message transmission.

The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light. Each of these distances and satellites' locations defines a sphere. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. This location is then displayed, perhaps with a moving map display or latitude and longitude.

Basic GPS measurements yield only a position, and neither speed nor direction. However, most GPS units can automatically derive velocity and direction of movement from two or more position measurements. The disadvantage of this principle is that changes in speed or direction can only be computed with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the random error of position measurement. GPS units can use measurements of the doppler shift of the signals received to compute velocity accurately.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.