Basic GPS Theory
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Basic GPS Theory
Back to Main Index of Markwell's GPS Page
The surface of the globe is divided into a spherical grid for the convenience of finding certain points. The grid consists of imaginary lines called latitude and longitude. Latitude is a series of concentric circles paralleling the Equator and extending to both poles. Longitude is a series of meridians, or longitudinal lines drawn between the poles at regular intervals that pass perpendicularly through the Equator. Where a particular latitude crosses a particular longitude, a pair of numbers, or coordinates, can be assigned. Every point on the earth has a set of coordinates that indicate its position relative to every other point.
Latitude is measured from zero at the Equator to 90 degrees north and south at the poles. Longitude is measured from zero to 180 degrees west and east. The reference lines for counting are the Equator, for latitude, and a line drawn through Greenwich in England, the prime meridian, for longitude. These are the zero lines. A degree of latitude is equivalent to about 112 kilometers (about 70 miles). Because longitudinal lines converge toward the poles, degrees of longitude vary according to the position on the earth. At the equator, one degree of longitude is the same length as one degree of latitude, and at the north and south poles, the distance between degrees of longitude is zero.
Degrees are divided into 60 minutes, and each minute is divided into 60 seconds. For example, the Eiffel Tower in Paris has the following coordinates: latitude N 48° 51' 32" and E 002° 17' 35". Sometimes, coordinates are expressed in decimal minutes instead of minutes and seconds, so the coordinates of the Eiffel Tower can also be written as N 48° 51.5333' latitude and E 002° 17.5833' longitude. Most official maps indicate latitude and longitude, so viewers know exactly what part of the earth the map represents.
Some maps have other special-purpose coordinate systems, such as the State Plane Coordinate System used on maps in the United States or the Universal Trans-Mercator (UTM) system used on many military maps.
Global Positioning Systems (GPS)
Global Positioning Systems (GPS) are devices that give a person an Earth coordinate position based on the reception of special satellite transmissions. Three or more satellites may provide the energy beams on which a handheld GPS, through triangulation, bases its calculation of exactly where on the surface of the Earth the GPS is located. 
Modern GPS units-some as small as cellular telephones-have a simple LCD screen similar to that on an electronic calculator, and a simple keyboard for entering codes. With a few keystrokes, these handheld instruments can be programmed to record and display the exact position of the unit, the user’s ground speed and direction of travel, and projected arrival times at certain points along a chosen route.
The current system used for GPS is administered by the United States, although the European Space Agency has plans for a similar system. Because of military security concerns, the U.S. government regulates the resolution of satellite transmissions available to the public for GPS use. Although GPS units are theoretically accurate to within several centimeters, their practical range today is about 3 to 30 meters (10 to 100 feet), not accurate enough for collecting precise map data. Another shortcoming of GPS for collecting map data is that trees and steep terrain block the reception of satellite transmissions. The most common and rapidly growing use of GPS is in aircraft and ship navigation, and some automobile manufacturers are beginning to install GPS in their cars. In Oslo, Norway, a taxi company is installing GPS in all its taxes to help track indiviual cars and provide security oversight for its drivers.
Despite its current limitations, cartographic use of GPS is on the increase. Among other purposes, GPS is being used to verify the location of boundary lines. A private, handheld GPS unit was recently used to substantiate that the legally defined US-Canadian border is actually 800 meters (2,625 feet) North of its traditional map location between Washington and British Columbia. GPS and related technology will play increasingly important roles in cartographic data collection as the accuracy of satellite-based positioning technology increases.