Mapping Then and Now

Determining Location in 2006

Determining one's latitude and longitude is incredibly easy now compared with how hard Lewis and Clark had to work. Now all you have to do is take a GPS receiver outside, away from tall buildings or trees, and wait a minute or two. The receiver makes all the observations and calculations for you. But how does the GPS receiver do it?

A GPS receiver uses the same principle to calculate location as Lewis and Clark used to calculate longitude—namely, measuring time to determine distance. A GPS receiver measures the time it takes for a signal to travel from a satellite to the receiver. This gives you the distance between satellite and receiver. To determine your true location, you need signals from three or more satellites. The signals are triangulated from the locations of the satellites to give accurate coordinates.