GPS and GIS
In Chapter 1 you learned a bit about the NAVSTAR Global Positioning System. Let’s explore some of the main reasons for making GPS a primary source of data for GIS.
- Availability—in 1995, the U.S. Department of Defense (DoD) declared NAVSTAR to have “final operational capability.” Deciphered, this means that the DoD has committed itself to maintain NAVSTAR’s capability for civilians at a level specified by law, for the foreseeable future, at least in times of peace. Therefore, those with GPS receivers may locate their positions anywhere on the Earth.
- Accuracy—GPS allows the user to know position information easily and with remarkable accuracy. A receiver operating by itself can let you locate yourself within 2 to 4 meters of your true position. (And using two GPS receivers, when one is positioned over a known (accurately surveyed) point, a user can get accuracies of 1 to 3 meters.)1 At least two factors promote such accuracy:
First, with GPS, we work with primary data sources. Consider two alternatives to using GPS to generate spatial data: the physical digitizer and heads-up digitizing on a computer screen. A digitizer is essentially an electronic drawing table, wherewith an operator traces lines or enters points by “pointing”—with “crosshairs” embedded in a clear plastic “puck”—at features on a map. Or, in heads-up digitizing, the operator manipulates the crosshairs on a computer screen with a mouse.
One could consider that the ground-based portion of a GPS system ...