Geographic Information Systems

Box 3: Global Positioning Systems.


Although global positioning systems (GPSs) have been in existence for 2 decades, only recently has this technology become an affordable and effective tool for wildlife managers and researchers. The Navigation Satellite Timing and Ranging (NAVSTAR) GPS developed and operated by the U.S. Department of Defense (DOD) consists of a network of 25 satellites in orbit about the earth (as of October 1993, all 25 satellites were operational), as well as ground operations support. After all the satellites are deployed, GPS will provide all-weather, worldwide, two- and three-dimensional (latitude, longitude, and elevation) positioning capabilities over a 24-hour period.

Wildlife biologists can obtain geographic coordinate position data with GPS receivers that can be hand-held or mounted in vehicles. A receiver manipulates signals emanating from the satellites passing overhead and provides positioning data with accuracies from a few centimeters to nearly 100 m. Currently, the level of accuracy depends on several variables, including the sophistication of the receiver's electronics, number of satellites available for a position fix at the desired time, obstructions to signal reception (e.g., trees), and degradation of the satellite signals by the DOD. Techniques are available to improve position accuracies, but these usually require additional receivers or a more costly individual unit, and they improve only certain problems. Selection of GPS receivers will depend on the desired accuracy levels for the intended applications.

The geographic coordinate data gathered by the GPS receiver can be viewed on the unit's display screen, stored for later analyses, or transmitted to another location for processing. Most receivers require about 2 minutes to obtain a positional fix and then are able to update the location of the receiver every second. Therefore, the GPS receiver maneuvered by a person or vehicle provides an effective method for obtaining data on point locations (e.g., nest site) and the geographic coordinate attributes of land features (e.g., wetland boundary).

The geographic coordinate data collected from the GPS receiver can be an effective means for establishing new layers, or augmenting and updating current data layers, in a GIS. Many receivers come with software that allows the user to directly input the coordinate data into a format usable by the GIS. This allows the data to be quickly verified and used.


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