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TECH GUIDE

Surveying: Global Positioning Systems (GPS)

Global Positioning Systems (GPS) are rapidly becoming an essential survey tool for workers in the environmental field. Handheld units made by Garmin and Magellan have come down in cost (most range from $100-$400 US), and the recent end to military scrambling of satellite signals has dramatically improved the precision and accuracy of a unit's navigation capabilities. Most handheld GPS units can locate a position to within 60-225 feet (garmin website). The accuracy depends upon the number of satellites the unit can 'see' and their locations relative to each other and to the unit. More sophisticated and expensive (around several thousand dollars) units (such as the Trimble units), can pinpoint and relocate an area the size of a penny, because they utilize more than one frequency of signal.

GPS units receive satellite signals, which contain a satellite's position information, from a network of satellites orbiting the earth. The GPS unit then calculates its own position by triangulating among the different satellites it 'sees', using the time it takes the signal to beam down from three or more satellites (in order to calculate distance) and using the position of the satellite. Because of this triangulatory mode of operation, suboptimal positioning of the satellites can introduce error into the calculated unit position. That is, the unit can better calculate its position if the satellites are spread out in the sky above the unit, rather than clumped together in a single area. With triangulation problems, the user might not know the unit location was miscalculated, so it is important to double check position information with a compass and map, or some other supplemental method. Additionally, the GPS needs the signal from at least 3 satellites to calculate the unit's position. Satellite coverage can be difficult in areas where terrain or other obstacles block the satellite signal, such as in vehicles, canyons, amidst tall buildings, or heavy trees.

Despite the above disadvantages, GPS units are fast becoming an essential tool for mapping, locating sample points, and orienteering in general. Most units have the ability to store location coordinates in the memory of the unit, and to point the user in the direction of a certain location, or 'waypoint'. GPS units also have the capability to track 'routes', where the user has been over the course of a trip, and to calculate elevations and distance traveled. After the field trip, the stored points can be downloaded directly to a laptop or desktop computer and can be imported into spreadsheets or other applications such as geographic information systems, for visualization and analysis.

Additional resources that gives a thorough explanation of how GPS works for the low-tech audience.

Garmin: What is GPS?

Trimble GPS Tutorial

LelandWest GPS Overview

 

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