|
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
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
|
