Around the globe and into your home: New innovations in GPS
By Jon Aschenbach

GPS, which stands for Global Positioning System, allows users to track their position anywhere on the earth. It has been available for civilian use since the 1980s. GPS satellites, 28 including spares, orbit at 11,000 nautical miles above the Earth and are continuously monitored by ground stations located worldwide. The satellites transmit signals that can be received by anyone with a GPS receiver. Using the receiver, you can determine your location with great precision. GPS is one of history's most exciting and revolutionary developments, and new uses for it are constantly being discovered.

Historically, GPS units have been used by foresters to calculate acreage of ownership and to map property lines, corners, roads, streams and other geographic features. They can collect any variety of point, line or polygon data. If coordinates are known, GPS allows users to find points such as property corners or plot locations in the field.

Several changes have occurred within the GPS system in the past two years. Generally, all of them have been favorable to family forest owners wanting to use GPS on their tree farms. The biggest change came about May 2, 2000, when President Clinton told the Department of Defense to stop using selective availability. This effectively stopped the purposeful addition of error in the signals received by all civilian GPS units. The reduction of error, while quite significant, still did not allow civilian users the accuracy needed for some mapping operations.

Between January 2000 and February 2001, we had a net gain of four satellites within the GPS system. Additional satellites help in multiple ways. With more satellites circling the earth, it is easier to acquire satellite signals, especially under tree canopy. Also, the newer satellites are better quality and have better atomic clocks and other electronics.

The most recent innovation concerning GPS-the addition of Wide Area Augmentation System, or WAAS- occurred in the past few months. WAAS originated from the combined efforts of the Federal Aviation Administration (FAA) and the Department of Transportation and was developed for precision flight approaches for commercial aircraft. The system corrects for GPS signal errors and effectively allows WAAS-enabled GPS units to achieve three-meter accuracy.

While not currently being used by the FAA, WAAS is available for civilian use. Two geostationary satellites positioned over the equator transmit the signal. Available only in North America, it is easily obstructed by trees and mountains. The signal is most available on open land and marine applications. Even with these constraints, most small woodland owners should find the WAAS signal available for at least a portion of their tree farms. Garmin is the primary manufacturer of GPS units with WAAS capability.

The major obstacle for using consumer grade GPS units is the effect of tree canopy on accuracy. The USDA Forest Service completed a study of four models of consumer-grade GPS units in two separate tests. The first test was conducted in an open area near Estacada, Ore. Accuracy was very good with the error ranging between 6 and 14 meters for all units. This is considered adequate accuracy under the national map accuracy standards (NMAS 1974). Testing at the Clackamas test site east of Estacada, which has a dense canopy of mature Douglas-fir trees, proved to be a different situation. The error for the best of the four units was 31 meters. At the worst, the error under tree canopy went as high as 68 meters. The full study can be viewed at http://www.fs.fed.us/database/gps/gpsusfs.htm. The best GPS model tested at the Clackamas Test Site used point-averaging mode to collect 60 readings per test point, which significantly improved its accuracy.

In summary, many innovations have taken place regarding consumer-grade GPS units over the past two years. The availability of inexpensive GPS units, having more satellites in orbit and the Wide Area Augmentation System will allow users to achieve three-meter accuracy in some situations. GPS users need to be aware of the effects of tree canopy to accuracy and should carefully look at how they use GPS data.

Jon Aschenbach is vice president of Atterbury Consultants, Inc. He teaches a GPS in Forestry seminar and can be reached at jaschenbach@atterbury.com.