METHODS

To determine some of the capabilities of the GPS III Plus and PC mapping products, a unit was purchased along with various cables, external antenna, and software products, equipment and services needed to acquire and use real-time DGPS.

The author was first introduced to the use of Garmin GPS units and PC mapping software in 1998 while on a fire detail in Florida. As a field observer and infrared operator, information, such as fire perimeter, hotspots, drop points, heliports, tower hazards, and other field data were gathered using GPS II Plus. Information was obtained by use of passenger vehicle, all-terrain vehicle (ATV), helicopters, and on foot. Waypoints and tracklogs were downloaded into DeLorme Street Atlas USA® 5.0 and used to produce incident maps.

In the spring of 1999, the author used GPS II Plus in Florida and Virginia. In Virginia, another mapping software program (Topo!™ Interactive Maps) was also used. In recent months, the author has mapped wildfires, documented field travel routes, and demonstrated the use of mapping software to develop terrain profiles for cable logging using a GPS III Plus, Garmin MapSource™ Topo, DeLorme Topo USA® 2.0 and Street Atlas USA® 6.0, MAPTECH® Terrain Navigator, Topo!™ Interactive Maps, and ArcView®. Each feature of the GPS III Plus was explored along with the use of Garmin MapSource™ products. Equipment and mapping software were tested in a variety of applications to ascertain a better operational understanding of products, explore possible applications, and make recommendations for their use while identifying any potential problems or critical limitations with either the equipment or mapping software.

TESTING EQUIPMENT AND SERVICES

To test position accuracy, the GPS III Plus was set on a known benchmark and waypoints established with the position averaging feature. Positions were marked using DGPS but not used to compare relative accuracy. Additional "side-by-side" field testing was done comparing traverses made using the GPS III Plus and cadastral survey-grade (sub-meter capable) GPS equipment, with both units utilizing real-time DGPS.

To test mapping software, products were reviewed and compared. Waypoint data from the GPS III Plus were downloaded and a variety of maps generated using the different software products. Mapping quality was compared to ArcView® mapping and USGS 7 1/2-minute quadrangle maps.

RESULTS FROM TESTING OF GPS EQUIPMENT

In testing the GPS III Plus at a known benchmark using the "positioning averaging" feature, the observed position without DGPS was predictably poor due to the effects of SA. (Positions were occupied for approximately 1 minute.) The display indicated average position accuracy of 36 ft (10.97 m). The observed position at the benchmark had an approximately .6 seconds of latitude and .7 seconds of longitude difference to that of the true benchmark location. With DGPS, accuracy of the GSP III Plus was improved indicating that the position was within 6.9 ft (2.07 m) of the true position. The recorded and true position were the same for the position precision available for comparison (i.e., to .0000 degrees). In field-testing, utilizing station-to-station locations to complete a closed traverse, the GPS III Plus performed well, with acreage calculations equal to the compass and tape survey and reasonably close (± 10 percent) to that produced using the survey-grade equipment.