MAPPING SOFTWARE

Several vendors market digital mapping products [figures 1, 2, and 3 (all map samples are taken from http://www.joe.mehaffey.com)]. The most commonly used products are designed for travelers and outdoor recreationalists. Products from DeLorme, Rand-McNally, and National Geographic are all similar in cost and features. When topography is not needed for mapping purposes, these products can provide a wealth of information, as well as allow users the ability to produce accurate and attractive maps.

Special software can be purchased that converts paper copy maps into digital maps (e.g., Fugawi). Other companies, such as Omni Map Resources, can produce customized digital coverage. Garmin produces its own MapSource™ products for use in its GPS units. This includes a road and recreation map base, an international map, and MapSource™ Topo.

Figure 1—Garmin GPS III Plus screen display using MapSource™ USA Topo downloadable software.

Figure 2—Digital map from USGS 7 1/2-minute quadrangle.

Figure 3—Area displayed using DeLorme Topo USA®.

SELECTIVE AVAILABILITY

Selective Availability (SA) is a process that involves the intentional degradation of GPS data of many applications. To correct these errors, post-processing data (differential correction) is required. On May 1, 2000, the U.S. Government disabled SA. Without the errors associated with SA, raw GPS data is now much more accurate and position location may be 10 times more accurate. The loss of SA will most likely eliminate the need for DGPS services.

Since the status of SA was changed after this article was written, the following sections, Position Accuracy and DGPS Services, have been left in this document so readers will understand the implications and remedies for SA.

POSITION ACCURACY

One of the primary concerns among GPS users is the accuracy of the lower-cost recreational-grade GPS units. Typically, these units provide approximately 98.42- to 164.04-ft (30- to 50-m) accuracy. With SA, signals can be greatly degraded and position accuracy may be even worse. Most GPS units are not capable of post-processing data to improve position accuracy. Without this, results from mapping can be quite varied.

The GPS III Plus has two means of dealing with the effects of SA and the inherent lack of post-processing capabilities. One feature is position averaging. This function allows users to average position samples overtime and save the results as waypoints. In tests conducted with and without position averaging, accuracy appeared to have improved significantly with the use of the position averaging feature (table 1).

Table 1—Garmin GPS III Plus versus cadastral survey-grade GPS equipment, and compass and tape closed traverse.

Type of equipment	Type of application (site)	DGPS/No DGPS	Observed position accuracy (remarks)
GPS III Plus	Position at benchmark	DGPS	U.S. Coast Guard and Geodetic Survey Station Benchmark at W84.8638 degrees X N35.1789 degrees (or W84 51' 49" X N35 10' 44") appeared as the same position on the GPS III Plus with DGPS "on" (actual display position for the GPS III Plus was W084.86362 X N35.17891 or W084 51'49.0" X N35 10'44.1").
GPS III Plus	Position at benchmark above	No DGPS	W84.8640 X N35.1787 or W84 51' 50.6" X N35 10'43.4" as compared to benchmark position noted above.
survey-grade GPS	Traverse acres	DGPS	GPS traverse was approximately 2 acres different than that of a compass and tape survey of the same area (approximately 34 acres with compass and tape).
GPS III Plus	Same traverse as above	DGPS	Closed traverse was less than 1 acre different than that of a compass and tape survey of the same area. Thirteen of 20 stations logged by the GPS III Plus were within 16.4 ft (5 m) of observed survey-grade GPS position locations.

DGPS SERVICES

The other means of obtaining better position accuracy with the GPS III Plus involves the use of real-time DGPS. Garmin sells the GBR 21 beacon receiver to interface with many of their GPS units. The use of this equipment requires FM reception of an operational U.S. Coast Guard beacon transmitter generally within 125 m (201.17 km) of the user's location.

The GBR 21 was designed primarily for marine, aviation, or automobile mounting and comes with 30 ft (9.14 m) of antenna cable that must be attached to the receiver at all times. It provides more than a 32.81-ft (10-m) position accuracy. The lack of complete geographic DGPS coverage by U.S. Coast Guard beacons and the need for higher accuracies, required in cadastral survey applications, has given private companies the opportunity to provide products, services, and equipment that interface with many brands and models of GPS units. The real-time DGPS signals are transmitted at various frequencies and translated into useable formats via a specially made vendor-supplied receiver. With the right equipment and vendor service, sub-meter accuracy is achievable with survey-grade and recreational-grade GPS systems.

The equipment tested was very compact and easy to use. While a commercial service is available in most states, there are still locations in the United States where coverage is limited or not available. The U.S. Coast Guard maintains a website (NAVCEN) that lists all beacon transmitter sites nationwide and their estimated coverage. Generally users must be within 45 m (72.42 km) of a FM broadcast station to receive DGPS signals. With the receiver and premium vendor service, the GPS III Plus appears to be able to deliver the 6.6- to 19.7-ft (2- to 6-m) accuracy as promised by manufacturers and DGPS supplier.