GPS and GIS: Enhanced Accuracy in Map Matching through Effective Filtering of Autonomous GPS Points

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ABSTRACT. The development of the Global Positioning System (GPS), along with ever more powerful geographic information systems (GIS), has spawned the development of public safety applications. One of the most noteworthy of these applications is the Mayday system in which a motorist can call for roadside assistance at any time, day or night, without knowing his location, so that a service center can send assistance to the site of the call. Some of today's Mayday systems provide national coverage which, while impressive in scope, can cause problems in assuring the reliability required to enlist the loyalty of customers and the cooperation of local emergency personnel. Customers whose calls for assistance are not answered promptly, or frequent calls that result in long searches by emergency personnel taking them away from their posts and endangering others who need them, are potential pitfalls of Mayday systems. This paper looks at the effects of filtering autonomous GPS points to enhance the reliability of positions when matched to maps in GIS databases.

KEYWORDS. GPS, GIS, map matching, Mayday systems

Technology and Travel Services

with advances in technology afforded by geographic information systems (GIS) and the Global Positioning System (GPS), new products are being developed for both consumers and specialized markets. A large number of these applications fall within the arena of transportation and intelligent highway systems. Vehicle tracking, fleet management, emergency vehicle routing, incident and accident notification, and motorist assistance have all been reported in recent literature. Three major United States trade publications, GIS Worm (Fort Collins, Colorado), GeoInfosystems (Eugene, Oregon), and GPS Worm (Eugene, Oregon) regularly devote column and feature space to these developments. Common to these applications is the requirement for precise to semi-precise location of vehicles using on-board GPS receivers. This study addresses one of the components of accuracy in a map-matching environment.

A specific transportation-related GPS/GIS operation known as a "Mayday" system utilizes GPS receivers carried in vehicles to identify the location of a disabled vehicle to a dispatch center, allowing the vehicle's occupants rapid automated access to help when needed. The initial Mayday system in the United States was developed in the Denver area in December 1994 by NAVSYS (Krakiwsky 1996a). The system marked the first reported public use of GPS and cellular telephone service to provide positioning information in a large-scale project. Under the Colorado Mayday system (Lacey and Cameron 1995), drivers send their positions to a central dispatch center and receive rapid assistance in emergencies. Other systems offering local coverage areas are also on line. Rockwell (Seal Beach, California) and ADT (Irving, Texas) have introduced the Micro Tracker system. Other systems include PuSHME in the Seattle area, the Tele Trac roadside assistance system in Los Angeles, Chicago, Detroit, and Dallas/Fort Worth, and the Mayday Plus system in Minneapolis (O'Brien and Balogh 1996).

The first national Mayday system was Ford's RESCU system which debuted in Lincoln Continental automobiles in January 1996. Under this system, motorists can, with the push of a button, notify the service center (Westinghouse Security Systems (WSS)--Irving, Texas) of an emergency or a need for roadside assistance. After an attempt to speak with the caller and verify the call via cellular telephone, WSS contacts appropriate local authorities, based on the call. This system was field tested across the United States by a team of researchers from Westinghouse Electric Corporation (Baltimore, Maryland), Westinghouse Security Systems (Irving, Texas), Ford Motor Company (Dearborn, Michigan), and Virginia Tech's Department of Geography (Blacksburg, Virginia) during 1995. Success rates in vehicle location were over 96 percent and times for local dispatch were generally under 11 minutes (Car-stensen 1995, Krakiwsky 1996a).

In February 1996, General Motors (Detroit, Michigan) announced its On Star system which is available on Cadillac models beginning with the 1997 model. Along with emergency and roadside assistance services, the On Star system assists in certain navigation tasks, in collision notification, and other useful services for motorists. General Motors offers this service nationwide. In addition to these there are numerous companies in the Mayday service arena in Europe and Japan (Krakiwsky 1996b). The potential market for such devices is enormous, and early demand is running well above that expected in the U.S. market.

Components of a National Mayday System

The basic operation of a Mayday system is fairly straightforward. A coordinated sequence of events passes a distress message from a disabled vehicle through a service center to a local authority which assists the driver of the vehicle to rectify the problem. The basic flow and pertinent accuracy issues that must be addressed in each step are described below.

1. A call is initiated by the driver of a vehicle needing assistance. For national coverage, this step is dependent on the cellular telephone system to make a connection from any location in the U.S. to the service center. Though a majority of the U.S. is covered by cellular service, expecting that every call will get through is overly optimistic. Some areas, especially those with irregular terrain that undulates out of sight of cellular transmission towers, have spotty service, some very rural areas have no service at all, and even well served urban areas have system overloads at times, making connection difficult. Mayday applications have little control of this factor, but areas of low or no service are rapidly diminishing.

2. As a part of the call, the on-board GPS receiver sends a location point (along with a variable amount of data on the customer and the vehicle) to the service center. To reduce transmission time and complexity, and hence the probability of transmission error, a single GPS location (the most recent "accurate" location) is sent. Currently, all of Earth has sufficient GPS satellite coverage …