Finding a Target with an Accessible Global Positioning System

By Ponchillia, Paul E.; MacKenzie, Nancy et al. | Journal of Visual Impairment & Blindness, August 2007 | Go to article overview
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Finding a Target with an Accessible Global Positioning System

Ponchillia, Paul E., MacKenzie, Nancy, Long, Richard G., Denton-Smith, Pamela, Hicks, Thomas L., Miley, Priscilla, Journal of Visual Impairment & Blindness

Abstract: This article presents two target-location experiments. In the first experiment, 19 participants located a 25-foot chalk circle 93% of the time with a Global Positioning System (GPS) compared to 12% of the time without it. In a single-subject follow-up experiment, the participant came within 1 foot of the target on all GPS trials. Target-location techniques are described.


Global Positioning Systems (GPS) contain three segments: (1) the component in space, which consists of 24 operational satellites; (2) the earth-based control sites that monitor the satellites; and (3) the user-receiver component, which interprets the radio signals from the satellites (El-Rabbany, 2002). The handheld device, which is often referred to as the GPS unit, also contains Geographic Information Systems (GIS) data, which is spatial, such as the locations of roads, railroads, and other features that are commonly displayed on print maps (Taylor & Blewitt, 2006). The two main functions of GPS-GIS navigation systems are to provide users with locational information by relating the users' position, as determined by the GPS, to the map information that is stored in the onboard GIS database; and to enable users to mark specific latitude-longitude positions with electronic markers, known as waypoints, which can later be used to relocate the specific positions. Both functions are extremely useful to individuals who are visually impaired (that is, those who are blind or have low vision) and are included in the commercially available adapted GPS products designed for people with visual impairments. Map-location information could give travelers who are blind their street location whenever they requested it, and the ability to mark and relocate targets could solve the problem of locating targets that are placed in unstructured environments, that is, areas on maps that do not contain streets, such as parks, golf courses, and woodlots.

Accessible GPS devices were first introduced commercially in 2000 (Golledge, Marston, Loomis, & Klatzky, 2004). Since then, it appears that the question of whether these devices are of sufficient accuracy for use by individuals who are visually impaired has not been addressed. This research was conducted to answer that question.

On the surface, it seems that these devices are not accurate. For example, commercial GPS devices are accurate, on average, to within 9.14 meters (30 feet) (Broida, 2004). Therefore, as users walk along a sidewalk or path with a device in search of a target, they may receive information from the GPS that the target is next to them, when it is more than 9 meters away. Since a 9-meter error could place a user in the center of a street instead of at the curb ramp, one would likely question the usefulness of GPS for travelers who are blind or have low vision. However, the 9-meter error reported for GPS devices does not necessarily equate with missing a target by such distances. Most important, the devices are used in conjunction with, not in the absence of, the myriad orientation and mobility (O&M) skills that are commonly taught to people with visual impairments (Jacobson, 1993; LaGrow & Weessies, 1994; Long & Hill, 1997). If travelers in the foregoing example were using GPS navigation units that were reporting the name of and distance to the upcoming cross street as they traveled along the walk, they could use cane skills to search for the curb after the device reported the distance as 100 feet or less, and they could stop when they found the curb.

Target-tracking technique

The BrailleNote GPS (BGPS), which was investigated in this research, has both the map-based orientation and the location-marking functions described earlier. However, since this study was aimed at accuracy, it focused only on marking and finding targets in unstructured environments. Two BGPS functions can be used to mark and relocate targets: the manual-route and the points-of-interest functions.

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