Dynamic Non-DGPS positional accuracy performance between recreational and professional GPS receivers

The purpose of this study was to measure and evaluate the dynamic non-differential positional accuracy of two global positioning systems (GPS) receivers. The two receivers used were the Trimble GeoXT handheld and the Garmin GPSMAP 76. Both units are single-frequency, twelve-channel GPS receivers. The units were tested for horizontal root mean square (HRMS) positional accuracy without real–time differential correction in the dynamic mode by recording the movements of an articulated truck across the Irish road network. The units were operated with their external magnetic-mounted antenna. The two antennas were fitted side by side to the cab of the truck. The articulated truck was a DAF XF95 model, 4 × 2 tractor unit with a tri-axle road-friendly air suspension Fruehauf curtain side trailer measuring 13.6 metres (m) in length and 4.2 m in height from the ground. Routes were travelled from the east of Ireland to the south and south–west of the country on six separate occasions during August 2005 giving a total of six consecutive routes. Over 50 hours of data, totalling approximately 6000 data points, sampled at 30 second intervals, were recorded for each of the GPS units. Of these 50 hours, over 30 hours were recorded as dynamic points, totalling approximately 4000 sampling points. The HRMS accuracy was measured at a confidence level of 63%. The HRMS results for the Trimble GeoXT ranged from about 6.9 m for the Cork 1 route to 3.2 m for the Cork 2 route (Table 1). Results for the Garmin GPSMAP76 varied from a much higher value of about 43.0 m for the Limerick 3 route to 56.9 m for the Cork 2 route (Table 1). With this highly variable level of positional accuracy between the two GPS units, it is clear which receiver unit can best be used for professional GPS data collection (Trimble GeoXT) and which is suitable for use as a recreational device (Garmin GPSMAP 76). The option to collect field data using inexpensive recreational GPS units may be sufficient for outdoor enthusiasts who simply require an occasional location fix of moderate (even uncertain) accuracy, but it is unlikely to be sufficient for the Geographical Information Systems (GIS) professional who requires consistently accurate locations of objects, lines and polygons so that data layers can be overlayed within a GIS. A position fix that is tens of metres in error can lead to distorted spatial data and hence incorrect decision making. In fact, for some applications, a very inaccurate position could be worse than no position fix at all.