An Examination of the Accuracy and Reliability of Three GPS Devices Cormac Powell, Alan E. Donnelly, Mark Lyons & Ross Anderson Department of Physical Education and Sport Sciences and Centre for Physical Activity and Health Research, University of Limerick, Limerick, Ireland
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Introduction Global Positioning System (GPS) is a satellite-based global navigation system that provides location information on the Earth’s surface. GPS is used in a wide variety of commercial and research applications. Errors in GPS data can arise from adverse satellite geometry, environmental conditions and device selection (Duncan et al., 2013). There are currently no best practice guidelines in place for collecting, processing and analysing GPS data on human performance (Kerr et al., 2011). The validity, reliability and feasibility of wearable GPS devices needs to be better understood if they are to become important measurement tools in physical activity research (Wieters et al., 2012). The aim of this study was to examine the distance measurement accuracy of three GPS devices and also the reliability and validity of said devices for positional measurement.
Methods Ten participants completed a surveyor measured 2.54km route at a self-selected pace, whilst wearing a Garmin Forerunner 110 (GF), an iPhone 4s (iP) and a VXSport Log 330a (VX). A researcher also completed the route eight times. In accordance with previous work by Wieters et al. (2012) the route included eight surveyed coordinates (Figure 1) where participants stopped for 20 seconds to create time points. An activPAL3 was worn on the right thigh to determine when a participant was stationary or in motion. Eighteen trials were included in the analysis. GPS measured distance was compared to the known distance. The Haversine formula (Duncan et al., 2013) and 95% confidence interval (CI) ellipses were used to examine positional accuracy. The Haversine formula was used to examine the distance between the known coordinates and the mean of the measured coordinates, while the 95% CI ellipses examined the spread of the coordinates around the mean of the measured coordinates. The number of satellites present and the position dilution of precision were recorded prior to the start of each individual trial. Data from the GF was excluded from the positional analysis due to its smart sampling technique.
Table 2 – Mean (SD) of 95% CI ellipses and distance from known coordinate for the iP and VX devices Known Device 95% CI ellipse Distance from known coordinate (m2) coordinate (m) iP 14.38 (21.33) 3.03 (2.04) 1 VX 17.40 (21.49) 3.27 (2.17) iP 95.73 (158.81) 9.66 (6.29) 2 VX 124.27 (140.75) 8.21 (5.90) iP 180.79 (256.51) 12.54 (9.52) 3 VX 247.64 (472.71) 8.72 (4.91) iP 48.41 (61.19) 6.73 (3.88) 4 VX 35.65 (33.88) 4.77 (3.88) iP 59.25 (49.44) 9.32 (4.47) 5 VX 122.02 (128.05) 6.93 (3.53) iP 52.80 (88.12) 11.32 (11.13) 6 VX 63.07 (43.74) 11.11 (16.58) iP 39.95 (45.47) 6.77 (3.28) 7 VX 54.75 (62.21) 11.40 (14.41) iP 19.50 (40.85) 4.77 (5.29) 8 VX 25.39 (34.95) 10.50 (26.10)
Discussion Numerous factors have an influence on the accuracy of GPS devices, such as environmental conditions, satellite geometry and device characteristics. Depending on device selection, there can be as little as 3% error when measuring overall distance. Large variations in positional accuracy were observed in this study, despite the number and spread of satellites falling within the recommended levels for each trial (Duncan et al., 2013). For the individual coordinates, accuracy may be influenced by different charateristics. Coordinate 2 (the most secluded known coordinate) has the largest mean area of the 95% CI ellipse for both devices (180.79m2 vs. 247.64m2), but point 4 produces an area for the VX that is more than twice the size of the area created for the iP (122.02m2 vs. 59.25m2), despite being in a less secluded location. Examining the mean distance from the known coordinates, both devices revealed large mean distances from known coordinate 2 (12.54m vs. 8.72m). While point 2 produces the largest distance for the iPhone 4s, point 6 produces the largest distance for the VX, a distance that is almost twice that of the iP for the same point (11.40m vs 6.77m). The discrepancy in distances observed here in relation to known coordinates raises questions about the suitability of these GPS units for research where greater positional accuracy is required.
Conclusions • Depending on device selection, error in overall distance accuracy ranged from 3.3 – 18.53%. • On average, the distance from the known coordinate was 8.02m for the iP and 8.11m for the VX. • On average the area of the 95% CI ellipse was 86.2m2 for the VX and 63.85m2 for the iP. • Despite the number and geometry of the satellites being within the recommended level for all trails, large variations were seen, suggesting that environmental conditions and individual device characteristics may have an impact on device accuracy.
Figure 1 – Route with known coordinates
Results The mean difference between the known distance and the GPS measured distances were 84.44m (3.3%), 234.44m (9.23%) and 470.65m (18.53%) for the VX, GF and iP respectively (Table 1). Table 1 – Mean (SD) for all variables for the VX, iP and GF Device
Measured distance Difference from known (m) distance (m)
VX
2624 (70)
84.44 (70.23)
95% CI ellipse (m2) 86.28 (150.40)
Distance from known coordinate (m) 8.11 (8.49)
iP
3011 (108)
470.65 (107.5)
63.85 (79.33)
8.02 (3.13)
GF
2774 (42)
234.44 (41.6)
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References 1. Duncan, S., Stewart, T.I., Oliver, M., Mavoa, S., MacRae, D., Badland, H.M. and Duncan, M.J. (2013) ) ‘Static Validity of Seven Portable Global Positioning System Receivers Under Different Environmental Conditions’, American Journal of Preventive Medicine, 44(2), 19-29. 2. Kerr, J., Duncan, S. and Schipperjin, J. (2011) ‘Using Global Positioning Systems in Health Research: A Practical Approach to Data Collection and Processing’, American Journal of Preventive Medicine, 41(5), 532-540. 3. Wieters, K.M., Kim, J-H. and Lee, C. (2012) ‘Assessment of Wearable Global Positioning System Units for Physical Activity Research’, Journal of Physical Activity and Health, 9, 913-923.
Acknowledgements The authors would like to thank the Physical Education and Sport Sciences Department of the
On average, there is a 95% probability that the coordinates collected will fall within an ellipse with an area of 63.85m2 for the iP and 86.28m2 for the VX (Table 1). With regards to the accuracy of the devices, on average, the iP was 8.02m from the known coordinate, while the VX was 8.11m (Table 1). Table 2 contains the 95% CI ellipses and the distance from the known coordinate for the iP and VX for each known coordinate.
University of Limerick for providing a research bursary which made this project possible.
International Conference on Ambulatory Monitoring of Physical Activity and Movement Limerick, Ireland June 2015
