# How Accurate is your GPS Watch?

BY Rebecca Johansson

We base much of our workouts on distance and elevation change, but how good are the numbers? Find out what factors might affect your GPS watch's accuracy with this peer-reviewed study's findings.

You are nearing the end of a marathon. Your GPS watch hits 42.2 km, and you expectantly look for the finish line, only to realize that it is still another 500 meters down the road. Is the racecourse too long, or is your GPS watch not accurate? While completing my Ph.D. at the University of Cape Town, I completed a study testing the accuracy of GPS watches in measuring distance.

The study took place at the Two Oceans Marathon, a 56 km road race held in Cape Town, South Africa. The distance, as measured by GPS watches worn by 225 participants, was compared to the IAAF-measured race distance. There were eight categories of watches including: the Garmin Fenix® Series, Garmin Forerunner XT® series, Garmin Forerunner® series, Activity watches, Suunto®, TomTom®, Polar®, and Cell Phones. Here’s what we found:

## Hilly sections, turns and tree cover may negatively impact GPS accuracy

We were able to determine accuracy on various segments of the race course thanks to timing mats placed at 16 km, 28 km, 42.2 km, 50 km, and the finish. You can see the relative error for each segment in Figure 1.

Figure 1. Relative error of GPS measured distance vs. IAAF measured distance for segments 2, 3, 4, & 5, respectively.  A.) n = 223; B.) n = 248; C.) n = 246; D.) n = 238.

GFX = Garmin Fenix series; GXT = Garmin XT series; GFR = Garmin Forerunner series; ACT = Activity Watches; STO = Suunto; TOM = TomTom; PLR = Polar; CEL = Cell Phones.

* p < 0.05 vs. GFX. # p < 0.05 vs. CEL. ^ p < 0.05 vs. TOM. ! p < 0.05 vs. STO. \$ p < 0.05 vs. PLR.

The median error in measuring distance was 0.6 – 1.9%:

Segment two is relatively straight and flat, with approximately 100 m of elevation change, while segments three, four, and five include larger changes in elevation (approximately 465 m, 300 m, and 150 m, respectively) and more turns.

You can see that the errors were greater in the hilly sections of the race with more turns (segments three, four, and five) compared to the flatter and straighter section (segment two). For example, the median errors in the TomTom® category for segments two, three, four, and five were 0.4%, 1.3%, -1.2%, and 1.7%, respectively.

This study isn’t an outlier. DC Rainmaker found an average GPS sports watch error of 1% on a 1.6 km straight path with a clear view of the sky; 1.7% around a 400 m track; and 1.7% on a 0.84 km loop route through a mix of open area and tree cover (Rainmaker, 2011). In line with this, there is a possibility the turns in segments three, four, and five affected the accuracy in measuring distance. Segments four and five also included significant tree cover compared to the other segments, which may have affected the accuracy. Finally, segments three and four include stretches of road surrounded by mountains on one or both sides, which can also affect accuracy.

Still, the GPS sport watches in this study had a relative error of just 0.6 ± 0.3 to 1.9 ± 1.5% (median ± interquartile range) in reporting distance covered. This indicates that GPS sports watches are a valid and feasible method for sports scientists and coaches to measure performance and track training load.

When considered in context, the relative error for segment two to the finish of the race (a distance of 40 km) was lower than in previous research using a GPS sports watch (Nielsen et al., 2013). Nielsen et al. (2013) showed the error in measuring distance using a Garmin Forerunner® 110 ranged from 0.8 to 6.2% (mean). The results from our study reflect better accuracy. This could be due to advancements in technology and GPS units since 2013, when the Nielsen paper was published. However, the small error associated with each brand needs to be considered when data are interpreted.

Figure 2.  Data are presented as median ± IQR.  Relative error of GPS measured distance vs. IAAF measured distance for segment 2 – segment 5 (16 – 56 km). The shaded region represents the ± 1% IAAF course measurement error. n = 212.

GFX = Garmin Fenix series; GXT = Garmin XT series; GFR = Garmin Forerunner series; ACT = Activity Watches; STO = Suunto; TOM = TomTom; PLR = Polar; CEL = Cell Phones.

## Cell Phones were less accurate than all other categories

Unsurprisingly, the cell phone category displayed a higher error compared to the other categories (Figure 2). One possible reason is that cell phones may have been held or worn on the arm with a case and strap, in a shorts pocket, or in a waistband. The placement of the GPS device on the body can affect accuracy.

The 3rd edition of the Garmin Fenix® series (31 out of 34 participants in the Fenix® category wore this watch) also ranked poorly. It was the first to offer GPS and Global Navigation Satellite Systems (GLONASS) (Hofmann-Wellenhof, Lichtenegger, & Wasle, 2007). The recent incorporation of GLONASS into GPS sports watches increases the availability of surrounding satellites. However, there is currently no research comparing the accuracy of GPS vs. GPS combined with GLONASS in the devices included in our study. Simply increasing the availability of surrounding satellites may not guarantee improved GPS accuracy.

Activity watches showed a lower relative error, but the data range was wider, meaning that accuracy could vary significantly between watches. There are many factors to consider which could affect this, such as the quality of the GPS chipset, as well as the quality of the manufacturer algorithm.

On the winning side, Garmin Forerunner Series, Garmin XT Series, Suunto, and TomTom watches all showed a low relative error with a small range, well below the 1% error mark. Polar also ranked well on average but with a range just over the ideal 1% error.

If you would like to read more about this study, it has been published in the International Journal of Sports Science & Coaching.  Johansson, R. E., Adolph, S. T., Swart, J., & Lambert, M. I. (2020). Accuracy of GPS sport watches in measuring distance in an ultramarathon running race. International Journal of Sports Science & Coaching, 1747954119899880.

## References

Hofmann-Wellenhof, B., Lichtenegger, H., & Wasle, E. (2007). GNSS–Global Navigation Satellite Systems: GPS, GLONASS, Galileo, and More. Springer Science & Business Media.

Nielsen, R. O., Cederholm, P., Buist, I., Sørensen, H., Lind, M., & Rasmussen, S. (2013). Can GPS be used to detect deleterious progression in training volume among runners?. The Journal of Strength & Conditioning Research, 27(6), 1471-1478.

Rainmaker, D. C. (2011). Sport Device GPS Accuracy In Depth: Part I. Retrieved from https://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth.htm