Following my long-term review of the Suunto Ambit3 Peak, friend and fellow guide Alan Dixon sent me an email with the title, “Suunto 200 hr mode not cutting it,” and shared that on a recent 8.2-mile hike his Ambit3 shortchanged him by 1.5 miles (18 percent). He had set the GPS accuracy of the Ambit at “Okay” and was comparing it to the distance measured with his iPhone using the GaiaGPS app.
Alan’s experience is consistent with that of Ethan, a reader who recently commented that his Ambit3 Peak was also 20 percent short when set to “Okay,” versus the listed mileage in the guidebook and on trail signs. (Importantly, he found it to be 75 percent short when the auto-pause was on.)
I’ve never intentionally and precisely tested the accuracy of the Ambit3 Peak in the “Okay” setting, but the observations made by Alan and Ethan sound intuitively correct: the recorded distance will be short of the actual distance. The exact margin of error will vary; 20 percent is probably a safe expectation.
In this post I will explain the cause of this inaccuracy and offer guidance on best uses of the Ambit.
The GPS accuracy of the Suunto Ambit3 Peak (and all other Suunto GPS watches, too, including all Ambit2, Ambit3, and Spartan models) can be set to:
- “Best,” or 1-second pings;
- “Good,” or 5-second pings; or,
- “Okay,” or 60-second pings.
It takes energy to acquire a GPS location, so unfortunately the watch’s accuracy is inverse to its battery life. The Ambit3 Peak is rated to 20, 30, and 200 hours when set to 1-, 5- and 60-second pings, respectively. I pushed the Ambit2 and Ambit3 Peak to the brink on multiple occasions (notably, during a half-dozen 100-mile races) and have found that these battery life estimates are reliable.
Note that the “GPS accuracy” setting is different than the “Recording interval,” which can be set a 1- or 10-second intervals. Ambit does not just record its GPS location — it also collects data from its accelerometer (e.g. cadance), barometer (e.g. pressure and altitude), thermometer, and any accessory devices like the Smart Sensor HR strap or Stryd power meter. The 1-second interval will generate the most accurate data set, but it will fill the internal memory faster. On longer outings with infrequent opportunities to download data, use the 10-second interval.
Most Garmin GPS watches have a similar battery-saving mode, known as Ultratrac. I’m less clear about how it works, but it will also extend battery life by a multiple. For example, the Garmin Fenix 5 is rated “up to 24 hours” in its most accurate setting, but “up to 60 hours” in Ultratrac. With older models, Garmin has been criticized for optimistic battery life estimates; I’m unaware of the consensus about the Fenix 5 models.
Accuracy of Ambit3 Peak at Okay/60-second GPS pings
Why did Ethan and Alan get shortchanged by 20 percent when using the Okay/60-second GPS interval?
One reason is that the Ambit fails to record every meander, such as turns and switchbacks. To illustrate this, last night I walked in a zig-zag pattern down the nearby frontage road, once with the Ambit at the Best/1-second interval and a second time with it at Okay/60-second. The recorded tracks are shown below. Notice how the red line (1-second interval) records my zig-zags perfectly, while the blue line (60-second interval) missed all of them.
On straight trails, the accuracy of the Ambit at 60-second intervals is probably quite good. But on trails with lots of tight turns and meanders, its accuracy will decrease.
In addition to missed meanders, I think there is a second explanation for the inaccuracy of the Ambit at the Okay/60-second interval: its GPS location data does not seem as accurate. For example, notice in the zigzag test that the blue line shows a path through the grass, even though I was on the road. And in the image below, notice the dramatic zigzag across Fifth Lake even though I remained on its west side.
I have a few theories about what’s happening here. It’s possible that less battery power is being sent to the GPS chip, resulting in less accurate location data. Or, with GPS pings far and few between, the Ambit struggles to remain connected with multiple GPS satellites, thus compromising its accuracy. If you have a better understanding of GPS and GPS watch technology, please share.
Accuracy of the Ambit3 Peak at Best and Good GPS settings
When using its Best or Good setting, I have been remarkably impressed with the accuracy of the Ambit3 Peak (and its predecessor, the Ambit2), especially in consideration of its $300 price, 2.50 oz weight, and battery life.
With the Ambit in Best/1-second mode, I have run three certified marathon courses: Colorado, Boston, and Houston. The Ambit recorded distances of 26.44, 26.39, and 26.51, respectively, versus the official marathon distance of 26.22 miles, or about 1 percent less. However, this difference can be partially explained by how I ran the course (e.g. swinging wide around tight corners, and not running in a perfectly straight line between apexes), not the accuracy of the watch.
With the Ambit in Good/5-second mode, I have run multiple 100-mile ultra marathons. Throughout these races, the Ambit has displayed reliable pace, heart rate, and vertical change data. The recorded distances seem fairly accurate, too. At Ultra Trail du Mont Blanc (UTMB) last year, my Ambit3 Peak recorded a total distance of 105.18 miles, versus the official 104.76 miles, or an overshot of just 0.4 percent. It recorded 31,821 of vertical gain, versus the official 32,398, or 1.7 percent less.
In other ultras, the difference between the official distance and my recorded distance has been greater, like 107.12 miles at Run Rabbit Run 100, which officially is 102.9 miles. That’s a difference of 4.1 percent. However, I don’t necessarily trust the official distances for ultra courses — they are not professionally measured or certified. (UTMB is the biggest ultra in the world, and I bet they have measured their course with more accuracy than most.)
Based on my experience with the Ambit, in terms of its accuracy, here are my usage guidelines:
Use Best/1-second mode if:
- Your activity will be completed in less than 20 hours; or,
- You need the greatest level of accuracy.
- Examples: marathons, 50-mile ultras or fast 100k/100M courses, day-hikes, trail mapping
Use Good/5-second mode if:
- Your activity may exceed 20 hours but will finish within 30 hours; or,
- You need reasonably accurate data.
- Examples: 100k or 100M races, 1- or 2-night backpacking trips
Use Okay/60-second mode if:
- Your activity will exceed 30 hours; or,
- The data needs to be only generally accurate.
- Examples: 3-day backpacking trips or longer
If the Ambit is not offering the level of accuracy you need for the duration that you need it, remember that it can quickly and easily be recharged in the field (or during a long stop in an aid station). It has a 500 mAh battery, so a portable charger like the Anker PowerCore+ mini could recharge it about four or five times (after accounting for the energy lost from current conversion).
When performing field research for my guidebooks, I use the Okay/60-second setting. It:
- Produces a functional GPX track of where I went;
- Displays roughly accurate average pace data, especially over longer distances;
- Records accurately my net vertical change, which is usually the limiting factor each day, not horizontal mileage; and,
- Gives me a generally accurate mileage read-out for the day and between landmarks.
However, the mileage data and GPX track are not suitable for public consumption. The listed mileages in my guides are measured in Caltopo later on. And I intentionally do not include a GPX track of the routes — if you need that level of assistance, you probably shouldn’t be on it. If I wanted to skip the Caltopo step and/or share a very accurate GPX track, I would need to use the 1- or 5-second setting.
Have questions about the accuracy of the Ambit? What’s been your experience with it at Okay/60-second GPS pings? Leave a comment.
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