How Accurate Are Running GPS Devices?
GPS (Global Positioning System) measures where you are on the planet and its accuracy varies by type of watch or phone; the technology they contain and the satellites they are connecting to. There are several private satellite networks that provide the service – GPS is the name for the original American one, which not all devices use now.
The quoted accuracy for GPS is that it gives your location to a 3 metre accuracy level 95% of the time. This assumes flat terrain and nothing to obscure the connection to the satellites, so no trees, tall buildings, large hills etc. GPS is less reliable at working out your elevation (how far up the hill you are).
In a GPS tracking device, positions of where you are are taken at frequent intervals (not constantly) and these are linked together by the watch or app to work out speed and distance etc. If you are on a particularly wiggly route you may find your running GPS watch “cuts the corner” as your stats were taken before the corner and after, and it plots a straight line from one dot to the next.
What Other Statistics Do Running GPS Devices Collect?
All running apps and watches track the time, from when you click start to when you click stop. They also track the time that your GPS position dots are taken, so they can work out the speed in more detail throughout the run. When you are doing an interval style session or a run with different paces, it’s useful to click “lap” when you change pace so you get to see clearly your paces for different parts of the run.
Most watches and phones now have accelerometers in them, this tracks the movement of the device. So it can count steps as it senses your arm going back and forth (and arm movement is directly linked to leg movement) and when combining with the time stats it can work out cadence; how often your feet are hitting the ground in a minute. This is a very useful statistic, as it links to running technique and efficiency.
Heart rate monitoring is included in most running watches now too, either via a chest strap or on the back of the watch face. At the moment the chest straps are more accurate. These rely on the conductivity of your skin as they are listening for the tiny electrical currents created by your heart. This means their accuracy is impacted by sweatiness; sweaty is better!
But My Running GPS Device Shows Me Loads Of Other Stats!
Lots of running watches and apps are showing loads of other information now too, so I looked up the details around how Garmin pull this data together and how they collect it. I picked Garmin as I have one, as do the majority of my runners.
With Garmin, stats like VO2 Max, recovery time, training effect, calories, etc are estimates. Garmin have access to a lot of laboratory data where loads of runners have had these stats scientifically calculated, and they compare your time, distance, cadence and heart rate stats to this data bank and show you the averages you line up best with. I see lots of runners getting worried about some of these stats presented to them, and it’s always worth taking into account they aren’t actually your stats, they are estimates based on others who’ve done a similar run.
As an example; to measure VO2 Max properly you need to run in a controlled environment with a mask on measuring your exhaled carbon dioxide, and they get pin prick blood tests throughout the run to measure levels of chemicals in your blood. So, your running watch can’t really know your actual VO2 Max…!
What About Heart Rate Zones?
The heart rate statistics from your running watch are usually presented to you in “zones”, usually coloured from green up to an alarming shade of red! Whilst the heart rate figure itself is usually fairly accurate, the heart rate zones vary a lot person to person. The frequently quoted 220 minus your age maximum heart rate is the basis for the zones used and this is actually proven to not be correct for many people.
What is a useful stat, especially if you wear an activity tracker all the time, is your resting heart rate when you wake up first thing in the morning. That’s a great way of tracking recovery and keeping an eye on overtraining.
Pros And Cons Of Running GPS Devices
Running GPS devices are great for tracking your running; knowing you are ramping up your mileage safely, knowing you aren’t going too far or too fast on recovery runs, knowing you’ve actually done the length of run you intended to. The statistics they collect about cadence, and pace are particularly interesting and useful, for runners and Coaches alike!
There is some evidence to show that this detail and data can lead some runners to become a little too focussed on the numbers, and potentially start aiming to improve the speed or distance each run; which isn’t a great training strategy.
Sharing data with others, via Strava or via linking to friends on other apps has also been shown to be linked to a risk of overtraining and exercise addiction.
Using your data as a useful tool for training, but not letting it provide all the direction to your running, is key.