GPS on the Run

I regularly give a talk about GPS and other electronics for my local Running Room marathon clinics. This is the content of my presentation, for those who missed it, or want a bit more information.

Personal Intro

A quick word about myself. I’ve been running with the Running Room marathon clinic for the last ten years. I’ve completed thirteen marathons. In addition to running, I am an inline speed skater. Along the way. I’ve gone through various GPS devices for both running and inline speed skating. I’m here to talk about how GPS and other electronic devices can complement a marathon training program


When starting out, runners just run. It’s great to just get out there and get moving. Runners training for a half or full marathon may want some additional training aids to provide quantified metrics to help them meet their goals.

The first of these is time, which can be fulfilled by any sports watch, such as a Timex Ironman. You can combine this with route maps to figure out pace, based on known points on the routes. That’s how we did it in the “old days” before portable GPS.

Next is distance traveled, which can be determined by counting steps and using stride pace to derive distance, or actual position, based on GPS in your watch or phone. The steps method has variable accuracy, depending on the accuracy and consistency of your stride length. GPS is much better, but has its limitations, which I will explain later. With distance, pace can be calculated, since it’s simply time divided by distance.

Finally, there are more advanced metrics like heart rate, cadence and running dynamics – how long you stay in the air, vertical oscillation and running power, etc.

There are a variety of electronic devices on the market that can track your run. They can be divided into  broad categories. 1) Pedometers 2) Phone apps 3) Smart Watches 4) “Fitness First” watches.


The step-based devices are essentially super-accurate electronic pedometers inside a wrist band or belt clip. Primarily used for daily activity tracking, most of them can be used for workouts as well. Fitbit is the most common brand, but other companies such as Garmin and Misfit also make devices.  All of these count the number of steps and derive speed, pace and distance based on a stride length that you either enter, or in some cases, it can calibrate on its own. Some of them have a display and all of them use an accompanying smartphone app to show and log workouts and daily activity. The more advanced models can track other health metrics like 24/7 heart rate and sleep. Smart watches without GPS, such as the Fitbit Blaze, also fall into this category.

GPS Watches

Dedicated GPS sports watches tend to be the most popular choice for runners. The position-based devices use a GPS receiver, similar to the navigation system in your car or in your smartphone. By repeatedly comparing position changes over time, they calculates pace and distance. These use signals from satellites to track your position, and by doing this at regular intervals it can derive your speed, pace and total distance traveled. The initial devices were dedicated to sports functions and later added fitness tracker and smartwatch functionality, such as all day activity and HR tracking and third party application support. The major players are Garmin, Polar, TomTom and Suunto.

Phone Apps

Modern smartphones have the necessary GPS and accelerometer hardware to track workouts and there are many apps that will do this. The phone based apps are convenient if you already carry a phone, although they can be a drain on the battery. Accuracy is can be both better and worse than a dedicated GPS. They can be less accurate because they don’t take position readings as frequently as a dedicated device. On the other hand, they can be more accurate if they use assisted GPS, which uses cell tower triangulation to help with the position fix. Also, they can be fiddly to get out and look at your phone in the middle of a run, especially if it’s wet outside. Speaking of wet, most phones aren’t waterproof and not designed to really take the beating that running can sometimes dish out. They can be a bit of a bother to carry during running.


Smart Watches

A number of smartwatches have recently been introduced, notably Android Wear (Samsung Gear, Polar M600, New Balance RunIQ, etc.) and Apple Watch. These pair with your phone app to provide a display on your wrist for notifications, but also fitness stats. Most of them also have optical heart rate monitors in the back, allowing for HR sensing without a chest strap. In my testing the GPS accuracy is on-par with a dedicated sports GPS watch and the optical HR results are mixed. The latest models also have a cellular connection, allowing you to take calls and receive messages and data on the run, without carrying your cell phone. Compared to traditional sports watches, smart watches tend to have better and brighter screens, at the expense of battery life. Some of them can be as low has 5h with GPS, optical HR and bluetooth music streaming.


The entry-level watches will have the basic metrics – stopwatch, pace and distance. The pace is can be very helpful, not only to measure progress, but also allow you to to relate it to a perceived effort. It can also help you run at a more consistent pace, which is a key training and race strategy. You’ll notice on your training plan that there are recommended paces for tempo, steady and LSD runs. A GPS watch can help keep you at the right pace, especially if there isn’t a pace leader handy.

The distance is handy to help you gauge how far you’ve run and allows for ad-hoc navigation during the run. For instance, if I need to add some mileage to a run, I can run out for half the extra distance on the GPS and just return.

More Features

The mid-range units will have heart rate, another key training tool, and can track 10 and 1’s. They can also be programmed for specific interval training, based on distance, time or both. The better units will allow you upload your workouts to the cloud (Strava, Garmin Connect, etc.) for automatic logging, more detailed analysis and sharing on social media.

Other often-used features are auto-lap, which will automatically trigger a new lap at a fixed distance interval, or when you return to a specific position. The former is good for segmenting your road runs into even 1 km or 1 mile chunks. The latter is convenient for outdoor track work, so you don’t have to push the lap button on every lap. Another feature is auto-stop, which will stop the recording if you stop moving. It’s handy if you want to measure moving time only, but be careful, since it will auto-stop if you lose the GPS signal.

Bluetooth music streaming is also becoming available in mid-to-high end watches and virtually all of the smart watches. They can beam music wirelessly to Bluetooth headphones, eliminating both your phone and headphone wires whipping around from your run.


The most advanced units use a combination of GPS and accelerometers to measure things like vertical oscillation, cadence, ground contact time, left-right balance and VO2 max. You’ll also find things like wireless connectivity and multi-sport features like swim tracking.

Some GPS devices can link to other accessories like a footpod, for use indoors on a treadmill, or as a backup if you lose the GPS signal in a tunnel, or near tall buildings. I’ll talk more about running downtown later on. They can also link to bike sensors like speed, cadence and power. Running power sensors are starting to become available as well, with Garmin, Stryde and Runscribe having units that either mount on your shoes or waistband.

Heart Rate Monitoring

In addition to GPS, another major metric that most of the watches will measure is heart rate. You can use HR as a gauge for effort, to make sure you are training at the right intensity. Some athletes will train using a HR target, rather than a pace target. In some cases, it can be much more useful, taking into account factors like wind, hills and snowy surfaces that can really alter your effort, for a given target pace.

Some HRM’s use a chest strap, which basically work like an ECG and measure the electrical impluses in your chest. A newer sensor is a wrist-mount optical sensor. It shines a bright LED into your wrist and uses a camera to image the blood pumping through your arteries near the skin. They are still relatively new and some sensors are geared towards workouts and others for all-day HR monitoring. You can find these either as stand-alone units that replace your chest-strap HRM, or built into the backs of a GPS watch like the Garmin 235 or TomTom Spark. There are a few oddball HRM devices that work on similar technology – headphones, caps, bike helmets, etc. I think eventually, all watches will have optical HR built-in.

How GPS Works

That said, they aren’t perfect and it’s really help to understand how they work to know how to interpret the results.

The GPS system is based on about 24 active satellites in low Earth orbit, similar to where the ISS and space shuttle orbit. Unlike communications satellites, they don’t “hover” over a fixed location and will track across the sky over time. At any given moment, there might be about a dozen satellites visible from any location. The number and location of satellites in view will change minute to minute. These satellites emit a super accurate time signal, which your GPS watch picks up. It looks at the differences in the time signal to determine the distance to each satellite. It uses the series of distances to triangulate your position on earth. On a good day, we’ll see 2m-3m accuracy, which is to say the GPS system can location your position to within a 2m circle.

The more satellites are used, the better the accuracy and the faster the watch can establish its position.

There are a some limitations. To get a proper signal, the receiver (i.e. your GPS watch) needs to have line of sight to each satellite, which is a bit of a problem in the steel and glass canyon around here. The buildings can block some of the satellites and reduce the accuracy. It also increases the amount of time it takes to get a position lock. That’s why you see runners waving their watches at the start of the run. Since the satellites track across the sky, the accuracy can vary moment to moment, depending on the number visible, where they are, and if they are blocked by buildings and other obstructions.

There are a few things you can do to help with getting a good lock. If you’re running from home and you live a condo, leave your watch on the balcony or near the window as you’re getting ready. By the time you’re ready to leave, it will have gotten a lock and a quick trip in the elevator usually doesn’t screw it up too much. If you’re running from your car, same deal, power it up first and leave it on the dash while you get ready. Finally, if you are running from the store, start your GPS up before you arrive to help it get a lock before you walk into the store. When you go out to run, it improves the chances of reacquiring the satellites quickly.

Pace Types

One of the key metrics you will use for your training is pace. Because of the way GPS works, it helps to understand how GPS calculates pace and the different types of pace metrics available on the watch. GPS only measures time and position. It doesn’t measure pace directly. Instead, it takes a number of position readings over a period of time and determine how far you moved. A good GPS signal will have an accuracy of 2-3m. That’s excellent for automotive applications, where you’re measuring speeds of 60-100 km/h. But runners move a lot slower, so the relative accuracy is much lower. That error of 3m takes the runner about a second to traverse. So, this leads to our different pace measurements, which differs by the amount of time the reading is taken.

First is pace, or really, instantaneous pace. The watch is taking the distance you’ve run in the last 10-30s and dividing by the time. It can fluctuate a lot and trying to “chase” it can be a challenge.

Second, overall pace. This is simply the total distance travelled in the workout divided by the total time. It a good indicator of overall performance, but it can be diluted by stoplights and position outages. If you don’t have a GPS position lock, like at the start of a run, or going through a tunnel, the time will count, but there’s no distance being logged.

Finally, there is lap pace, which is only applicable if you have the device set to lap during regular intervals, like every km, or every 10 and 1 interval, or every lap on the track. It’s the cumulative distance within the current lap, divided by the cumulative time in that lap. It’s a pretty good indicator – once you’ve gotten past the first minute to smooth out the speed variations, as we saw in the instantaneous pace.

Racing with It

If you haven’t gotten to the pacing strategy talk yet, a common way to approach a race, when you have a time goal, is to plan to run specific sections of the race at a certain pace. That ensures that you properly allocate your energy over the race distance and not run overly fast in the first part of the race, which is very common. You can use a pace band with stopwatch, but you’ll need to remember to check at the km or mile markers on course. With a watch, you can check a lot more easily and frequently.

For instance, a common strategy is to negative split, meaning you run the first half of the race slower than your goal pace, and the back half faster. In order to do this, you can use a pace band and a stopwatch. Or you can use your GPS for more frequent feedback and more frequent adjustments.

GPS watches tend to read “long”, i.e. they show a longer distance than what you actually travelled. There are two major reasons. First, if the accuracy lock is not great, the position recorded swings back and forth from the “real” position, and that makes it appear that you traveled more than an actual pure straight line. The second is that during a race, you’ll rarely run the perfect racing line that the course was certified against. That can actually add up to a few hundred metres over the marathon distance.

It’s key to remember though, the distance markers and timing mats are what counts in a race, not what your GPS says. The log is a great tool for post-mortem analysis of how things went.

Distance Errors

You’ll often find that your GPS will read long, compared to the race markers. That is, it shows longer than your actual distance. That’s because any errors in the position tend to swing your position off the actual track and the zig zag line ends up being longer than the exact route, which is the shortest possible distance. It’s also common to run wide in turns, either by crowds preventing you from getting on the perfect racing line, or general inattention. So remember, it’s the race markers that matter.

Random Tips

  • Keep your GPS on the charger. It uses Lithium based battery chemistry. These will perform best if they are kept topped off. They don’t have the memory effect that other Nickel-based battery chemistries have and keeping them charged reduces the number charge cycles you put the battery through, which will increase their lifespan. Plus, you’re always sure to have full battery capacity at the start of each workout.
  • The chest HR strap will often read wildly at the start of a run. That’s because it’s dry and doesn’t have good electrical connectivity with your chest until you work up a decent sweat. Wet the strap or lick it (ewww!) before you put it on.
  • Don’t put the chest strap in the washing machine.
  • Conversly, optical HR is not sensitive to moisture, but can accidently pick up your cadence. If your HR is reading 170-180 bum at a relaxed pace, you can bet that’s actually cadence. Sometimes, lifting the sensor off your wrist will help the sensor relock on your actual HR.
  • It helps to be standing still to acquire the signal lock. The GPS chipsets seem to be faster at acquiring and more accurate if the initial lock is from a stationary position.
  • If you must start the run immediately (like during a group run), learn the offset trick. Typically, the watch will start picking up position sometime within the first few km, especially as you run out of the downtown core. If you know the exact 1, 2 and 3 km markers on the route, you can figure out the offset of the GPS distance reading. For instance, when you reach the mapped 1km point, look at the distance reading of the GPS. If it says 0.6km, you know your watch started at 0.4km into the run. During this run, you’ll need to add 0.4km to the distance reading on the watch to get your actual distance. This presumes that you’ll have a good position lock for the remainder of the run, of course. But it’s much more common to have accuracy issues at the start of a run, rather than at the end.
  • Learn the reset sequence of your watch. It’s an electronic device, and sometimes will lock up in the middle of the run. Knowing how to reset it can sometimes get you back up and running while you are still up and running. Other times, it just needs a full overnight charge to restore things.
  • Check the internet forums for your device to learn about other tips and tricks, as well as common issues your unit might have.

Current Watch Recommendations

These are my watch recommendations as of sprint 2018. Technology changes rapidly and if you’re reading this later, there may be new models to consider.

Entry Level

  • Garmin Forerunner 35, Vivosmart 3
  • TomTom Spark 3


  • Garmin Vivoactive 3
  • TomTom Spark 3 Cardio+Music
  • Polar M430
  • Polar M600, Apple Watch (Series 3)


  • Garmin FR645 Music, Fenix5 series


I hope this talk has given you some insight into how GPS and HRMs work and how to apply it to your runs and training program. Good luck and good signals!

Useful Links

A basic primer on GPS

DC Rainmaker is the pre-eminent blogger for sports technology reviews and also has excellent articles on GPS accuracy and pacing. Very simply, do not buy a GPS watch without reading his review first.



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