GPS … It’s A Love/Hate Kinda Thing

I’m obsessed with data.  A lot of runners are.  In a sport where you’re most often competing against yourself, there’s something about the black and white nature of numerical feedback that can be motivating.  In my opinion, it’s why a lot of “Type A” personalities are drawn to the sport.

If I’m running, there’s a 99.9% chance my Garmin is along for the ride.  Having pace feedback has driven me to some of my best workouts and races.  There’s nothing quite like the feeling of floating along, checking your wrist, and realizing you’re 15 seconds faster than goal pace.

Except when you’re not.

Or when your GPS watch is just straight up lying to you.


Most runners these days have some sort of GPS watch – be it a Garmin, Suunto, Polar, IMG_7177Fitbit, Apple Watch, etc.  Watches can vary greatly in terms of features but under the hood, they all rely on GPS.  At a high level, it works like this: the watch is determining your position on a periodic basis from the location data provided by the GPS satellite.  It then interpolates between those points to plot your path and determine how far you’ve run and how fast you’re moving.  The problem is that this position data isn’t 100% accurate.  Even in good conditions, GPS has a margin of error of around 3-5 meters.  Certain conditions, such as a cloudy sky or being near tall buildings, can make it even worse.  (Source)

This doesn’t sound like enough to matter, but over the course of a run, it can add up.

I was lucky enough to spend some time in Flagstaff, AZ this year – a magical, forest-filled, oxygen-depleted city that is home to many professional or aspiring-to-be-professional runners.  One of the many perks of running in Flagstaff (aside from the thousands of miles of trails to explore, or the fact that the high altitude whips your butt into amazing shape) is that some of the more frequently traversed routes for workouts have been meticulously wheeled out by Ben Rosario, head coach of the NAZ Elite team.

lakemaryLake Mary Road is a go-to spot for many pros as well as weekend warriors out for their long runs, and it’s precisely where I headed for my first long run at altitude.  Spray-painted distance markers line the shoulder of the road every 1/4 mile — but I noticed something weird by the time I hit mile 3.  My watch was almost always off.  I’d cross the mile marker, but my watch distance would still be reading 0.99 or 0.98.

Doesn’t sound like much, right?  Who cares about one one-hundredth of a mile?

In general, if you’re just out for an easy run – sure, no problem, no need to care.  But if you’re trying to hit specific splits within the context of a workout, that 0.01 can mean an additional 4-5.5 seconds, assuming you’re running somewhere between 7-9 minute pace.  If you’re like me, and you’re supposed to be running 7:30s, and your watch keeps telling you you’re running 7:35s… you’re damn well going to push harder.  I HATE YOU GARMIN, AND YOU WILL SAY 7:30 ON THE NEXT ONE.

kiltieloopAnother real life example: 6×1 mile repeats on a wheeled 1-mile loop.  Garmin was spot on for the first one, and clicked in at 6:58.  Each subsequent mile measured a little longer, a little longer, a little longer.  The last mile beeped, showing me a 7:20.  I threw my hands up in frustration – I thought I had only gotten slower and slower in the workout.  But as my ever-so-patient coach pointed out, I finished about 80m past the actual mile mark.  Again – doesn’t sound like much, right?  Well, covering 80m at a 7-minute pace would take you about 21 seconds.  Turning that 7:20 into a 6:59.  I hate you, Garmin.


GPS watches can provide false feedback in the opposite direction too, and the most frequent case of this is at the track.  Recall the inherent 3-5 meter error – this is exacerbated by the tight turns on a standard 8-lane track.  In a recent workout, I was running 6:40s (as determined by a good old fashioned Timex and counting laps) but my watch was telling me I was running anywhere from 6:07-6:19 pace during the session.

garmindataIn the past, though, I didn’t understand how much of a discrepancy there actually could be.  For almost an entire training cycle, my ignorance led me to believe I was running much faster than I actually was during track workouts.  If the workout was something along the lines of 8x800m with 1 minute recoveries, I would program the workout into my watch.  This meant I was relying on the Garmin tell me when I had covered 1/2 a mile by its own measurement and when the 1 minute was up, rather than counting laps and manually lapping the watch at the end of each repeat.

Although I can’t be sure of the exact delta in retrospect, I know that for many threshold workouts I thought I was running around a 6:45-6:50 pace (or faster!), when in reality, it was probably closer to 7:05-7:10.  One could argue that in the end it didn’t matter – my long run workouts were done on the roads, where the GPS is a little bit more accurate and I was able to hit prescribed paces in those.  For track workouts, I was still running at the correct intensity for each session and therefore deriving the same expected benefit.  But it did skew my perception of my own fitness and gave me a false impression of my current ability, which is not something I personally enjoy.  (Give me harsh truths please. Garmin, I didn’t pay $250 for you to rub my shoulders and tell me I’m awesome.  Have I mentioned that I hate you?)


All this being said, I recognize that my Garmin-hate is superficial.  I truly love my Garmin, I want it in my life, and I very especially want the data it provides.  I just don’t want to be driven crazy by it.  Maybe you’re like me in that regard.  I’m betting you are.  So what can we do?

1) Take your workouts to a measured route.  A lot of rail trails or bike paths through more densely populated areas have spray-painted mile markers.  Talk to other runners in your area and see if anyone has found a hidden gem.  Or, if you have the apparatus, measure out and mark a few miles yourself.  (A few ways to accomplish this: if you can drive the route, measure it with your car’s odometer.  Map the route using a couple different online tools, such as MapMyRun – be careful about where you place points around curves or turns.  If you’re super anal, buy a Jones Counter.)

2) Set your watch to manual lap at the track.  I am sure this seems SO FLIPPING OBVIOUS to many people but I legitimately didn’t realize what a difference it would make and so I’m guessing (hoping?) I am not the only one.  It’s a little more mental work (to remember to hit lap and to not lose track of reps) but it’s very, very worth it.

3) Learn to run by effort.  If you’re not able to frequently run on measured routes, try to teach yourself to be less reliant on your watch for pacing feedback and more tuned in to your body.  (Bonus – this will come in super handy if you ever run a race in a big city, where skyscrapers wreak havoc on the reliability of GPS.)  If you feel like you’re putting in the appropriate effort, it’s highly likely that you are – no matter what the watch says.

4) Be kind to yourself.  We all have off days.  We’re not perfect and we’re not going to nail every workout.  Before you beat yourself up, consider the fact that you’re relying on imperfect technology to “grade” your effort.  GPS watches are great tools, but they shouldn’t be the only way you assess your performance on any given day.