The Basics: Tweaking the GPS display for races

Posted by Dave on October 25, 2011 | 11 Comments

Last week at the 7 Bridges Marathon in Chattanooga, the unthinkable happened to my friend Todd: The battery died on his GPS trainer, 15 miles into the race. He had been running at near PR-pace, but without the feedback he was accustomed to, it was difficult for him to adjust, and he ended up missing a PR (personal record) by less than five minutes. He’s not sure he would have made it even with his GPS, but for many of us nowadays, running without GPS is like running blind, so it was bound to have a big effect.

Sure, there are mile-markers, but since Todd wasn’t wearing a wristwatch, he couldn’t time himself on each mile, so he really had no idea how fast he was going. It wasn’t always that way. Back in the 1980s when I ran my first 10K race, there was always a volunteer at each mile marker, calling out the times for that mile. That happens occasionally even today, but in my experience, more often than not, the times are wrong!

Yet even running with a fully-functional GPS can lead to its share of problems. It’s rare that the distance on my GPS matches the official distance of the course: In my case it’s usually a little longer than the official distance. Last weekend at the Bridges Half-Marathon, for example, my GPS measured the course at 13.34 miles (compared to 13.11 for a true half-marathon). If I had been shooting for a particular time — say, the 1:30 I would need to qualify for guaranteed entry to the New York Marathon in 2012, then that extra 0.23 miles would have added over a minute and a half to my time, and that could be the difference between success and failure.

Depending on what type of running you’re doing, there are many possible reasons a GPS can go wrong, but the fact of the matter is, it’s never going to be perfect. When you’re being timed in a race, what you care about is managing your pace over the official race distance.

So how, beyond making sure your battery has enough juice to make it to the end of the race, can you set your GPS device to match the official distance? It’s not an easy task. I usually have my primary GPS screen set up as follows:

  • The top-left corner is what I pay attention to the most. That is my average pace for the current mile. Note that it’s not my “current pace,” which GPS typically does a poor job calculating since it’s affected by trees, buildings, and other obstacles. Average pace eliminates a lot of that uncertainty.
  • On the top-right is the time for the current mile, so I know how far I’ve got to go in this mile assuming I maintain my current pace.
  • At bottom-left is the average pace for the previous mile, which is useful in case I didn’t happen to look down at the timer when I finished that mile.
  • At bottom-right is total distance run (which isn’t very far in this example because I only ran a hundred yards in order to generate a plausible mid-race display for this picture).

This works great for training, because I can easily adjust my pace as I run to match my goal for the workout. But because the GPS measurement might not match the official distance in a race, it can lead to problems.

Consider my race last week. According to the GPS, I ran an average pace of 6:52 per mile. Over 13.11 miles, that works out to a total time of 1:30:01. But because the GPS wasn’t perfectly accurate measuring the distance, when my GPS measured 13.11 miles, I still had 0.23 miles left to run! If I had been trying to run a 1:30 half-marathon, I would have missed the mark. My official time ended up being 1:31:37, for a pace of 7:00 per mile. To run the race in 1:30, I would have needed to run GPS “miles” of 6:44, not 6:52: 8 seconds faster per mile.

In two and a half weeks I will be running a marathon with a target time: To qualify for the 2013 Boston Marathon, I need to complete the Richmond Marathon in 3:25 in or faster. I expect that, once again, my GPS won’t be perfectly accurate measuring each mile. But by how much? Should I just run each mile 8 seconds faster than my goal? Richmond is different from Chattanooga, with different buildings and trees, so it’s unclear whether that would be fast enough. Or maybe that would be too fast, and I’d end up unnecessarily wearing myself out in the first half of the race.

To handle this problem, I plan on making two subtle changes to my Garmin display when I run my next race:

The first change is fairly easy to see. In the lower left, instead of my average pace for the previous mile, it displays total time for the previous lap.

The second change isn’t visible on a screen shot. I turn the system’s auto-lap setting off (on a Garmin Forerunner 305, this is found under the “Training – Training Options – Auto-Lap” menu). This means that my GPS won’t automatically record a lap time at the end of every mile. Instead, I will need to manually indicate when I pass each mile marker.

Obviously I can’t do this when I’m training because none of my training routes have mile markers, but in a race I can record each official mile as a lap, whether my GPS measured it as 0.98 miles or 1.03 miles. In this way, my GPS will stay in sync with the mile markers as I run. After each mile I can easily see if I’m hitting my target pace, and adjust accordingly.

Let’s take the example of last week’s race, where if I had been shooting for a 1:30 overall time, I would have needed to run a 6:52 pace. I run mile 1 by GPS in 6:52, but I see that I don’t actually reach the Mile 1 marker until 7:00. Since that’s 8 seconds slow, I decide to speed up to a 6:44 pace, and when I pass the Mile 2 marker, my lap time for the official mile is 6:52, just what I need. I continue to monitor the pace for each mile, adjusting as necessary. Within a few miles I will have a good sense of the GPS pace I will need to run to make my official target time. I can then check my overall average pace for the race (which I keep on a separate screen), and see if I’m hitting it (remember, I need to make up for those lost 8 seconds in Mile 1).

In the real world, it is still probably best to give yourself at least a 5-second-per mile cushion, just in case you have to stop to tie a shoe or deal with some other emergency, but with this method, you don’t have to worry as much about accounting for GPS error.

If you have any additional tips or tricks for monitoring your pace with GPS, feel free to share them in the comments.


11 Responses to “The Basics: Tweaking the GPS display for races”

  1. Matt
    October 26th, 2011 @ 3:54 pm

    Sadly, this is another example of me reading or hearing about a feature on the Garmin 305 (in this case, auto lap) and saying to myself, “Wow, I didn’t know it actually did that.” Always read the user manual.

  2. Rufus
    October 27th, 2011 @ 1:16 pm

    I’m not really happy using the current lap average pace, since at the beginning of the lap it is more sensitive to your instantaneous pace than at the end of a lap. I wish there was a moving average window, something that says your pace over the last half mile or so, then the sensitivity would remain constant.

    For me I stick with standard old distance, instant and last lap pace, an auto-lap (but I hit the lap button to level it up at the mile markers), and of course heart rate. I also have a screen that is ONLY time, for when I need to stop ruminating on the pace or heart rate or what not and just run. (in this case, I want a blank screen. I think it would be great if Garmin released a dev-kit for this beasty and let us users develop new features for it)

  3. Ben
    October 28th, 2011 @ 9:41 am

    I find that these discrepancies in distance between the course and what your watch indicate are from running a less than optimal line. Over at he has several posts about how a course is officially measured and how to best approach running the optimal line.

  4. The House of Husar
    November 17th, 2011 @ 10:49 pm

    I also have the 305. I have mine set to Time, Average Pace, Distance and HR. For some reason I like to see Average Pace and not current pace. I think it has to do with not getting freaked out about current pace so much.

  5. Joe
    November 22nd, 2011 @ 8:13 pm

    What Ben said. I think there is a higher chance of introducing errors by manually trying to mark each mile then there is with running the shortest line possible.

    I also set my watch to scroll through the various data fields and use total time. I carry an arm timing sheet that I can refer to based on nothing but time elapsed if need be.

  6. Halam
    December 3rd, 2011 @ 12:57 pm

    Do the foot pods help? I use a Nike sportsband and a 305 and find the sportsband smoother on pace indication but the 305 more accurate on distance. Last weekens ultra 35 miles, 32 miles in that order. The two swing past each other on pace and can disagree by as much as 2 minutes per mile so you got to decide which to trust, or neither.

  7. Sylvain
    July 19th, 2012 @ 6:25 pm

    The most important field for me is the average pace of the race to date. I also look at the current lap pace and the instant pace, as secondary info. As far as any discrepancy between the GPS distance of the watch and the actual race distance, I simply mentally calculate how many seconds discrepancy there are between the kilometer bip of the watch occured prior to the Kilometer marker on the course. Say, the watch bipped ten seconds earlier than the 10k marker, that would represent one second per kilometer early, meaning I need to run a second faster than my goal pace throughout the race. I am not sure there is something simpler than this, perhaps I am missing something.

  8. Marcus
    October 23rd, 2012 @ 8:02 pm

    In the case of the 7 bridges marathon, I’m pretty sure the error was due to a problem with the course and not your GPS. At the end of the first mile, I heard several nearly simultaneous beeps from GPS watches including my wife’s, but the first mile marker was still a ways ahead. When I got to the first mile marker my wife’s watch read 1.2 miles. For every mile after that, her watch consistently showed 0.2 miles over the mile marker indicated. At the end of the marathon, her watch read 26.39 miles. My personal belief is that the starting gate was put at the wrong place. When the scheduled start time of 7:00 came, people were still working on putting the antennas onto the starting gate. The gate was then moved into place on the street. At this point, things seemed pretty disorganized, and I think the gate was just put in the wrong place. I’m somewhat surprised that I haven’t read more people complaining about this issue.

  9. Marcus
    October 23rd, 2012 @ 8:03 pm

    Hmmm, and now I see you’re talking about last years marathon. Seems a little strange.

  10. wayno
    November 7th, 2012 @ 4:55 am

    I use a 405 and had it reboot itself during a marathon once at mile 15 and missed my target by 10 seconds. Since then I always wear a light/cheap/standard stop watch on my other arm and put one of those pace tattoos or write the splits on my arm as a backup. Its helpful anyway as my garmin and I deal in KM but some races deal in mile markers, so using the stop watch time and the pace tattoo (which comes in miles and kms) I can usually cross reference targets on the fly…plus it gives you something to do with your mind. Its also worth noting on a big busy marathon if you dont run the exact racing line, especially on corners, you could actually be running longer, so it may not be the gps thats out. For the london marathon which is 40,000 people I’ll usually run a few hundred metres longer as I use the corners to overtake the masses.

  11. Aaron Olson
    November 26th, 2013 @ 3:06 pm

    My garmin 210 and 310 have been incredibly consistent. Half marathons aprox .15 tto long and marathon .3 too long. So when running a half I just tell myself I’m running 13.2 and make appropriate adjustments.