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.