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Going to the track? Ignore the GPS
by on Thursday, April 2, 2015  (4 comments)

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It doesn't take much to create very poor measurements on a track

Last week, one of the runners I coach decided to take her workout to a track. Personally, I think this was a good idea as there are advantages to the track that she could benefit from. However, she decided to still keep track of her pace using her Garmin. With her permission, I'd like to share some of her experience because I think she got caught in a worst case scenario of what can go wrong with GPS devices at the track.

In short, she was doing a 2x2 mile workout. She knew to run 8 laps for 2 miles* but, in her first 8 lap repeat, Garmin recorded her as only running 1.76 miles. On the second, I believe she forgot to hit the lap button at the end of her 8 laps and she actually ended up running roughly 9.25 laps before her Garmin registered her going 2 miles. As a result, when she checked her Garmin to see the pace she was running, it was reporting her running about 30 seconds per mile slower than her target pace. Even though she was in fact running about 30 seconds per mile faster than her target pace. She finished the workout very frustrated until I looked at her Garmin tracks and recognized what was going on.

So what happened? How were her distances and paces off by so much? Well, the picture at the top of this post tells most of the story. Garmin recorded her route as being inside the actual track the whole way around. Below are a few close-ups that demonstrate what went wrong.

Again, I think this is a worst case scenario of what can happen but I think it serves as a good warning of what you can experience while running on the track. So, here's what went wrong:

A little off adds up to a lot:

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Garmin was "a little" off. Over 8 laps, a little each lap can add up to a lot.

The first problem was what was happening on the straights. As you can see in the above picture, Garmin tracked her as running down the straight on the inside of the track. This may not seem like a big deal. It's not when you're running down a straight road or trail. Whether it tracks you right where you are, 5 feet to the right or 5 feet to the left doesn't really make a big deal. On the track, though, if it's tracking you consistently 5-10 feet (or more) inside the track, that adds up when you're running multiple laps.

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Worse yet, Garmin recorded her inside the track on both sides

This might not be a problem if Garmin consistently tracks you 5 feet off to the same direction. That's not what she was experiencing, though, as you can see in the image above. Garmin was consistently tracking her on the inside - on both sides of the track. That's where you get inaccurate distances from.

Cutting corners

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Nobody in their right mind would run like that!

The other big problem with GPS devices on the track is what you see happening above on the turns. GPS devices aren't constantly tracking you every millisecond. They record your location once every several seconds and "connect the dots" to estimate the route you actually took. Again, this isn't a problem when you're running straight ahead on a road or trail as we do most of the time. It's a big problem when you're spending half of the time turning.

As you can see, Garmin assumed that she took perfectly straight lines between the points it recorded her location at. In this case, it meant Garmin recorded her running well into the infield, significantly shortening her route over 8 turns per mile.

These two factors combined resulted in Garmin roughly turning her 8 lap repeats into the distance of an actual 7 laps and greatly messing up her reported pace and distance. This is an extreme example of what can happen but I've seen this happen with other runners.

What to do?

So how do we avoid these inaccuracies? Well, you don't have to avoid the track. There are some good reasons for hitting the track. However, you have to understand that tracks and GPS devices don't get along well together. Instead, just use the stopwatch. If you want to check your pace, know what lap splits you're going to be looking for and you can check every 1/4 mile if desired.

* Note: Yes, I know 4 laps on a track isn't exactly 1 mile. It's roughly 9 meters short of being an actual mile. 9 meters, though, is less than 3 seconds at 7 minutes per mile. Unless you have some need to be extremely precise, it's close enough to serve the purpose.

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4 comments
Ryan

For the record, the runner mentioned above went back to the track this week, ran by lap splits and absolutely nailed her workout.

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Ed

That is odd. My Garmin has been right on the money. My 1/4 mile "beeps" or laps, occur at very nearly the same exact spot on the track every time. Not that I have been on the track this year yet. I wonder if I have been lucky, or was she going around people?

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Ryan

Not going around people as far as I know and I'm quite sure not running on the inside of the track. What you see on the straights is not very common but what you see on the turns is quite normal to be honest. The GPS system tends to want to draw straight lines between measured points. Since the points are only measured typically once every 3 seconds at most, you get tracks that look at least somewhat like what you see in that screen capture I took and you end up with short measurements.

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Ed

Next time (it'll be a while) I hit the track I'll take a closer look at my tracked path and see what it shows. I still usually keep an eye on my start and finish location of each lap ensuring they are almost identical.

But this is important to know in case something seems off- I'll know to ignore the GPS and go with more of a stop-watch method as you stated.

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