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Yesterday's Run - Why I Don't Trust a Recreational GPS

Thursday, August 22, 2013

So, MapMyFitness was running a promotion where you could sign up for a month of VIP membership for free. I signed up two days after it ended. But I still got it for free because they sent me an e-mail saying, "Last chance to get a free month of VIP!" so I took the bait.

One of the features it added (I think) was the ability to use coaching on the MapMyRun mobile app. I decided to try it out on Tuesday for my 5 mile run. I ran that same route just two days earlier on Sunday as my long slow run. I was tired and sore and ended up running it very painfully slow (for me) at 11:12min/mile pace.

So, I decided that since Tuesday was not supposed to be a slow run day, I would try out the coaching feature and have it help me run a faster pace. I decided to be conservative and have it help me run a 10:30min/mile pace - because I'm not really back to running below 10min/mile for my longer runs, yet.

So, what it does is, you set a time interval (in this case I set it to 2 minutes), and then it yells out your pace at that interval. So, every two minutes my phone would announce to me what my pace was. This way I could adjust my pace every fraction of a mile to keep myself at my desired pace, even if it meant exerting myself a little more than what I felt was comfortable - which was kind of the point.

So, off I went and after 2 minutes my phone tells me I'm running at 10:38 pace. Not bad, but I want to be under 10:30, so I picked it up a bit.

After 2 more minutes it tells me I'm running 10:28 pace. Cool. Right on pace.

2 minutes later it tells me I'm running 10:22 pace. Sweet. I feel like I'm running faster than that, but hey, I just ran a sluggish 11:12 pace on Sunday, so who am I to judge. Most likely I'm still feeling sluggish, just this time I'm pushing myself harder.

2 more minutes later and my phone tells me I'm running 10:08 pace. Hmm, I don't remember speeding up that much. As far as I can tell, I'm still running the same pace that I was.

I hit the 2 mile mark at an intersection and 4 cars pass me so I don't hear what my 1 mile split is. At the end I do hear my phone tell me "Good job!" so that must mean I'm at least close to running a 10:30 pace.

2 minutes later my phone tells me I'm running 9:38 pace. What the heck? I know I haven't sped up THAT much.

2 minutes later and my phone tells me I've back off to 10:28 pace. Ok, so now my mind starts spinning with my knowledge of how GPS's work, and frankly, I'm not really surprised that my pace is wavering.

My pace evens out for the next reading at 10:23, but I can't hear the next two because I'm now running on a busy street. I forgot to mention that I'm not wearing headphones and my phone is clipped to my waistband which makes it hard to hear when cars are driving anywhere within 500 feet of me at 45mph.

So, I pass 2.5 miles and that's when all hell breaks loose. It tells me that I'm running at 9:08 pace and that I am way under my pace and should slow down. However, I know that I haven't sped up much, if at all. Yes, I am running on an ever-so-slight downhill, now, instead of the ever-so-slight uphill i was running on, but nothing THAT drastic.

I hit 3 miles and it tells me I'm running at "29 minutes-" but I don't hear the rest because of road noise. So much for all those 10:28 and 10:22 minute pace read-outs.

After that I don't change my pace at all and my phone then tells me I'm running 8:05 pace and two minutes later that I'm running 11:15 pace. Holy hand grenade!

I run most of the next 1.5 miles on a busy street and I mostly hear "Pace-....-8 good job!" with everything in the middle drowned out.

The last half mile I get two readings before I hit the 5 mile mark and both times I am told I am running way under pace.

So, finally, I finish. My phone tells me I ran 5.11 miles. I've plotted the course on-line and I know it is only 5.02 miles. So, right there my phone's GPS is telling me I ran nearly a 10th of a mile further than I really did.

My final overall time came out to 49:43 for the supposed 5.11 miles. My splits were 9:58, 9:59, 9:39, 9:50, 9:24, and 45 seconds. Most of those extra 45 seconds should be tacked on somewhere else.

So, how did the coaching feature fare?

Well... not good.

For the first mile it kept telling me I was running at 10 min mile pace when in the end I ran it in 9:59 pace. Of course, if the pace was wrong, and my total mileage was wrong, could it also be possible that it located the mile markers wrong, too? Very possible.

And during my second and fourth miles, I got wild pace swings even tho I was not changing my pace at all. From 9:38 to 10:28 during the second mile and 8:05 to 11:15 during the fourth mile.

So, did the coaching feature help me run faster than I did on Sunday? Absolutely. It kept me on my toes and constantly thinking about my pacing and making sure I didn't slow down.

Did the coaching feature help me run the correct pace? No way. After the first 2 miles I quit trusting it, completely. If I sped up every time it said I was slow and slowed down every time I was fast, I would've ended up doing an interval workout.

Knowing the inaccuracy of recreational grade GPS, I will likely never use the coaching feature, again, and will likely also never buy any type of Garmin or other brand GPS watch. I know I just can't trust it to accurately gauge my pace or even the total distance.

I will continue using the GPS on my phone, however, because it at least gives me a close estimation of my splits (despite the slight inaccuracy) and accurate total time, since I broke the band on my running watch in July and don't have the money to replace it, yet.

At the end of it all, tho, I had an awesome workout where I pushed myself a little and ended up running my fastest long run of my current training season. So, maybe I should try it again next time I need a mental boost...
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Member Comments About This Blog Post
    I've used mapmyrun for about 2 years, not the VIP version, just the free one. I knew the GPS was slightly off and the pace always said I was going 20-40 seconds faster than I really was. This was my experience during several races. I recently got a Garmin since I'd like to train more accurately with paces. I used them both on a recent run to see the difference. mapmyrun would say 1 mile for every Garmin's 0.9 or 0.8 mile. That adds up when you're marathon training. And mapmyrun makes me feel speedy. I think the Garmin is more accurate.

    Sorry it was a bad experience for you. Happy running!
    1250 days ago
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