This post is inspired by a great conversation with one of my runners after we discussed his past couple marathons (including Boston – well done Rob).
There have been a number of blog posts on the accuracy of GPS units (here, cool one here, good one here too, very good one here…). If you read all the fine print that came with yours (regardless of maker) they also make it clear there is a margin of error.
I’m not going to rehash what these very good articles have already beautifully articulated. Instead I want to address GPS units and address the use, reliance and non-use of them. Rob noted that at the end of his marathons his GPS unit was reading 26.5 or 26.6 miles. As stated in the articles, certified courses follow an 87 page manual on measuring requirements and are measured multiple times with all tangents in the course considered, etc. The courses were not 26.5 – 26.6 miles long. They were 26.2 miles.
The practical implication is that on race courses you cannot rely on your GPS unit to “pace you” to your new PR. Your GPS unit thinks you’ve gone further than you really have and therefore the displayed pace will show faster times than your real physical pace. Rob’s indicated he was on 8:01/mile pace at 35K while his official split yields a 8:06/mile pace.
Though you may not think this is much let’s look closer. So, you want to qualify for Boston and your target time is 3:30:59. 8:01 pace yields a 3:30:11 and a 8:06 pace yields a 3:32:23. Up until this year (2011) with the new standards going into place (eliminating the 59 seconds), this meant that your GPS unit had you qualifying for Boston with 48 seconds to spare when in reality you missed qualifying by 1:24!
Far fetched? Not so. Rob a consummate pacer used his GPS in a marathon last year. Late in the race he calculated that he was well under his qualifying time and running comfortably. Then time continued to tick away but there was no finish line. The buffer his GPS lulled him into believing he had was nonexistent. Panic started to set in as the finish line appeared in the distance – more than a quarter mile BEYOND his GPS marathon mark! He did qualify… be the skin of his teeth! At the time he swore that the course was long like so many other GPS wearers.
The message to all runners is clear.
GPS units are good for a general gauge of pace and distance. It (like a regular watch) is accurate on the amount of time you are out there. Certified courses are accurate. Individual mile markers may or may not be accurate depending on how the race organizer set them. No not all courses have gone through the certification process and so your local 5K may not be as accurate and in fact your GPS may be more accurate (no way to know for sure).
Use your GPS units for training. But I advise that you modify your pacing by about 1%. If your goal pace is 8:00/mile you should look for about 7:55/mile on your GPS. You are not really running faster. You are merely correcting for the margin of error in your GPS.
And finally my advice is to not become so reliant on a GPS unit in races. Stop trying to match your GPS pace every instant. Instead – use a watch (or at least pay attention only to the timing function of the GPS). And focus on getting to the finish line as fast as possible.