Hello again and welcome to “Questions with the Footnote!”

As I said in last week’s blog please send me email me any questions you might have regarding running. This week ‘monkeytrousers’, who is training for next year’s Virgin London Marathon, got in touch and asked the following question:

“Hey Footnote, can you help with some questions on timings & PBs? I ran a 10k today which my Garmin measured as 10.2km... can I take the 10k time as the time my Garmin measured 10k or do I have to use the race finishing time even if it was longer than 10k? PB either way so not that fussed, but I would like clarity on this!”

Firstly, a very good question Monkeytrousers! For those that have not seen or used one yet a Garmin is a GPS device/watch that uses satellite signals to calculate your current speed and distance from one satellite signal to the next. This comes in handy for runners who want to know how far you ran or how fast you are currently running. Garmin and other GPS tools are a great running tool if they are accurate!

How accurate are they?

I have used many different GPS’s and the simple truth is that GPS or pedometer style devices are not 100% accurate. Sometimes they are accurate (mainly when i run PB’s) but, a lot of the time they are out ever so slightly. I calculate your Garmin’s supposedly “over” measured distance as just under 2% which, is actually not that bad for a GPS device!

There are also many factors that affect the accuracy of your results:

• Multiple Signals — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
• Clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
• Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

Source: http://www8.garmin.com/aboutGPS/

It is therefore reasonable to say that your Garmin’s 10.2km measurement of a 10k course could be incorrect because of the above factors. However your specific results might suggest another reason.

"Run your tangents, soldier!"

Marathons are measured in the shortest distance possible, plus a bit more to make sure they aren't short (only about 40-50 meters for a marathon, 4 to 5 meters for a 5k.)
More likely, you didn't run all the tangents (that is an acquired skill) and in fact unknowingly ran longer than you needed too. In big races running the tangents is very hard to do without cutting up fellow runners. My advice would be to keep your head up and look out for future corners so you can position yourself accordingly (I think of myself as a racing car going into a corner!).

In conclusion, I’m afraid to say you will have to go off the races finishing time as this is official and the most accurate measurement of your course (assuming the race has been accurately measured correctly). Congratulations are still in order for still achieving a PB and your Garmin PB may be a mental incentive that you can actually run this in your next race!

P.S. Got a question for me? If so send me a message on my profile page!

 I feel really old looking back and remember buying the first Garmin that had a watch and a separate satellite receiver box that was the size of my old school lunch box!

New poll for you- Have you ever run a race that was clearly measured incorrectly? 

Tags: garmin, gps, pedometer, distance accuracy, was my race accurate?

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