Some months ago a cyclist in the UK did the first ever 100 mile TT at an average speed of over 30mph.
Sadly the course was found to be a little short so the record does not count.
Garmins measured the course to be only 99.8 miles long. But recently the course was re measured using very accurate wheels and the course only measures 99.575 miles long.
It would appear Garmins measure long by approx 0.25 miles over 100 miles.
What distance measuring accuracy do Garmin claim?
I’ve never seen any specific documentation for it, but I imagine if you traveled in a straight line, it’d probably be within a couple meters… standard for a GPS.
But since routes are never straight, it’s hard to put a +/- on their accuracy. They ping the satellites at intervals, which can alter the way the route is tracked… sharp turns getting cut short, for example. I’ve compared distances with friends after the same race, and it’s typical for us all to have different distances showing on our watches.
They ping the satellites at intervalsThey don’t “ping” satellites, they listen for the GPS satellite broadcasts.
I read somewhere that “recreational” GPS receivers can be expected to be accurate to 10 meters 95% of the time. More than good enough to get your car, airplane or boat where you want to go. For example, 10 meters of accuracy on car means you are getting data within 1-2 car lengths. On a 120 foot long airliner, the “error” might mean the GPS is telling you where seat row 9 is instead of where the pilot is actually sitting, i.e. completely immaterial to the intended use. But, general GPS units are not accurate enough to prepare a detailed accurate survey or map.
There are highly accurate ways to use GPS to measure distances down to inches which surveyors and folks doing detailed mapping use but they require special equipment that can eliminate almost all the error. But that kind of accuracy is not needed for navigation so its not part of most GPS units used for navigation (unless maybe your vehicle is involved in firing missiles at something).
I read somewhere that “recreational” GPS receivers can be expected to be accurate to 10 meters 95% of the time.
In North America, if the device supports WAAS, you can expect an accuracy of ± 3m, 95% of the time.
I don’t know if the Europe has an equivalent correction program.
My view is that people generally credit Garmins, Suuntos and anyone else that makes a handheld GPS device with far more accuracy than the devices merit.
I have a few run routes which are either straight A-B routes, or A-A circuits. I set them using bikely.com (the accuracy of which I also doubt, but importantly if I map the same route many times over on bikely I get the same distance). Most of the routes have overhanging trees. On is reasonable free of buildings, being alongside a river, but runs under a couple of road bridges. Others are in built up areas.
My Suunto distance measurements are all over the place: today on my 750m route, I got distance measurements for my various reps between 610 and 750m. This was running circuits of a city square - the distances were repeated to within a max of 2 metres variance on the ground each time. This sort of variation is typical.
I have a pretty good feel for pace now, and I know my pace was consistent, and sure enough over my reps the spread was no more than 4 seconds. So consistent time plus perceived consistent speed tells me that the variations in the distance measured are wrong.
People get fooled a bit on GPS accuracy because car GPS units we are all familiar with “snap” the car icon onto the nearest road unless they are really way way off in terms of accuracy. In reality, the signal quite often has you slightly off the road but the units are smart enough to assume you are not really driving on someone’s lawn. So, it ends up looking to the user like the GPS is super accurate enough to always be able to place you right on the narrow road you are driving on. Bike and running GPS units are not mapping units like car GPS’s so its more likely you will actually see the inherent error.
I have one of the latest and greatest Garmin running watches. The only place I run where I know with reasonable accuracy the actual distance is when I do my tempo run at the local school track. The Garmin consistently underestimates distance. 4 miles should be about 37.5m longer than 16 laps if I stuck to the shortest path next to the white line. I have to leave the inside lane a few times but still the Garmin typically doesn’t finish the 4th mile until about one third of the way down the back straight of my 17th lap - so about 25m per mile under estimating actual distance. I figure it is because it takes a shorter line on the bends than the track actually is.
Anyone else have the same experience?
I have a run a marathon in each of the last two months, both on certified courses. My 910 recorded 26.58 and 26.57. Seems like the accuracy should be better then that?
I have a run a marathon in each of the last two months, both on certified courses. My 910 recorded 26.58 and 26.57. Seems like the accuracy should be better then that?
Unless you ran at the inside of every single corner of every turn, you are going to run a bit further than 26.2, since the measurement for a course assumes an inside track. That alone could add up to a fair bit if the course is twisty.
You think so? Its off 1585 ft out of 138,336ft. GPS does not read in a perfectly straight line. It should always be over. One reading maybe 2m to the right, next 1m to the left. .
You got the exact same measurement on courses that are the exact same length. I would think that means the GPS is pretty accurate and you are just running too far outside of the turns.
Edit: Ooops…should read Kenny’s post before I hit send. He already said it.
I find my old garmin 405 is pretty accurate in that I use in on out and back courses with exact measures out and back, and do the same courses frequently and get dead bang on 99% of the time. Strava on the other hand…
Also remember that a properly measured marathon has a .1% short course prevention factor applied, so your 42.195k/26.21876 mile marathon should measure 42.237k/26.25 miles.
In North America, if the device supports WAAS, you can expect an accuracy of ± 3m, 95% of the time.
1m with a good receiver. Also, there’s no “±” The error measure is a horizontal radius, so there is no sign…negative radial distance wouldn’t make much sense… Would have nitpicked, except you did the same for the guy who used the term “ping.” 
except you did the same for the guy who used the term “ping.”
Kudos to you. You can cross me off your list of people that require retribution.
What distance measuring accuracy do Garmin claim?
Depends on which Garmin. Which one(s) are you referring to?
Also there was a recent paper that suggested that most (all?) GPS units overestimate distance travelled. This might explain the issue.
NY Times article claims some bias in the programming of the units that can lead ton inaccuracy.
I think I saw a bit of a rebuttal to some of the claims on DCRainmakers site.
I used to do track workouts with my Garmin 310xt and it was always off a little. Bend doing my speed work on an old park road since I got my 910. I think I will go and see tomorrow it is supposed to be nice out.
.25 miles in a hundred is a 0.0O25 percent error. That seem pretty accurate to me.
Track workouts are one of the harshest situations you can test a GPS watch in; you’re usually running very quickly around relatively tight bends with few straight sections.
Most watches will perform worse in these situations (e.g. in terms of distance accrual) than on your average road or trail run, all other things being equal.