GPS watches underestimate distance?

[simonwilliamstutor]

I've noticed a lot of parkruns on Strava that show the distance as just under 5km. My two have been recorded as 4.98 and 4.94. I try to start my watch at the start line and stop on the finish line and I'm guessing a lot of others do the same. So do GPS watches always underestimate distance?

[dagshar Graduate]

Not sure about this. When I previously did park run my watch was spot on; on Saturday it was 30m short (they had changed the route a bit). I guess if you have a course with a lot of bends in it though, then it may depend on where you run. If they have measured it in the middle of the path and you run the corners a bit tighter etc, it's probably easy to lose 30 metres. On the other hand I completed several events in the autumn that were labelled as 10k, but worked out as 9.6km which is 6 miles, so I think the labelling was out. In fact on some of those occasions I noticed that it was the first few k that were short and then later on it matched exactly. At the London Winter run Garmin said at the 8k marker that we were only at 7.7, but in the end it was just over 10k. Other people doing the same run were also either slightly over or slightly under - again I think it's the fact that over the distance it's easy to gain/lose a few metres here and there depending on your exact route.

[Rignold]

Yes, it is very easy indeed to vary by 20-60m over 5000 depending on the line you take on corners etc. I seem to receall reading something about this with olympic runners and their positioning n the pack. 50m over 5k is only 1% but if you have the equivalent of a 50m headstart you have a distinct advantage. And that is hugging the inside line of a track as closely as possible. In open space, with dozens or hundreds (or more) of other runners you can have 20m variance on every bend.

That said GPS watches are not contantly updating by the millisecond. They take a position, then another, and work out the distance between. This does not account for kinks inbetween. If you ran in tight circles for ten minutes you would have run a lot of distance in footfall terms but the GPS would ot know this. Alo there are going to be spot where interference from trees, buildings etc blocks staellite reception and the GPS will have to guess for a while until it gets a new fix, at which it will corrct itself, but again can only do this as the crow flies. You were at point A and are now at point B.

That is really the natre of how GPS devices work. Understandng their limitations and what they are best suited for is important to get the most out of them. For interval training, for example, they are not much cop.

There is a very good article on it here:

fellrnr.com/wiki/GPS_Accuracy

If pinpoint accuracy of distance is your thing then a footpod or similar is the way to go.

Having said all that race distances can vary considerably anyway, particularly cross-country. I have done 10ks that turned out to be 14,plenty that varied up to a km, and on one occasion a 10 miler that ended up just short of 15, athough that was partly deliberate on the organisers part.

Its all running, though.

[Curlygurly2 Graduate in reply to Rignold]

Why are they not much good for intervals particularly Rig?

[Tomas Graduate in reply to Rignold]

Excellent explanation about the limitations of GPS accuracy, Rig. But I think the "foodpod is the way to go" is a bit out dated. Foodpods work simply as a pedometer by counting steps (which the watch can time and do clever things with). Many GPS watches also have an accellerometer nowadays, so they do the same thing without the need for a separate footpod.

Also, I respectfully disagree on the use for interval training. I think they're perfect. If you're doing timed intervals (5 minutes fast, 3 minutes slow) then the accuracy doesn't matter at all. And if you're doing distance based intervals (400m sprint, 200 recovery) then the small inaccuracies does not in any way make the interval less beneficial.

[nhs2015 Graduate]

I have notice this and I think it is the run that is not properly calibrated. Sometimes I am a few metres under sometimes I am a few metres over.

[John_W Graduate in reply to nhs2015]

Easy way to check and calibrate you watch/phone - run 12.5 laps (5000m) around a standard 400m athletics track and see what your device says

[Tomas Graduate in reply to John_W]

Won't work, because whatever issues the watch/phone has with interference (or for the phone, other apps running at the same time hogging the processor) won't be exactly the same all the time

[John_W Graduate in reply to Tomas]

Yes, I'm aware of that - any phone or watch GPS will struggle to give the same value for a fixed distance or route every time. I'm not talking about the inherent *variation* of the measurement.

However, I think many people (and it isn't important to most), don't realise how inaccurate their GPS distance really is. Given that 5 or 10k (or longer) race routes are often not *exactly* the specified distance, then a running track is the best solution for demonstrating just how your inaccurate GPS measurements can be. You are at least taking away the uncertainty of any inaccuracy in the race/route measurement by its organisers.

[Tomas Graduate in reply to John_W]

Sorry John, I misunderstood you. Yes, if the purpose of the exercise is to demonstrate just how inaccurate GPS measurements can be (rather than trying to calibrate and hope to calculate more accurate distances using the GPS in the future), then comparing against the track is an excellent way of doing it.

[baronblaze]

The GPS is also affected by the weather conditions.

[poppypug Graduate in reply to baronblaze]

Ah, that's probably why my Garmin hasn't been able to locate a satellite the last two times I have been out , very windy and rainy . xxx

[John_W Graduate]

Hi Simon,

Some over-estimate.

My TomTom Cardio Runner consistently under-estimates. Using Google Maps to calibrate (it measures a 400 athletic track as 401m - 0.25% out), my own 5km route is measured as low as 4.8 km sometimes - 200 meters!.

Generally, it's about 1.5-2.2% out.

Cheers

John

[Tomas Graduate]

I guess it should also be pointed out that online maps (google or any of the others) are also not accurate. There is no guarantee that the roads are EXACTLY where the map says they will be (guess how Google and the Ordnance Survey gets their data). And the thickness of the line on the map is not a true representation of the width of a road.