This is the GPS map from an activity I recently did. Can you guess what it was? It starts at the green triangle and ends at the red square.
The answer is I was skipping on the same spot for a minute or so. The Yellow star marks where I was yet you’d never know it from what you see. It looks like I’ve started at the top of the garden, gone over the fence and ended up a couple of doors down. It provides a good example of how inaccurate GPS can be. Not only do I appear to move, but the trail never even touches where I was actually skipping.
GPS (or Global Positioning System) was originally set up for the U.S. military and comprises a set of satellites in geostationary orbit that are able to pinpoint a position to within a few metres. In mid-2023 the system comprises 32 satellites with more planned. it was opened up to commercial interests during the 1990s and these days it’s not the only system available. In recent years other groups of satellites have been launched by Russia (GLONASS – GLObal NAviation Satellite System), China, Japan, India and the European Union. Note that the commercial version of GPS is deliberately not as accurate as the military version.
While GPS navigation units were previously available, the earliest GPS wristwatch was released in 1999 by Casio but this was simply a watch that could tell you where you were standing. It wouldn’t be until the early 2000s when companies like Garmin began to sell watches allowing people to track their runs. Initially only used by hardcore runners, the market has expanded with people now tracking their training by phones or smartwatches.
GPS can pinpoint you to within 3 metres which is incredible when you consider the satellites are situated at an altitude of 11,000 miles (eleven thousand!) or 20,000km in metric. And yet it’s often not accurate enough for runners. I’ve frequently heard runners proclaim after races “This course is long” or “this course is short”.
As you can see in that original picture the accuracy of GPS in any one moment is questionable. It’s good enough for your car’s SATNAV to get you from A-to-B when you just need to know what road to drive down, but runners want it to accurately represent where they’ve just run. And for the most part it does this – but only to within a few metres.
The first thing affecting accuracy happens when you turn on your GPS watch. It needs to lock onto the satellites. My old Garmin FR610 claims to remember where I last was and that it’ll be ready within thirty seconds; but I’ve always ended up leaving it out on the recycling bin while I’m putting my shoes on. Even when the “Locating satellites” screen says it has locked in, if I check the Accuracy it can still be 15 metres away. If you set off before the satellites have truly been located, the distance is going to be inaccurate. Now, in the case of my skipping picture, I’d already been out for a run so the satellites were locked in.
When accuracy is at its best of 3m it means you can only be 1.5m to the left or right which is barely more than arm’s length. But if the accuracy drops to 6-7m that’s like being stood in the middle of the road and the watch thinking you could be on either pavement.
This is where GPS can start to read long. While you may be running a straight line, GPS may be zigzagging left, right, left, right either side of you as it tries its best to read exactly where you are. The straight line is always going to be the shortest route, any digression left or right will add distance. While a centimetre added here or there won’t impact matters a lot, on a longer run it adds up.
It’s harder for GPS to read short unless there is some kind of signal dropout. That happens if you run under a bridge, under trees or between tall buildings which all stop the watch receiving the satellites’ signal. On a couple of rare occasions I’ve had my watch lose signal for no explicable reason.
If you’re on a twisty course going round sharp corners or doglegs then there’s a good chance your watch may think you’ve cut the corner. While it checks your location frequently, probably every second, the quicker you’re running the more ground you’re covering and the greater chance of it being off with its moment to moment measurements. This could take distance off.
Generally though GPS is accurate enough. When you go to a race what matters is the distance that has been professionally measured and certified. even if your GPS tells you the course is shorter or longer. Most likely it will read long because road races are measured using the shortest line through twists and turns – usually to within a metre of the curb. Most runners just follow the herd in front or find themselves dodging left and right to pass others.
For some the days of the GPS watch never arrived. They’re happy to use a smartphone app like Strava which is more than capable of recording their runs. And it doesn’t just use GPS it boosts accuracy by using the mobile phone signal.
In the old days, runners stood around waiting for their mates to turn up, maybe bouncing up and down for some stretches before setting off. These days, they’re waiting for the watch to lock on to satellites or scrolling through their phone apps to press start the moment they set off.
Big Red Running 