Which is more accurate?

[LemonieJ]

I plot my routes/distances out on 'Plot a Route' and also on 'Google Maps'. When I run I have Strava on my phone but there is always a discrepancy between the distance I should be running and the distance Strava says I have.

Today I ran 4.4 miles which is correct on both of the online maps. When I finished, Strava said I had only done 4.1 miles? Which one should I trust? Please say the longer one lol

Need to get my new GPS watch sorted!!

[Irishprincess Graduate10]

Definitely the longer one! I have no idea but all I know is that the watches aren't accurate either. In some races my Gamin says one distance and the official kilometer markers say something else. I'd be inclined to say the online maps are more accurate than Strava but I know nothing 😕

This is a question for MarkyD .

[MarkyD Graduate10 in reply to Irishprincess]

Thanks IP, I'm here!

[IannodaTruffe Graduate10]

GPS phones are not as accurate as GPS watches, in general. Plotting on an on screen map is open to a lot of error. I have just discovered magic.gov.uk website which has large scale Ordnance Survey maps and drawing and measuring tools, enabling precise plotting.

I have run with phone and watch at the same time and occasionally they agree with each other.

[MarkyD Graduate10]

Good news LJ! You can trust the measured route at 4.4 miles 😀.

There are a couple of sources of error in a phone GPS system. First, and perhaps most importantly, the GPS needs to get an accurate 'fix' on the constellation of satellites. The official GPS system takes quite a while to get a fix (a minute or two) and this is dependent on a clear view of the sky, without trees and buildings in the way. Of course the radio signals from the satellites are not blocked by these, but you'll get multipath propagation effects. Which confuse the phone a bit, so it does not know where it is starting from.

The phone might look for nearby WiFi signals to help it get started: it may know the geographical location of a wifi network from the wifi sniffing that Google did when they made the StreetView survey.

Sorry, got a bit tangential there. My point is that your phone probably doesn't have a good idea of where it started from at the beginning of the run, particularly if you simply started strava & then ran. So then it guesses your route until it finally gets a lock. So you distance never has a hope of being right.

Next, GPS is not very accurate: 10 metres is a good rule of thumb. So on a winding running route, or even one with several turns and corners, the run is going to be estimated. GPS simply can't deal with a tight corner. So whilst you run round a nice radius, GPS plots a route from the start and finish, effectively 'cutting the corner'. And cheating you on distance.

Don't get too hung up on all this. The exact distance during training is not 100% critical. Use whichever measurement is most convenient for you. I always use the distance from my running watch. It gives me a number, and it may be more or less than my previous run, and subject to some errors. But I assume it is always wrong by the same amount.

Running watches tend to acquire a fix faster than a phone, typically by using some tricks and they also use GLONASS constellation with better positional accuracy. So they are inherently more accurate.