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Apr 2018
7:35am, 23 Apr 2018
64 posts
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willdowney
So I appreciate that with weaving and not following the race line it will be quite rare that someone would run a marathon on 26.2 miles.
Having done London before from Green and Blue Start and other marathons I generally look to finish closer to 26.3 miles
I was on Red start yesterday and ran an extra half mile. I looked at my “flybys” on strava and pretty much all of them were clocking close to 27 miles, some were even over?
So yesterday I appreciate that the heat meant more people were walking so maybe more weaving then normal. Personally I made a point of getting water from every station so a slight diversion off the blue line 23 times yesterday but would all that add an extra ½ mile (More for some people)?
Whats people’s previous experience of starting from red start?
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Apr 2018
8:00am, 23 Apr 2018
19,223 posts
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fetcheveryone
It's an added penalty for $trava users 
One thing you could try is looking at the VLM thread, which lists quite a few of the runners who started on Red, and then looking at their GPS traces for the day. Might show a pattern?
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Apr 2018
10:22am, 23 Apr 2018
19,226 posts
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fetcheveryone
Just looked at the traces for some of our red start runners:
FergusG: 26.32 miles
Brunski: 26.72
jlucas: 26.24
jomo_richmond: 26.31
HOD: 26.5
Maccatheknacca: 26.37
DMZ: 26.26
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Apr 2018
10:40am, 23 Apr 2018
9,281 posts
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Badger
It's highly likely that GPS issues are the reason for most of the difference. The London Marathon route is hell on GPS in places, especially around Canary Wharf, where all the glass buildings are like mirrors for the GPS signals. I ran it some years back wearing a Garmin 310xt, which is one of the most accurate units Garmin have ever made, and I think my distance came out over 27 miles, but it was clear from the track that it was all GPS jumps.
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Apr 2018
2:46pm, 23 Apr 2018
2,464 posts
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FergusG
For what it’s worth, I had both GPS and GLONASS enabled on my watch yesterday in an attempt to minimise signal loss. Jlucas and DMZ must’ve stuck to the blue line like a pair pros
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Apr 2018
2:55pm, 23 Apr 2018
12,243 posts
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Autumnleaves
Not logged yet but I have 26.8ish I think. My watch was .1mi out for the first few miles and then got steadily more adrift so I assume it's just the GPS.
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Apr 2018
3:11pm, 23 Apr 2018
4,503 posts
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larkim
Weaving doesn't add as much distance as you might think.
Let's assume a marathon in a completely straight line, 42.185m long. The shortest route is therefore 42.185m.
Now, let's assume that this is a 10m wide road, and the ideal line is running down the middle of it.
If for every 84.37m you head for the outside of the road, and then back to the middle, then outside to the other side and then back again (i.e. creating 500 triangles with straight sides 84.37m and 5m, and a hypotenuse not yet calculated) then Pythagoras would tell you that the distance from the middle of the road to the edge would be 84.52m (sqrt of ((5^2)+(84.37^2))). So doing that 500 times would cost you 7500cm, or 75m over race distance. So 42.260m or 26.26 miles.
I reckon weaving that much across the road persistently throughout the race would be a fairly extreme version of weaving, but if you do it 1000 times instead of 500 times it would still only add 295m to the total distance run.
Looking at various strava feeds etc, you can see where the distance goes haywire in Canary Wharf etc.
I'd be fairly confident that VLM haven't mis-measured their route!
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Apr 2018
3:12pm, 23 Apr 2018
4,504 posts
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larkim
(Waits for someone to tell me my maths is wrong...)
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Apr 2018
4:00pm, 23 Apr 2018
7,685 posts
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becca7
27.5 miles for me off the Green start and I was able to follow a pretty good racing line throughout. It's just what everyone else has said about tall buildings, tunnels, turns. Normally the interference around Canary Wharf sees my Garmin trace going for a dip in a pontoon.
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Apr 2018
4:14pm, 23 Apr 2018
4,392 posts
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Metro_Nome
I measured long at Paris marathon, which has similar tunnels. At one point, according to my Garmin, I ran a 4 minute mile followed by a 16 minute mile, all while running steadily. After that, my Garmin was out from the mile markers. That was a pretty clear demonstration if I needed it of Garmin inaccuracies.
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