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Comment by lpolovets | Hacker News Reader

The article says the tolerance is "3 centimeters in each lane," which feels surprisingly high. The longest race is the men's 1500m freestyle, which is 30 lengths of a 50m pool. That means that in the worst case, pool dimension errors can add 0.9m to someone's race -- about half a second of time in the 1500m. I bet there are races where 0.5 seconds makes a big difference.

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Tolerance works two ways, so the difference could be 1.8m.

However, I would expect that there is a high correlation between the size lengths of lanes in a single pool, so that one doesn't have one lane 3 centimeters too long and the one next to be 3 centimeters too short. It's weird that the standard doesn't make any requirements on that.

Also, touch plates get installed after the pool is constructed. I would expect those could easily be made to compensate for any size differences. That would leave a size difference at the start, but that's easily fixed, too.

Having said that, every few years or so there's a newspaper article of a swimming pool that gets constructed to be 50m, where the constructors forget about those touch plates.

Maybe I misread the article, but it sounds like the effective length of lane can change depending on various environmental conditions. I suppose you may the touch pad distance easily adjustable, but that would be lots of measuring while the swimmers are in the water.

What environmental factors would change the lengths of the lanes?

The concrete of the pool being hot/cold... It could, theoretically I guess, cause it to contract/expand by a non-insignificant amount.

Concrete on a footpath gets slots made every step or so, to prevent cracking from heat expansion. It's hardly surprising that concrete the length of the pool could have a centimeter or so of wiggle room.

From TFA:

(Attempting to construct a concrete pool to any tighter a tolerance is nearly impossible; the effective length of a pool can change depending on the ambient temperature, the water temperature, and even whether or not there are people in the pool itself.)

You could maybe have some sort of system to measure the distance between the two sides of the lane in near-real-time (laser rangefinders can be accurate to ~1mm).

better just to call it a tie than play lawyer ball and break out the lasers for measurements

You got me thinking about what other factors could affect the result at various degrees of precision. By my math [1], the time it takes for the sound of the starting buzzer to reach the ears of swimmers at opposite ends of the pool could actually affect the outcome measured in hundredths of a second. But it's an order of magnitude smaller effect than the lane lengths varying by 3cm. Of course, they could remedy this (and for all I know, maybe they do!) by having one starting buzzer speaker placed at the same distance behind each swimmer.

[1] Each lane is 2.13m wide, so the swimmers in lanes #1 and #8 are 14.91m apart. If the buzzer were placed right next to swimmer 1, it would take ~0.044s longer to reach swimmer 8's ear (using 340.29 m/s as the speed of sound). Or if it were right in the middle between swimmers 4 & 5, it would reach swimmers 4 & 5 ~0.019s before it reached 1 & 8.

For both swimming and track events, each starting block has a speaker to broadcast the gun sound. In the 2012 Olympics, the starting gun was replaced with a fully electronic device.[1]

1. http://www.theatlantic.com/technology/archive/2012/07/the-sp...

Ah, cool, thanks for filling me in on that. The fact that the organizers indeed go to that length/expense to remove the ~0.02s advantage a swimmer would otherwise have due to the speed of sound makes it even stranger to me that they'd accept a 3cm difference between lanes, which seems to have an order of magnitude larger effect in long races (per OP's calculations).

I'd also wonder if there isn't some advantage/disadvantage to being one of the two swimmers on either end (#1 or #8) versus any of the others (#2-7) which each have two swimmers on either side of them. I have no idea which way the effect would go, but it would seem to me that there must be some effect at the level of hundredths or thousandths of seconds. Edit: seems there's indeed an advantage to being in the inner lanes, and they account for it by seeding [1]. (This was probably well known to anyone who pays any attention to or knows anything about competitive swimming at all. :)

[1] https://www.quora.com/In-competitive-swimming-why-are-the-in...

I found this related Physics StackExchange commend in the articles: http://physics.stackexchange.com/questions/99318/is-it-fair-...

They do, although for some reason I can't find anything more official than http://entertainment.howstuffworks.com/olympic-timing2.htm

Differences in wall-to-wall distances at middle and longer distances are less important than in shorter distances. The article uses a 100m race. But consider a 50m race. 3 cm over 5,000 cm is a .06% difference. That's easily 1/100 of a second in a 24 second race.

But in longer races, there are many other factors that add noise to this. For one, swimmers aren't competing in a pool where one has to swim 1,500m and another 1,499m pool. They are making turns. A 3cm different is well within the 'noise' of a flip turn.

Also, there is drafting and bow waves, and other things going on that have a bigger effect than the 3cm per length.

I don't think the length matters because you do the same lane multiple times. So the error should always be a consistent fraction of the distance.

This would only matter for national and world records - swimmers competing in finals are swimming against each other at the same time, under identical pool conditions (ignoring any psychological differences between lanes - e.g. breathing towards your cheering fans and family on odd laps vs. even ones, etc).

Remember that concrete expands and contracts, and the pressure of the water influences that process.

0.5 seconds in the 1500 is almost nothing though. In shorter races the difference is less meaningful

I thought so, too, but looking at https://www.rio2016.com/en/swimming-sw-mens-1500m-freestyle-..., there were quite a few differences smaller than that in Rio. 38 vs 39 is 0.03 seconds (in different heats, but qualification for further rounds is on time), 3 vs 4 is 0.24 seconds in the same heat.

And I don't see how the race length would be a factor here, as it doesn't affect the relative difference.

It's only at the really short races that the error becomes smaller than the accuracy of the timing.

My reading of that was that every lane must be within 3 cm of every other lane.

As they mention the alternative is to not have swimming.

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