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I was under the impression that line calling is done in the following way.

Freeze time at the moment the base of the ball first touches the ground.

Take a "bird's eye view" of the ball, ie imagine looking at it from directly above. (In essence, project the 3D object to the 2D court surface.)

If any of the ball is intersecting the line, then the call is in.

In particular, _the base of the ball need not touch the line, as the 'back' of the ball may be 'overhanging' the line.

This seems natural enough: "is any of the ball intersecting the line?". In particular, the trajectory of the ball doesn't matter: if it's hit flat, almost horizontal, then the cross-section at the point of impact is exactly the same as if it were falling horizontally; one need only see the snapshot, not the whole trajectory.

Now, in the Fritz/Isner match yesterday, it seems that this is not the case.

enter image description here

The cross-section is clearly intersecting the line -- the hawk-eye image on the left is at different time to the video on the right.

Compare this, though, with a Federer/Nadal match of which most of us are aware (it was in Summer 2008...): https://www.youtube.com/watch?v=Y3Ufsrx5J-4. Again, at impact, the ball clearly 'overhangs' the line. However, at no point is the ball actually in contact with the line. This ball was called in by hawk-eye.

So it seems that there is inconsistency. Can anyone share any light on this?

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Your impression of line calling is incorrect. The ball must physically touch the line to be counted in. An "overhang" as seen from directly above the ball (used in other ball sports such as football or hockey) is not enough.

This is easiest to see with clay court games. If the ball clips or skids on the line, it leaves a dusty mark showing this. For close calls, the umpire can leave the chair and go to inspect the line themselves. If there is no mark, the ball didn't touch the line, even if it bounced such that there was a direct overhang.

Hawkeye's accuracy is on the order of three millimetres (3mm). Video might be claimed to show a ball not touching the line when Hawkeye returns an "In" result. This is accounted by the accuracy limitation, the lack of display of which has been a long-standing criticism of the use of such systems in sport.

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  • Thanks for your input! :) -- I had considered clay courts in this, and wondered how they entered into the consideration. Let's not get into the inaccuracies of clay court calling here! \\ My only concern with your hypothesis is the shape of the ball in the hawkeye reviews. Hitting with heavy spin "oblongates" the ball, so that it is in the shape shown by hawkeye. If it is, as you suggest, the entire path of the ball that counts, then the ball would look like that. – Sam OT Mar 1 '20 at 16:09
  • Look at it this way: if the ball were covered in wet paint (and the paint didn't fly off with spin!), what shape mark would it leave on the ground? Answer: not the shape shown in the hawkeye images. (This can easily be seen from the YouTube video link in the original post.) How do you feel that this affects your claims? – Sam OT Mar 1 '20 at 16:12
  • The shape shown by Hawkeye is exactly the shape expected of a deformable ball as it bounces with horizontal movement. "Heavy spin" is not needed, and doesn't even account for much of the elongation, and even a totally non-spinning ball will develop 'forward' spin when it bounces, due to friction with the ground creating a pivot point. – Nij♦ Mar 1 '20 at 21:40
  • I feel that the video in the original post quite clearly disproves what you claim, but I don't have any genuine measurements, just "eyeballing it", so I'm happy to agree to disagree :) -- also, I haven't accepted your answer (yet), as I'm hoping for an evidence-, rather than anecdote-, based one. If you can include links to ITF rules websites, as opposed to just what you believe is the case, then I can certainly accept yours! Just currently you give no external justification for your claims. I'm not saying they're false! Just I'd like to see evidence :) – Sam OT Mar 3 '20 at 22:05

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