Shot calculations - off the back boards

by IRockTheRed on Feb 2, 2010 12:55 PM EST

Some questions came up in another post that I can't find about where the shot distance is calculated from. I thought I'd do a bit of research on the topic, and see what I could come up with.

*Distance from the Boards to the Goal Line*

Per the 2009-2010 NHL rulebook, the distance from the boards to the center of the goal line as 11'.

The goal line itself is 2" wide.

Here's a diagram, taken directly from the rulebook:

*Looking at a Recent Goal*

*Nicklas Backstrom*'s goal in the recent Caps-Lightning game is categorized in the Play-by-Play as "GOAL WSH #19 BACKSTROM(23), *Wrist, Off. Zone, 15 ft.* Assists: #8 OVECHKIN(41); #14 FLEISCHMANN(21)"

Here's a picture:

In this image, Backstrom looks to be about 5' in front of the goal mouth (given the 4' distance from the goal line to where the crease begins its curve), and the 6' distance from the goal line to the top of the curve. That, plus the 12' to the boards behind the net, is about 18' distance. However, the image is not precise, given I don't have frame-by-frame access to the sequence, so Backstrom shifting closer to the goal mouth before he actually puts the puck into the net is entirely possible.

So it appears, at least to me, that the distance is measured off the boards behind the goal, and not from the goal line. An enquiry to the NHLOA BBoard produced only a, "we aren't sure on that one" response, so I am enquiring further, and will repost here if any change to this is required.

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If my quick mental math is correct then it appears to me that they are measuring the distance from a point directly on the goal line, dead center of the goal mouth, straight to Backstroms aproximate center of mass on the ice.

But keep in mind, I did get an F in pre-algebra once…..

by crafty on Feb 2, 2010 1:19 PM EST reply actions

The next question becomes, if it’s a deflected shot, do they calculate the distance the puck traveled or the hypotenuse of the puck-traveling triangle, which would be the distance from the shooter to the goal?

Q: Your rival, Sidney Crosby: dislike him?
Ovechkin: "I love him."
Q: You send him cards on his birthday?
Ovechkin: "I send him cards every day."

by Marshall Pirate on Feb 2, 2010 1:35 PM EST reply actions

I buy it, but there are still some problems, for example, how do you get a shot distance of 7 feet (see post and comments here)?

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by J.P. on Feb 2, 2010 1:40 PM EST reply actions

Wrap around?

by David Getz on Feb 2, 2010 2:01 PM EST up reply actions

My guess right now is a typo...

OK, the Marc Savard shot?

Let’s take a look at the play-by-play and see if that’s around anywhere…

Savard’s Goals

- Snap, Off. Zone, 13 ft. (BOS vs. CAR – 10/3)
- Snap, Off. Zone, 26 ft. (BOS vs. NYI – 10/10)
- Wrist, Off. Zone, 16 ft. (BOS vs. DAL 10/16)
- Slap, Off. Zone, 48 ft. (BOS vs. DAL 10/16)
- Wrap-around, Off. Zone, 7 ft. (BOS vs. TBL 12/2)

That must be the one… if he’s doing a wrap-around, then he is technically behind the goal when the puck goes in, if I understand wrap-arounds correctly… if it’s calculated off center mass of the player, and his center mass is 5’ behind the goal line, it’s still possible, given reach and the length of his stick, etc.

I need a snappy signature...

by IRockTheRed on Feb 2, 2010 2:09 PM EST up reply actions

Here's Savard...

It’s not very clear, but he is halfway between the goal line and the back wall, and he flipped the puck around behind him in from the side of the net.

I need a snappy signature...

by IRockTheRed on Feb 2, 2010 2:12 PM EST up reply actions

Adding some further information...

I was able to locate (FINALLY) the information on where goals came from, here. It’s neatly charted, with little video cameras showing every goal, and other images for shots and hits. The “video cameras” appear to be positioned slightly back from where the actual play took place.

I measured, pixel-by-pixel, the distance from the boards to the goal line; it is 20 pixels wide, so my scale for this is 20 pixels = 12’.

From there, I measured out 20 pixel lengths to the approximate center of the “video camera,” and guesstimated based on that. Here are my results. For actual video, check the link above, but I think this supports my claim.

I need a snappy signature...

by IRockTheRed on Feb 2, 2010 4:00 PM EST reply actions

The truth is those stats are recorded with no standard metric and are ridiculously, wildly inaccurate.

by Bushwood Bushwhacker on Feb 2, 2010 5:26 PM EST reply actions

I don’t think that’s true, I think they’re recorded via computer on a diagram of the rink and the program calculates the distance.

by David Getz on Feb 3, 2010 7:30 AM EST up reply actions

You give the NHL far too much credit. All facets of the league are far less sophisticated than one might imagine, especially when it comes to recording statistics. (For all who clamor for GPS inside pucks to settle goal disputes… you’re dreaming. They just don’t have enough money to employ that kind of technology.)

All stats are taken by hand. Here are a few posts on the wild inconsistencies in stat recording:

http://www.puckprospectus.com/article.php?articleid=351

http://docs.google.com/viewer?a=v&q=cache:LwgmHvaTyOMJ:www.hockeyanalytics.com/Research_files/SQ-DistAdj-RS0809-Krzywicki.pdf+who+records+shot+distance+nhl&hl=en&gl=us&pid=bl&srcid=ADGEESi-ylXnPjQGVitDvVBk4xTcDq-SGNvQnx9j4iTPdgZbdw8zo8GNMrLk3R8XP_bQFEhmNnbOdobEJfXWzEaU8-1_Ixmfo6O42ReKMMGnLJsApPuFK68qqchkaSiBhiaQ4mRrl0ej&sig=AHIEtbQ—Ee3GEfJw3lvxLlZeobuREBmlA

If that massive second link doesn’t work, google “removing observer bias from shot distance.”

by Bushwood Bushwhacker on Feb 3, 2010 11:09 AM EST up reply actions

Yep, that massive link borked itself.

by Bushwood Bushwhacker on Feb 3, 2010 11:09 AM EST up reply actions

The puck prospectus article is about shot counts, not distance to the goal.

This was a fairly simple question: how does “distance to the goal” when it is visually about 2’ or less become 14’?

It therefore had a simple answer: you measure off the back boards.

Yes, it’s probably based on eyeballing to a large degree; otherwise, we’d hear about those superfancy GPS-enabled pucks – and considering that the NHL changes pucks every two minutes of playing time throughout the course of the game in order to keep the pucks appropriately frozen – this from the NHLOA BBoard and my question on the topic – and that they keep somewhere around 15 pucks per frame in the freezer – there’s just no way.

Of course eyeball measurements can vary wildly, so there will be some variation. But the difference between a 2’ shot (where the shot was taken from 2’ in front of the net) and a 14’ shot (where that same shot was taken from 2’ in front of the net, plus 12’ in front of the boards behind the net) is not an “eyeball error”. The question I was trying to answer was, “where is that measured from,” and it definitely looks to me like that answer is the back boards.

I was not in any way stating that the measurements were accurate in any way; even the most exaggerated “measuring like a guy” wouldn’t mistake 2’ for 14’!

I need a snappy signature...

by IRockTheRed on Feb 3, 2010 11:43 AM EST up reply actions

Hahaha.. I totally agree there is serious weirdness going on with the distances, and measuring off the backboards is an interesting hypothesis. I haven’t personally looked at the numbers enough to weigh in. I would guess that if you found any shots that are less than 3 feet, that would nix the notion.My response was pointed at DMG’s idea that this type of stat is generated in any other way besides a human observation. The first link is much more about shot total, but the second link (pdf) is a crazy extensive study on shot distance recording bias.

by Bushwood Bushwhacker on Feb 3, 2010 12:09 PM EST up reply actions

serious fail. negative rec

by Bushwood Bushwhacker on Feb 3, 2010 12:10 PM EST up reply actions

Check this out

http://www.hockeyanalytics.com/Research_files/SQ-DistAdj-RS0809-Krzywicki.pdf

by Bushwood Bushwhacker on Feb 3, 2010 12:11 PM EST up reply actions

I’d use RFID not GPS. Really cheap too.

I look to the future because that's where I'm going to spend the rest of my life.

by zephyr on Feb 3, 2010 2:56 PM EST up reply actions

What’s RFID? How would it work if you could set it up?

by Bushwood Bushwhacker on Feb 3, 2010 6:43 PM EST up reply actions

And...

Multiply it by 45 pucks (minimum) per game… how cheap is it then?

I need a snappy signature...

by IRockTheRed on Feb 4, 2010 8:11 AM EST up reply actions

Looks like it is a subjective measure

I read this and was intrigued. A little ‘net searching found This article from Hockey Analytics. It shows that the measurement is done ’in house’ and that there is, in fact, some bias as a function of the arena. It’s way too late for me to dive deeply into the article, but if there is a subjective bias by arena, it appears to be less than scientific.

"Right now, I can't wait for the playoffs." -- Mike Green 1/18/2010

by VaMedic on Feb 2, 2010 11:12 PM EST reply actions

Oh quite probably...

…given that they can’t get out there with a laser measuring tool.

But still – the basic question was, “how can a goal from 2’ away from the net be 14’?”

so the basic answer was, “it’s measured roughly off the boards behind the net.”

I need a snappy signature...

by IRockTheRed on Feb 2, 2010 11:22 PM EST up reply actions

Tom Kowal (NHL Referee) has agreed to make enquiries in the next few days to see if he can find out the “official word” on this. I will post his findings when I have them. Yay, NHLOA BBoards!

I need a snappy signature...

by IRockTheRed on Feb 4, 2010 10:53 AM EST reply actions 1 recs

Wow. That is dedication. Rec’d

by Bushwood Bushwhacker on Feb 4, 2010 1:04 PM EST up reply actions

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