Why That Single Camera Angle Doesn't Work for VAR

I thought this would be a good place to share a moment from a recent game I watched. The decision on the field was offside, the ball was shot and entered the ball frame. Which of course as a result 50% of the players on the field appreciated and 50% of the players on the field were frustrated by. It's worth noting that although there was a quick reaction "come on", there was nothing that rose to dissent and the game continued without incident.

To start, lets recall the two major elements to offside.
1: Where players were in relation to the offside line at the moment the ball was played by a teammate.
2: If the player became involved in play. In this case the player does shoot the ball, so we know that condition is met.

This moment from the video stream is as close as we can get to the actual moment the ball is kicked, it's currently blocked from view by the white player (1). With higher quality tools and feeds I might be able to get it closer, but this is what we'll have to work with. So, given this is the information we have available, make your decision and where is the offside line?

From this view it appears that (4) is offside, (3) is our offside line, and (2) is onside.

But, what we need to do is consider the location of the camera, the distortion of this angle, and even the lens and zoom being used. To do this we actually need to draw lines out from the actual field to find a center.

To accomplish this, I first drew my three red lines simply to orient the graphic. Because this is on a multisport field I utilized the hashmarks used by American Football. Even looking at the 41yd marker on the left of the screen you can already see the distortion from the 40yd line.

Without these markers I may need to rely on the Goal Area, Penalty Area, and Goal Line markings. This particular field has a burgundy which are not as visible and even hard to spot at times while on the field.

So now I've got an apex in the photo from these three lines. At this point forward every line I draw will cross through this point.

So the next line I drew was the dotted green. I did cheat a bit and used the football markings again at the 38yd line. This happens to be the back foot of players (2) (3) (4). So although in the initial view it looked like (4) was in an offside position with the line drawn, now it's much closer. Because (4) never becomes involved in play we'll remove her from our offside discussion here.

Now wait a second, Dan, you used the back foot. -Why?- Well in this case we need an equal point of measure. We know the turf is flat, so we're not incorporating anything for height, this makes it equal for purposes of this measure.

We measure the offside line from the location of the players body that COULD play the ball legally. This appears at this moment to be the forehead of attacking player (2) and the right shoulder of defending player (3). 

Now, based on how players are standing it appears that there is a slight lean forward. So, if we were to imagine the players position (2) in a forward stride leaning towards goal and position (3) standing more upright with a narrower stride we see how tight of a decision this is.

This is why AR positioning is so critical. Having a square angle with play is necessary as a couple of feet will quickly throw off the ability to make tight decisions. Sometimes even when we glance at a photo or video what seems from one perspective may not be the same view that we have as the official on the match.

Christina Unkel had a much for succinct Tweet on the topic while I was authoring this post.

Credit and acknowledgement are due to Corey Rockwell and Jonathan Johnson who have both shared similar examples in the past and helped to educate me using examples like this.