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# The Computer Science Behind the First Down Line

What is the science behind that moving first down line that appears superimposed on the field but underneath the players?

By
Sabrina Stierwalt, PhD,
November 23, 2015
Episode #171

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It’s Thanksgiving this week here in the U.S., which means it’s time for food, family, and football. So let’s tackle (see what I did there?) a question that I’ve always had as a casual football viewer: what is the science behind that moving first down line that appears superimposed on the field but underneath the players?

In football, almost every play is aimed at passing that first down line, a marker that indicates a team has successfully moved the ball forward for a total of at least ten yards. For every possession, a team has four chances (or plays) to cross that line. Ideally, a play ends with the ball all the way in the end zone for a touchdown, but falling short of that, passing the first down line at least means a fresh start—resetting their number of chances (or downs) up to four.

Since 1998, fans watching from home have been provided with a bright yellow or orange line that conveniently marks exactly where this first down line lies during each play. That way, fans can easily tell how far their team has made it (or how far they still have to go) on their march forward toward their competitor’s end zone.

So the challenge becomes how do you mark a line on the field that needs to move repeatedly throughout the entire game? How do you make sure that line appears in the correct spot no matter the viewing angle of the current television broadcast camera? Even as a single camera pans across the field, tracking a running player or a thrown ball, how do you make sure that line doesn’t move? Finally, how can you set up the line so that the players appear to run over it, rather than the line crossing over the top and blocking the view of the action?

## Pre-game 3-D Computer Modeling Is Key

To get the first down line on the field for viewers at home, a lot of work is done before the game even starts. A detailed three dimensional model of the field is made, which includes the gentle slope most football fields have in order to encourage water to drain away from the center. This model also notes where every single possible first down line could be placed in relation to the field and its close surroundings.

Then, special mounts are made for the main television broadcast cameras that track each and every movement the camera makes so that the camera positions can be modeled as well. When the camera makes any kind of movement—from manual movement by the camera person or panning across the field to even zooming or focusing—the extent of that movement is carefully recorded and the resulting perspective on the field is modeled in detail.

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