<Li> The back judge is positioned deep in the defensive backfield, behind the umpire . He ensures that the defensive team has no more than 11 players on the field and determines whether catches are legal, whether field goal or extra point attempts are good, and whether a pass interference violation occurred . The back judge is also responsible for the play clock, the time between each play, when a visible play clock is not used . </Li> <Li> The head linesman is positioned on one end of the line of scrimmage . He watches for any line - of - scrimmage and illegal use - of - hands violations and assists the line judge with illegal shift or illegal motion calls . The head linesman also rules on out - of - bounds calls that happen on his side of the field, oversees the chain crew and marks the forward progress of a runner when a play has been whistled dead . </Li> <Ul> <Li> The side judge is positioned twenty yards downfield of the head linesman . He mainly duplicates the functions of the field judge . On field goal and extra point attempts, he is positioned lateral to the umpire . </Li> <Li> The line judge is positioned on the end of the line of scrimmage, opposite the head linesman . He or she supervises player substitutions, the line of scrimmage during punts, and game timing . He notifies the referee when time has expired at the end of a quarter and notifies the head coach of the home team when five minutes remain for halftime . In the NFL, the line judge also alerts the referee when two minutes remain in the half . If the clock malfunctions or becomes inoperable, the line judge becomes the official timekeeper . </Li> <Li> The field judge is positioned twenty yards downfield from the line judge . He monitors and controls the play clock, counts the number of defensive players on the field, and watches for offensive pass interference and illegal use - of - hands violations by offensive players . He also makes decisions regarding catches, recoveries, the ball spot when a player goes out of bounds, and illegal touching of fumbled balls that have crossed the line of scrimmage . On field goal and extra point attempts, the field judge is stationed under the upright opposite the back judge . </Li> <Li> The center judge is an eighth official used in the top level of college football . He stands lateral to the referee, the same way the umpire does in the NFL . The center judge is responsible for spotting the football after each play, and has many of the same responsibilities as the referee, except announcing penalties . </Li> </Ul> <Li> The side judge is positioned twenty yards downfield of the head linesman . He mainly duplicates the functions of the field judge . On field goal and extra point attempts, he is positioned lateral to the umpire . </Li>

What's the length of a football field in feet