Intervisibility

A sightline (also sight line) or visual axis is a normally unobstructed between an intended observer (or spectator) and a subject of interest, such as a stage, arena, or monument. Sightlines are a particularly important consideration in and stadium design, road junction layout and urban planning. In cities such as London, construction within sightlines is restricted to protect the key views of famous landmarks.

Subjects that have a direct line of sight with one another are said to be intervisible.

Good sightlines allow spectators to see all areas of a venue stage or field of play. To ensure this designers utilize the C-value.

The vertical distance from a spectator's eyes to sightline of the spectator directly behind is called a C-value. The C-value is a value that helps produce a rake. The stadium bowl rake if based on consistent C-values will follow half of a parabola in section. The curvature will be greatest closest to the pitch and will become an increasingly flatter curve as it moves to the upper rows. As a general rule the rake or curve of the seating bowl will flatten as the first row of seats move away from the side-line for a constant C-value. It is impractical to make different step heights for every single row. In practice the riser height is the same for a set of 4 or 5 rows at the bottom of the bowl with the number of rows in each same height set increasing for higher up rows. The uppermost tier could have only 2 or even 1 different step heights.

It is the lowest rows of seats that are the most sensitive to the impact of the C-value because of the very flat view angle to the touch line. The higher seats can have a reduced C-value without impacting as seriously on the clear view of the field of play.

Sightline criteria in theaters can include: the "isacoustic curve" defined by John Scott Russell in 1838 and applied at the Auditorium Building in Chicago and the Emery Theatre in Cincinnati; alternate row sightlines where each patron sees between the heads of patrons in the row in front and over the heads of patrons in the second row in front; next row sightlines where each patron sees over the heads of patrons in the row immediately in front; and basic considerations like pointing the chairs more or less toward the performance, being able to see the conductor in the pit, being able to see other patrons, being able to see actors on elevated scenery, and not being obstructed by a wall, railing, column, ceiling overhang, loudspeaker cluster, or any other obstruction. The design of sightlines includes considerations of how much of the stage and scenery each patron can see. For example, can each patron see the top of the scenery or not, and can each patron see the whole stage floor or not? Design of sightlines is divided into two related exercises, vertical sightlines and horizontal sightlines. Design of proper sightlines includes resolving both technical and aesthetic issues. At issue is the emotional response of the audience: whether a performer can hold the audience's attention or not; whether the patron feels they had a "good" seat and their ticket was fairly priced or not; whether the audience gets the emotional impact of the performance or not; and whether the patron wants to come back and see another performance in that theater or not. Building codes restrict the maximum and minimum rise per row, limit the minimum width per row, limit the maximum deviation in the size of steps from row to row to achieve the curvature of the rake, and limit other aspects of sightline design.

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