A tall bridge measurement is from the highest point of the structure, such as the top of a suspension bridge tower, to the ground or water below it. Only a bird or maintenance worker can visit the lofty heights of these towering structures. A high bridge measurement is from the road or rail deck elevation to the ground below it-the maximum level at which you and I are at while traveling across the span. There are many variables that should be considered when attempting to find the true height of a bridge. The most obvious of these is the level of the water below the bridge. Elevation drawings drafted by bridge engineers usually refer to the water level surface below a bridge as NW for normal or mean water level. The heights of the bridges within this chapter are often between 5 ft and 10 ft (3 m) higher or lower than the mean water level figures that are listed.