Break lines involving a ditch

$($a$)$ always require additional spot elevations between top and bottom of the ditch.

$($b$)$ allow fewer survey measurements to be made when surveying it than if only spot elevations are

measured on it$.$

$($c$)$ are perpendicular to the direction that water flows in the ditch.

$($d$)$ are only required at the bottom of the ditch.