What term describes the central survey line in the design of a road or ditch?

The term that describes the central survey line in the design of a road or ditch is "centerline." The centerline serves as a fundamental reference point for the layout and alignment of the road or drainage feature. It is critical for ensuring that the design is symmetrical and functions properly, as it dictates the overall geometry of the structure. Engineers and designers use the centerline to calculate grades, determine widths, and ensure adequate drainage and accessibility.

In contrast, other terms do not serve the same purpose. A sump refers to a low space that collects water or other liquids, often used in drainage systems but not as a central design reference. A borrow pit is an excavation site where material is obtained for use in construction, while a channel describes the physical pathway that water follows. Neither of these terms relates directly to the central survey line in the design context.