What is a parabolic curve primarily used for in surveying?

A parabolic curve is primarily used in surveying to create a gradual transition between two adjacent grade lines, which is essential for ensuring smooth changes in elevation or alignment where the road or path changes direction. This is particularly important in the design of roads and railways, as parabolic curves help to minimize abrupt changes in slope, preventing sudden accelerations or decelerations that can be dangerous for vehicles and improve overall safety and comfort for users.

Unlike other options, like checking line of sight or establishing a 90-degree angle, which pertain to different surveying principles and techniques, the use of a parabolic curve focuses on the geometry of alignment and elevation changes in transportation design. The survey of existing terrain conditions relates more to the assessment of the land rather than the construction or engineering design, which is where the parabolic curve plays a crucial role.