Which principle is similar in purpose to profile levelling?

Profile levelling is a method used in surveying to establish a profile line along a predetermined route, allowing for the determination of the elevation of points along that line. This technique provides crucial information for tasks such as road or railway design, where understanding the terrain's vertical changes is essential.

The principle similar in purpose to profile levelling is differential levelling. Both methods aim to establish height differences between various points. In differential levelling, surveyors measure the vertical distance between a benchmark and other points using leveling instruments to ensure accuracy in height determination over distances. This is directly related to the objectives of profile levelling, where elevation changes along a profile are necessary for design and construction purposes.

Other options like spot elevation measurement focus on individual points rather than a continuous profile, cross-sectioning involves creating a cross-section of land rather than measuring height differences along a route, and triangulation is primarily used for determining horizontal positions rather than elevation changes. These distinctions clarify why differential levelling is the most suitable answer in relation to profile levelling.