What does a vernier scale measure?

The vernier scale is an instrument used primarily for measuring precise lengths, and it can be particularly useful in applications requiring high accuracy. While it's traditionally associated with measuring length, one of its specific uses is to measure angles when attached to the angular measurements of a transit or theodolite.

In this context, when the vernier scale is utilized with a transit, it helps in determining the angle of turn, which is essential for accurate surveying and construction practices. With the capability to provide finer readings than conventional protractors, the vernier scale enhances the precision of angle measurements, making it crucial for applications in various construction tasks.

The other options reference measurements that do not align with the primary function of a vernier scale. For instance, piping length, depth of excavations, and total height of a structure are typically measured using other tools or devices more suited for those tasks, such as rulers, measuring tapes, or level gauges. Thus, the vernier scale's relationship with angular measurement makes it specifically apt for determining the angle of turn in a transit, confirming why this option stands out.