What part of an engineer's transit is set to 0°0'0" reading?

The correct answer is that the vernier scale is set to the 0°0'0" reading. The vernier is a small sliding scale that allows for more precise measurements. In the context of an engineer's transit, it is used to read angles more accurately.

When the vernier scale is aligned with the main scale, it provides a reference point for measurement, typically starting at 0°. This alignment is critical for ensuring accurate angle readings during surveying tasks. As a surveyor adjusts the instrument to measure specific angles, setting the vernier to 0° enables the user to take subsequent readings from that initial point, facilitating precision in establishing bearings and laying out projects.

The other components have different functions. The telescopic sight aids in aiming the instrument at a specific point, the leveling bubble is used to determine if the transit is horizontally level, and the base plate serves as the foundation upon which the instrument rests. These components are essential for the overall functionality of the transit but do not specifically relate to the initial setting of the angular measurement.