Which device is primarily used to define the line of sight in modern surveying?

The device primarily used to define the line of sight in modern surveying is the compensator. This optical tool is integral to instruments like the total station or the modern level, as it automatically adjusts for tilt or changes in alignment, ensuring that measurements are taken with the line of sight precisely leveled and aligned. By effectively stabilizing the line of sight, the compensator enhances the accuracy of observations and measurements in survey work, which is crucial for obtaining reliable data.

In contrast, while devices like the bull's-eye bubble and spirit level bubble can aid in setting up equipment on a level surface, they do not actively adjust or maintain the line of sight during survey operations. A plumb bob, similarly, serves to establish a vertical reference but is not used for defining a line of sight during measurement. Therefore, the compensator stands out as the most appropriate device for ensuring precise line-of-sight measurements in modern surveying practices.