What is the term used to describe the line of sight observed through a level instrument?

The term "line of sight" specifically refers to the straight path along which an observer can see through an instrument, such as a level. In masonry and construction surveying, establishing the line of sight is crucial for achieving accurate measurements and ensuring that structures are level and properly aligned.

When using a level instrument, the line of sight helps in determining the elevation differences between various points, thus ensuring that the intended construction is exactly as designed and within proper specifications. This term encompasses the direct observation feature that is essential for any work involving precision layout. The other terms presented do not accurately describe this fundamental concept in surveying with a level instrument.