What does the stadia interval refer to in surveying instruments?

The stadia interval specifically refers to the distance between the crosshairs, or hairs, in a surveying instrument such as a theodolite or level. This concept is significant in stadia surveying, which allows for distance measurement based on angular measurements and the length of the interval between these hairs. When surveying, the stadia interval aids in calculating the horizontal distance to a target by using the observed angle and scaling based on the distance between these reference points. This method is particularly efficient for measuring distances in areas where traditional tape measures may be impractical.

Understanding the stadia interval is crucial for precise measurements and calculations in surveying, as it connects angular measurement with distance. The other options do not accurately represent the concept of stadia interval, as they pertain to different aspects of surveying and measurement.