What does the stadia interval refer to in leveling?

The stadia interval refers to the interval between two rod readings on a distant stadia rod. In surveying, particularly in stadia leveling, the technique uses a leveling instrument that is designed with additional crosshairs that can read distances based on the spacing of the crosshairs on a rod, called a stadia rod.

When a surveyor takes a reading on a stadia rod, the additional readings made between the upper and lower stadia lines (the crosshairs) allow for the calculation of distances based on the known ratio of the distance between those lines. Thus, the stadia interval essentially serves as a means of transforming angular measurements from the leveling instrument into horizontal distance measurements. This method can be very effective for measuring long distances quickly, which is why understanding the concept of the stadia interval is crucial for accurate distance and elevation readings in civil engineering and surveying applications.

This understanding also supports the use of stadia rods but does not directly relate to the physical distance between any two arbitrary points or the overall length of the leveling instrument, nor does it pertain to the heights of rods used. Such distinctions clarify the specific role that the stadia interval plays in the leveling process.