In the stadia method, what is fixed while measuring distance?

In the stadia method, the angle is fixed while measuring distance. This surveying technique involves a theodolite or leveling instrument that is aligned at a specific angle to the horizontal. When using stadia rods, the observer measures the staff readings at two points on the rod, which are situated a known distance apart. The distance to the staff is calculated based on the readings taken, along with the angle of elevation or depression from the instrument.

Fixing the angle allows for the determination of distances based on the principle of triangulation. The relationship between the angle of the line of sight and the distance to the target points is fundamental to accurately applying the stadia formula. By keeping the angle constant, the measurements become reliable and repeatable.

In contrast, other parameters like length, height, and direction are subject to change during measurements, which can introduce variability in calculations. Therefore, anchoring measurement to a fixed angle ensures precision in determining distances within the stadia method, making it a vital characteristic of the technique.