What must a survey instrument be equipped with to measure both distance and elevation?

• A. Stadia hairs
• B. Vernier
• C. Compass
• D. Plumb bob

Answer: (A)

Stadia hairs in the telescope enable distance measurement directly through the stadia method. The two horizontal stadia lines in the reticle form a fixed interval, and when you sight a graduated staff, the difference between where the upper and lower hairs cross the staff (the stadia interval) is proportional to the distance to the staff. Using the instrument’s known constants, you convert that interval into a horizontal distance. Since you also know the staff’s height (and the instrument’s height above ground), you can combine the distance with vertical position data read from the staff to determine the elevation of the point relative to the instrument. The other tools don’t provide this built‑in distance-measuring capability (vernier is for fine subdivisions, compass gives direction, and a plumb bob helps with vertical alignment but not distance or elevation).