Which point has a lower elevation, given two rod readings 3.5 m and 4.0 m from the same level?

• A. The point with rod reading 3.5 m
• B. The point with rod reading 4.0 m
• C. Both have the same elevation
• D. Elevation cannot be determined

Answer: (B)

When you measure from the same level with a fixed instrument height, the rod reading is the amount the point falls below the line of sight. The point’s elevation equals the height of the instrument above the reference level minus the rod reading. Because the instrument height is the same for both points, a larger rod reading means a smaller elevation.

For example, if the line of sight is at 5.0 m above the reference, a rod reading of 3.5 m gives an elevation of 5.0 − 3.5 = 1.5 m, while a rod reading of 4.0 m gives an elevation of 5.0 − 4.0 = 1.0 m. The point with the 4.0 m reading is lower in elevation.

So the point with the higher rod reading has the lower elevation.