What is the minimum number of NGS first or second order benchmarks used for vertical control?

• A. One
• B. Four
• C. Three
• D. Two

Answer: (D)

Vertical control relies on having stable, independent references to anchor elevations. Using two first- or second-order NGS benchmarks provides the necessary redundancy and a way to verify measurements. If you relied on only one benchmark, any small error in that benchmark’s published elevation or in the leveling process would directly affect all tied elevations with no cross-check. With two benchmarks, you can form a closed leveling path or compare measurements between the two references, detecting and correcting inconsistencies. Because these benchmarks are high-precision, two is the minimum that yields a reliable and testable vertical control network.