What is the minimum number of NGS First or Second Order benchmarks required to establish the Elevation/Vertical control network?

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

Answer: (C)

The need is for a vertical reference that is tied to the national system. Elevation control relies on fixed, highly accurate marks so that all elevations can be propagated consistently. With only one benchmark, you’d know the height at that single point, but you couldn’t define a connected vertical network or test for consistency across multiple points. Having two First or Second Order benchmarks gives you a defined vertical baseline between two well‑documented elevations, allowing you to propagate the vertical reference to other locations, connect to the national vertical datum, and perform closure checks to verify accuracy. More benchmarks add redundancy, but two is the minimum needed to establish a usable Elevation/Vertical control network.