On a USGS map, what is the recommended reference point to determine elevations in the field?

• A. Contour line
• B. Ridge line
• C. Benchmark
• D. Stream gauge

Answer: (C)

A fixed elevation reference point called a benchmark provides the absolute elevation used on USGS maps. Benchmarks have been precisely surveyed and assigned an elevation relative to a standard vertical datum, so you can determine field elevations by locating the benchmark and transferring its known height to your location or by using it in conjunction with surrounding contour lines. Contour lines show lines of equal elevation but don’t give an absolute height at a new point without a reference. A ridge line is just a landform feature, not a fixed elevation reference. A stream gauge measures water level, not land elevation.