What is the term for a marker placed by the USGS to officially designate a location's elevation above sea level?

The term for a marker placed by the United States Geological Survey (USGS) to officially designate a location's elevation above sea level is a benchmark. Benchmarks are physical markers, often made of metal or stone, that are installed at surveyed locations to indicate their precise elevation. They are crucial for various applications, including mapping, construction, and land surveying, as they provide a reference point for determining the height of other locations relative to sea level.

In contrast, a datum is a reference frame or surface used for measurements, but it does not refer specifically to a physical marker indicating elevation. An elevation marker might seem relevant, but this term is more general and does not specifically signify the formal designation that a benchmark does. An altimeter, on the other hand, is an instrument used to measure altitude, but it is not a physical marker or a reference point established by the USGS. Therefore, the correct answer, benchmark, reflects the precise and official designation of elevation as established by the USGS.