Set and measure elevation above sea

Taken literally, it varies every second but usually what is meant by the expression is the vertical distance to the datum of the currently used map coordinate system. Historically it has been determined as the median sea surface level over a time at a specific location. It used to be marked, often with a metal rod inserted in a rock.

The largest World Heritage site is the so-called Struve Geodesic Arc. It is a chain of triangulation points that stretches from northern Norway to the Black Sea.

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