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Length, when referring to measurement in one dimension (as in the length of a line segment or a or a piece of string), has a specialized meaning. So does height when it is used in conjunction with base.
If an elevation is marked on a map, but there is no physical mark on the ground, it is a spot height. The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark. A fundamental benchmark is a point with a precisely known relationship to the level datum of the area, typically mean sea level.