Complex sentence: The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a "fundamental benchmark", a point with a precisely known relationship to the level datum of the area, typically mean sea level.
Split into simpler sentences: 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.