Fundamental Benchmark

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A fundamental benchmark

An underground chamber topped with a short granite pillar. The pillar contains an easily accessible height reference point, but the accurately measured level is underground where it is less likely to be disturbed. There are approximately 207 FBMs across the country and whilst a few have fallen into disuse, most have been incorporated into the National GPS Network as Passive Stations.

For a detailed breakdown of the different types of FBM, see Ian Richardson's FBM styles - A Pictorial Guide.