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Ordnance Survey Benchmark

Ref: BM042

This benchmark is located on the north-western corner of the old Central Police Station in Irish Town.

Unlike the more typical benchmark which is found chiselled directly into masonry this one is a wall-mounted metal benchmark, referred to technically as a Flush Bracket. Cast in relief on it are the initials OS for Ordnance Survey and BM for benchmark with its corresponding 5-digit BM number 109.63. The slots were used to provide a steady and permanent fixing point to apply a levelling staff. Flush brackets are rarely found in Gibraltar these days and according to existing records this particular one is one of only three surviving ones.


Ordnance Survey Benchmarks

Ordnance Survey benchmarks can be seen generally in locations throughout Gibraltar. They first appeared in the OS map of Gibraltar carried out in 1865 by the Royal Engineers, represented by the broad arrow symbol. The Royal Engineers used these benchmarks to calculate each position relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark which was a point with a precisely known relationship to the vertical datum of the area, typically mean sea level. A number of benchmarks can still be seen on the base of the Line Wall which marked the extent of the land mass just above sea-level. The location and height of each benchmark was then shown in the army OS maps right up to the 1930’s.

Demolition of many old buildings, road enlargements and the return of dense vegetation to many parts of the Upper Rock has destroyed or obscured many of these benchmarks, but many marks remain extant to this day.