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Ordnance Survey Benchmark

Ref: BM004

This benchmark is located at the base of the Moorish Castle keep.

 It is shown on the 1866 OS Survey Map of Gibraltar (updated up to the 1940’s) and is represented by the broad arrow symbol and its corresponding BM number 316.24.

 

Ordnance Survey Benchmarks

The broad arrow symbol topped with a horizontal line can be seen chiselled on masonry in locations throughout Gibraltar denoting Ordnance Survey benchmarks which first appeared in the OS map of Gibraltar carried out in 1865 by the Royal Engineers. 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.

Ordnance Survey Benchmark Image

MAP LOCATION