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Tide Gauge bolt marker, King's Bastion

Ref: GM007

King's Bastion gauge pin marker located on South West corner.


Tide Gauge Marks

Tide gauges have a practical application in the shipping and fishing industries where low or high tide levels can hinder or prohibit access to shallow bays or locations with bridges.

The Royal Navy operated a ‘float and stilling well’ tide gauge system at Gibraltar since 1961, however by 2003 the system was considered obsolete and an exercise to update the system was carried out. In 2004 the Proudman Oceanographic Laboratory (POL) was commissioned by the Gibraltar Port Authority to provide a state-of-the-art radar gauge system at the North Mole.

Previously to this date the Royal Navy had fixed a number of bolts and pins on several parts of the Rock in order to establish a permanent fixed reference for what was known as Chart Datum, the fundamental intention of which was to identify a reference level for soundings (depths below the sea surface) below which, other than in the most exceptional circumstances, the sea would not fall. In that way, given the height of tide, navigators could relate the soundings on their charts to the actual depth of water the ship was in.

Today the Chart Datum and Principal Datum are no longer on the same level, the former being below the latter by 0.146m. However, at the beginning of the 1860’s both the Chart Datum and the Ordnance Datum appear to have been one and the same, although each used for different purposes.

King's Bastion pin location.

King's Bastion South West corner.

Tide Gauge bolt marker, King's Bastion Image