§ Mr. Barry Field
To ask the Secretary of State for Defence what plans he has to revise chart datum; on how many occasions since 1 January the charted depth has been lower than the lowest astronomical tide; and by how much charted low water depths were below chart datum during the storms and spring tides of early 1996. 
§ Mr. Arbuthnot
This is a matter for the chief executive of the Hydrographic Office. I have asked the chief executive to write to the hon. Member.233W
Letter from John Clarke to Mr. Barry Field, dated 6 March 1996:I am replying to your Parliamentary Question (0528H) to the Secretary of State for Defence about chart datum as this matter falls within my area of responsibility as Chief Executive of the Hydrographic Office Defence Agency.There are no plans to revise chart datum in UK waters. Chart Datum (CD) is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom this level is taken as approximately the level of Lowest Astronomical Tide (LAT). LAT is defined as the lowest level which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions.The UK Hydrographic Office (UKHO) maintains and monitors records for three permanently established tide gauges, at Plymouth (Devonport), Portsmouth and Rosyth. These records show that at all three ports the water level has fallen below CD/LAT on one occasion since 1 January 1996. In each case this occurred on 20 February 1996 when at Plymouth the recorded level was 0.18 metres below CD, at Portsmouth 0.10 meters below CD and at Rosyth 0.13 metres below CD. Tidal records for other ports are not held by the UKHO on a permanent basis.I hope this response is of assistance.