Datums in ABP South Wales Ports and the River Usk
On the 1st January 2009 there was a change in South Wales datum for
tide measurement at each of the five ABP ports – Swansea, Port Talbot,
Barry, Cardiff and Newport - datum changed from local datums to Admiralty Chart Datum. (ACD)
Two principal Datums are widely used across the UK. Ordnance Datum
Newlyn (ODN) which is applicable across the whole of mainland Great Britain,
is a convenient zero value to transfer across the whole country and is used by
civil engineers, builders, mountaineers and other land based users. Values
are given in metres counting positive upwards from zero.
At sea, Admiralty Chart Datum (ACD) is used. This is a level declared for a
port or harbour and it’s approaches and is based on Lowest Astronomical
Tide (LAT). LAT is that level below which the predicted tide will never fall,
although in practice wind and high atmospheric pressure may occasionally
force the actual water level below this. Values are given in metres counting
positive downwards and negative upwards from zero.
Admiralty Chart Datum is used as the basis for soundings on all UK
navigation charts. This means that any sounding immediately shows the
minimum depth of water available - with any known tide level being a
‘plus’ which may be added with care.
The enclosed docks in South Wales have various datums, some based on
lock inner cills, some on constructed steps. In some places the dock is
dredged to datum, in others a Maintenance Level (ML) above or below the
datum is dredged.
Tide tables –
Since 2009, only ACD tide heights have been shown in the tables.
The individual port information text has been updated to highlight the changes
and warning reminders inserted. Relative level tables for each port should
help clarify the change.
The widely used Arrowsmith’s Bristol Channel Tide Tables have also
Changed the five ABP ports to ACD, so matching all their other port
Local Survey Charts –
UKD charts are mainly internal to ABP Marine
Management, Dredging and Pilotage, but some are used elsewhere.
From 2009, all regular South Wales tidal-water sounding charts have been drawn using ACD as the datum for soundings.
Tide Gauges -
There was a program of replacement ( completed in 2008) of visual tide boards for all five ports, new boards being made of laser-cut galvanized steel. To avoid confusion, the letters ACD are at the top of each of the new gauges.
Automatic recording gauges and readouts are all ACD.
Various tables and drawings from UK Dredging, both constant (Tidal
Reduction Tables & Curves showing heights before and after HW) and annual
(Acceptance Tables showing a year’s lock availability for vessels of various