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| Admiralty Charts |
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At sea, Admiralty Chart Datum (ACD) is used in relation to water height. This is based on the Lowest Astronomical Tide (LAT) and is the level below which the predicted tide will never fall, although in practice wind and high pressure may occasionally force the actual water level below this. Values are given in metres and are used as the basis for sounding 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.
Port of Swansea. The datum for tidal predictions in ABP South Wales Ports is Admiralty Chart Datum. Previous to 2009 these ports used an obsolete datum, usually based upon the dock entrance cill.
The ACD zero is 2.82 metres above the cill gauge zero. Typical tide readings will be 2.8 metres above the previous Ordnance Datum of Predictions. | |
| Swansea Marina Tidal Curves Swansea Bay Dredged Channel To Marina |
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Swansea Channel Maintenance Level (ML) is 4.2m below ACD. Swansea Ferryport Maintenance Level (ML) is 4.9m below ACD.
Pilotage Notes:
In Swansea Bay tidal streams flow anti-clockwise for 91/2 hrs (Swansea HW–31/2 to +6), with at times a race off Mumbles Head. From HW–6 to–3 the stream reverses, setting N past Mumbles Head towards Swansea. Keep seaward of Mixon Shoal. When N of SW Inner Green Grounds (SWIGG) SCM lt buoy, Q (6) + L Fl 15s keep to W of the dredged channel and clear of commercial ships. Yachts must motor in harbour and approach, maximum speed 4 kn.
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Marina & Tawe Locks
Enclosed docks and marinas 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 above or below the datum is dredged.
Note: ACD is also referred to as Chart Datum (CD). This is safe enough if there is only one chart datum about.
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