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Levels and Datums

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Objectives

By the end of this lesson you will be able to:

Understand what causes tides

Define and determine Chart Datum, MHWS and HAT

Uphold basic navigation safety – through determining whether your ship can safely navigate into harbour and/or under a bridge

Calculate an LDL

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Tidal Ranges in UK

Portsmouth 3.9m

Plymouth 4.7m

Avonmouth 12.2m

Channel Islands 9.6m

Dover 6.0m

Spurn Head 5.7m

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The Earth-Moon System

Moon

Earth

Gravitational Attraction

Barycentre

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Daily Tides

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Tidal Period

The Lunar Day lasts just under 25 hours.

In each period of 24 hours there will be two HW and two LW.

Each HW and LW will occur approximately 6 hours 10 minutes after each other.

The predicted times of HW and LW get progressively later each day by about one hour.

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The Sun’s effect on Tides

Although the Sun has a much greater mass than the Moon, the Sun’s Tide Raising Force is only about 46% that of the Moon.

Although of lesser magnitude, the Tide Raising effects of the Sun on the Earth are similar to those of the Moon. Thus, Tides caused by the Sun will vary.

Earth’s Rotation. The Solar Day is approximately 24 hours; thus when the

Sun’s Declination is zero, the Semi-Diurnal ‘Solar Equilibrium Tide’ will

have two HWs 12 hours apart, interspersed with two LWs also 12 hours apart. The time interval between successive HWs and LWs will be 6 hours. Thus, Tidescaused by the Sun will vary according to the following parameters:

• Change of Sun’s Declination. The Sun’s Declination (that’s the distance of the moon above the equator) changes much more slowly than that of the Moon and reaches a maximum of about 23½/ North and South of the Equator on about 22nd June and 22nd December respectively, these dates being known as the Solstices.

• Distance of the Sun. It takes the Earth about 1 year (approx 365¼ days) to complete its elliptical orbit around the Sun. Perihelion, when the Earth is

closest to the Sun, occurs on about 2nd January, and Aphelion, when the Earth is furthest away, occurs on about 1st July. Thus, the Sun’s Tide Raising Force will be at its maximum in January and at its minimum in July. However, this variation in magnitude is very small, in the order of 3%.

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Sun- Moon System at Springs

New Moon

Solar Tide

Lunar Tide

Full Moon

Twice every Lunar Month, the Earth, Moon and Sun are in line with each other when viewed in the Ecliptic Plane the dotted line on the screen). At ‘New Moon’, the Moon is passing between the Sun and the Earth (which is not visible to an observer on Earth - effectively a ‘black’ Moon); the Moon’s and Sun’s Tide Raising Forces are thus working in ‘Conjunction’. About 14¾ days later, at ‘Full Moon’ (when the Moon is seen as a bright full disc), the Earth is between the Moon and Sun; the Moon’s and Sun’s Tide Raising Forces are thus working in ‘Opposition’. The net result in both cases is a maximum Tide Raising Force, producing what are known as Spring Tides, when higher HWs and lower LWs than usual will be experienced; Spring Tides occur shortly

after New Moons and Full Moons take place.

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Sun- Moon System at Neaps

First Quarter

Solar Tide

Lunar Tide

Last Quarter

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The Monthly cycle due to Moon orbiting the earth

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The daily cycle – due to earth rotation

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Spring and Neap Tides

In each Lunar Month there will be a New Moon and a Full Moon.

At this period the range will be a maximum range which is called a Spring Tide.

Spring tides occur 2 days after New and Full Moon in the UK.

The Moon will also pass two periods of quadrature and the range will be a minimum which is called a Neap Tide.

Tides of this pattern are called Semi-Diurnal Tides and occur throughout the Atlantic region.

For an appreciable Tide to be raised in a body of water, it is essential to generate a large enough Tide Raising Force; to achieve this, the body of water must be large. The great oceans of the world - the Pacific, Atlantic and Indian Oceans – are large enough to permit Tides to be generated.

The Atlantic is more responsive to Semi-Diurnal Tide Raising Forces; thus Tides on the Atlantic coast and around the British Isles tend to be Semi-Diurnal in character (ie two HWs and two LWs per day) and are more influenced by the phases of the Moon than by its Declination.

‘Large’ Springs Tides occur near Full or New Moons with ‘small’ Neap Tides near the First and Last Quarters. The largest Tides of the year occur at

Springs near the Equinoxes when the Declinations of the Sun and Moon are both zero (ie they are over the Equator).

The Pacific is generally more responsive to the Diurnal Tide Raising Forces, and so Tides here tend to have a large Diurnal component. In these areas, the largest Tides are associated with the greatest Declination of Sun and Moon (ie at the Solstices). Areas in the West Pacific off New Guinea, Vietnam and in the Java Sea are predominantly Diurnal with one single HW and LW per day; on the North / East coasts of Java, Tides are purely Diurnal.

Mixed Tides, where Diurnal and Semi-Diurnal Tide Raising Forces are both important, tend to be characterised by a large Diurnal Inequality (see Para 1015, Fig 10-11). This may be apparent in the heights of successive HWs, LWs or both; such Tides are common along the Pacific coast of the USA, the East coast of West Malaysia, Borneo, Australia and the waters of South-West Asia. Occasionally, the Tides may even be purely Diurnal.

The bodies of water of the Mediterraneanand Baltic Seas are too small to enable any appreciable Tide to be generated.

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Typical Tidal Ranges

Open Ocean 0.5m

Largest

Bay of Fundy 17m

Bristol Channel 15m

Smallest

Mediterranean,

Baltic and Caribbean <1m

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Levels and Datums

Except for Vertical Clearances, heights on Admiralty charts are given above MHWS. Mean Sea Level (MSL) is used where there is negligible Tide.

Since 2004, Vertical Clearances have been quoted above Highest Astronomical Tide (HAT) in all areas where there is an appreciable (tidal) Range. MSL is still used where there is negligible Tide. The ‘Heights’ statement beneath the title of each information panel makes it quite clear which Vertical Datum

(HAT, MHWS or MSL) is being used for Vertical Clearances on that particular chart. Where a ‘Safe’ Vertical Clearance has been obtained to avoid the risk of electrical discharge from overhead power cables, it is shown on charts in magenta.

Mean Sea Level (MSL) is the average level of the sea surface over a long period, preferably 18.6 years, or the average level which would exist in the absence of Tides.

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Lowest Astronomical Tide

LAT is the lowest water level predicted under average meteorological conditions and under any combination of astronomical conditions.

Abnormal meteorological conditions may effect this

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Water surface

any height of tide

LAT

Chart Datum

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Chart Datum

Chart Datum is used to reference Depth of Water and Height of Tide for a given area.

The Hydrographer sets Chart Datum at a level which best approximates LAT for the area.

As such, depth information on the chart can be considered a ‘worst case scenario’.

CHART DATUM AND OTHER SPECIFIC REFERENCE POINTS

The most important level to understand is CHART DATUM. Chart Datum is used by the Hydrographer to simplify the chart and to allow the information printed to follow a principle of ‘worst case scenario’.

By international agreement, Chart Datum is defined as a level so low that the Tide will not frequently fall below it. In areas for which UKHO is the authority, Chart Datum is the approximate level of Lowest Astronomical Tide (LAT).

By approximating LAT, Chart Datum does not have to be changed for different areas of the chart.

However, because Chart Datum is so close to LAT it can still be assumed, by and large, that the water level is unlikely to drop below it.

By referring depth to Chart Datum we can therefore assume that we have at least that depth of water in a given area regardless of HoT.

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Water surface

any height of tide

Chart Datum

MLWS

MHWS

MHWN

MLWN

Mean High Water Springs

Mean High Water Neaps

Mean Low Water Neaps

Mean Low Water Springs

Highest Astronomical Tide

HAT

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Tidal Prediction - Publications

Admiralty Tide Tables (4 volumes)

Volume 1 - UK and Channel Coast

Volume 2 - European and Atlantic Waters

Volume 3 - Indian Ocean

Volume 4 - Pacific Ocean

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Admiralty Tide Tables – Table V

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Exercise

1. What is the MHWS for London Bridge?

2. What is the MLWN for Lerwick?

3. What is the MSL for Liverpool?

4. What is the HAT for Belfast?

5. What is the LAT for Galway?

6. What is the LAT for Calais?

7.1m

0.9m

5.3m

3.9m

-0.2m

0.3m

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Charted Depth

The depth of water below Chart Datum.

Blue, Light Blue or White areas

Written as a straight number – Metres, with decimals in subscript

10.5m Depth below chart datum

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Any High Water

Chart Datum

MLWS

MHWS

MHWN

MLWN

Any Low Water

Spring Range of Tide

Neap Range of Tide

Range of Tide

Charted Depth

Height of Tide

HAT

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HEIGHT OF TIDE

Chart Datum

MLWS

MHWS

MHWN

MLWN

Charted Depth

Height of Tide

Actual Depth

Charted Depth + HOT = Actual Depth

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Underkeel Clearance

Draught

Height of Tide

Charted Depth

Tide + Depth - Draught

Chart Datum

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Minimum Clearance 1m

Draught

Minimum Height of Tide

Charted Depth

1m minimum

Chart Datum

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Drying height

By understanding the reference points - Chart Datum and MHWS, it can be seen that the area in between may be covered or uncovered by water.

The area below Chart Datum on a chart appears as blue or white.

The area above MHWS (land) appears as yellow.

The area in between is green.

This is the area that dries out as the tide goes down and therefore has a drying height.

This height is measured above Chart Datum

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Water surface

any height of tide

Chart Datum

MLWS

MHWS

MHWN

MLWN

Drying Height

Charted Depth

Rock

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Drying Heights

Charted Drying Height 0.8m above Chart Datum

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Charted Information

4

6

2

1

4₂

3₂

0₂

6₃

3₂

2

1

2₂

Bank dries one to two metres when tide falls to Chart Datum

Sounding

Charted Depth

Chart Datum

MHWS

High Water Line at MHWS

Height of Tide

Inter-tidal Zone

Chart Datum

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Sample problems

Chart Datum

If the charted depth was 5.0m and a rock was 7.0m high

What would be its drying Height?

Rock

Sea Bed

Height: 7m

Charted Depth:

5m

Drying Height = 2m

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Sample problems

Chart Datum

Rock

Sea Bed

Charted Depth: 5m

HoT: 1.5m

Given the Drying height of 2m and a HoT of 1.5m, what will be the height of the rock above the surface?

Drying Height: 2m

0.5m

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Sample problems

Chart Datum

If the HOT is 2.7m what would be the actual depth above the rock?

HoT 2.7m

Rock

Sea Bed

Drying Height 2m

0.7m

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Limiting Danger Line (LDL)

The LDL only relates to a specific HOT or time.

It is normal to work out the worst case LDL (lowest water for the period) for an entry or exit of harbour.

It is recorded on the chart by the use of a continuous line hatched off towards the danger

Draught + Safety - Height of tide = LDL

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LDL

= Draught + Safety – HOT

In this case drawn on at 2.8m

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LDL

= Draught + Safety – HOT

In this case drawn on at -1m

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Charted Elevation

Lights have a charted elevation

When we conduct a coastal passage and expect to see a lighthouse, we need to know its charted elevation to be able to calculate at what range we will first observe it.

These heights are all based on MHWS

Charted Elevation

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Charted Elevation

MHWS

Chart Datum

Charted Elevation

Height above waterline

Height of Tide

Height of MHWS above Chart Datum

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Charted Height

All other structures have a charted height.

These are measured from the ground level or a described other datum

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Charted Clearance

Bridges have a charted clearance

We need to know the amount of clearance for the mast and superstructure under a bridge.

Clearances are all based on HAT – the worst case scenario

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HOT

Chart Datum

MLWS

MHWS

MHWN

MLWN

Charted Clearance of Bridge above HAT

MHWS measured above Chart Datum

BRIDGE

Charted elevation of light above MHWS

HoT measured above Chart Datum

HAT

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Masthead Clearance

Maximum Height of Tide

MHWS

Charted Height

Masthead Clearance

Chart Datum

Masthead Height

HAT

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Charted Elevation - Sample problems

0.7m

A vessel with a waterline to masthead height of 23m is to enter Milford Haven when the HoT is 0.7m.

She has to pass under a bridge with a charted clearance of 21m.

Chart Datum

a. By how much will she clear the bridge?

HAT

7.9m

21m

23m

23.7m

28.9m

= 5.2m

A vessel with a waterline to masthead height of 23m is to enter Milford Haven when the HoT is 0.7m. She has to pass under a bridge with a charted clearance of 21.

a. By how much will she clear the bridge? (Bld 1)

To address these types of questions, we must resolve the fundamental problem. Namely, that the height of the bridge, the HoT and therefore the masthead height, are referenced to 2 different things.

Q. What is the charted clearance related to?

A. MHWS. (Bld 2)

Q. What is HoT related to?

A. Chart Datum. (Bld 3)

To solve this problem, we must relate both the height of the bridge, and the height of the mast to one point, in this case, Chart Datum. First we must ascertain the height above CD for MHWS at Milford Haven from p.318 of ATT. From this we can tell that MHWS Milford Haven is 7.0m above Chart Datum. (Bld 4)

The charted elevation of the bridge is 21m. (Bld 5) Therefore, the total clearance of the bridge above Chart Datum is 28m. (Bld 6)

We must next ascertain the height of the mast above Chart Datum. We know that the HoT is 0.7m. (Bld 7). As HoT is measured above Chart Datum and the masthead height is measured from waterline (Bld 8) it is straight forward to calculate the masthead height above chart datum. In this case 23.7m. (Bld 9)

Once the total clearance, and height of the mast have both been related to Chart Datum, it is only a matter of subtracting one from another. Ie, 28-23.7 . Therefore, she will clear the bridge by 4.3m. (Bld 10)

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Charted Elevation - Sample problems

5.9m

23m

A vessel with a waterline to masthead height of 23m is to enter Milford Haven.

She has to pass under a bridge with a charted clearance of 21m.

Chart Datum

HAT

7.9m

21m

28.9m

b. What is the first time, after 0800 on 22 March 2004 that the vessel can pass under the bridge?

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Objectives

You now:

Understand what causes tides

Can define and determine Chart Datum, MHWS and HAT

Uphold basic navigation safety – through determining whether your ship can safely navigate into harbour and/or under a bridge

Calculate an LDL