International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 2

Number 2

June 2008

149

Tidal Level Predictions in Reference to Their

Datum, Based on Particular, Selective Sources

P. Kabzinski

Gdynia Maritime University, Gdynia, Poland

ABSTRACT: The article presents differences between tidal datum of predictions taken from six official data

sources (British, American and Caribbean comprising both paper and electronic form) for three pairs of ports

located on the Atlantic coast of North, Central and South America. Two day of predictions represent spring

and neap tide. It can be treated as a continuation, a deeper stage of works focused on tidal comparisons. The

most essential trial of tidal datum unification for particular ports gives the more adequate base for comparison

of Height Deviations from reference values (for particular ports: local or mean) and shows wide variety of the

results.

1 INTRODUCTION

One of the most essential elements of safe

navigation within limited waters and harbour

approach phase is the under keel clearance, which is

closely bound with (among others) the level of tide

as a correcting factor to depths read from

navigational chart directly. In some harbours the

phenomenon of tides seems to be the only friend

enabling safe call of a ship.

In that type situations it is unusually essential, to

get exact prediction of changes of water level in

strategic places. The safety of ship and crew

depends on accuracy of tidal predictions in even

degree with the possible quantity of cargo to

transport, and also the time and speed of a vessel on

stage of entry to/ exit from harbour and its adjacent

waters.

2 LIST OF TIDAL DATA SOURCES

2.1 British (worldwide):

− Traditional (paper) – Admiralty Tide Tables

[ATT, vol.2] issued annually [1];

− Electronic (PC software) – SHM (Simplified

Harmonic Method of Tidal Prediction for

Windows) DP560-Edition 2 (2004) [4] and

ADMIRALTY TOTALTIDE DP550, Version 5

(2004); all the above mentioned released by the

U.K. Hydrographic Office [5].

2.2 American (worldwide):

− Traditional (paper) – Tide Tables 2004 issued

annually by International Marine, formerly by the

NOS (National Ocean Service) – a division of the

NOAA (National Oceanic and Atmospheric

Administration) and accepted by the U.S.Coast

Guard [2];

− Electronic – archive data to be found on official

NOAA web site: www.noaa.org [6];

150

2.3 Trinidadian (local):

− Traditional (paper) –Tide Tables 2004 issued

annually by Trinidad and Tobago Hydrographic

Unit [3];

3 PORT SELECTION

Due to tidal sources available on board a vessel,

where the origin of the researches took place, Six

ports were chosen forming 3 geographical pairs,

located as follows:

− SE Caribbean Sea: PORT OF SPAIN and

Scarborough (Trinidad and Tobago);

− East Coast of South America: PUNTA LOYOLA

and Bahia San Sebastian (Argentina);

− East Coast of the United States: CHARLESTON

and Eastport (United States);

Every pair consists of PRIMARY and Secondary

station within their geographical region.

Last two ports represent NOAA water-level

observation stations, that offer additionally rich and

detailed database (available on Internet [6]) including

for example 6 minute step tidal predictions. Such

additional information allowed to:

− use them as a reliable and accurate background

for other tidal data (predictions);

− compare Datum of predictions (heights) – next

paragraph;

4 ASTRONOMICAL AND OTHER ONDITIONS

The term ‘astronomical’ covers all elements and

factors creating the origin of tides. Actual Moon &

Sun condition-as two the most important deter-

minants of tides were specified for days used in

predictions:

*20th December 2004: – Moon’s First quarter [on

18th]; – Moon on Equator (slightly N); – Moon

between Apogee and Perigee; – Sun’s Winter

Solstice [on 21st];

*27/28th December 2004: – Full Moon [on 26th];

– Moon farthest N of Equator [on 26th]; – Moon in

perigee [on 27th]; – Sun’s Winter Solstice [on 21st];

However, one must be aware of particular

weather conditions, such as: heavy rainfall, unusually

low/high barometric pressure, strong on/offshore

winds, etc. Their exact influence on tide (both time and

height) is difficult to evaluate, but may be significant (!)

5 DATUM OF HEIGHTS PREDICTIONS

All predicted HEIGHTS originally represented as

they usually refer to Chart Datum of the largest scale

chart for the locality.

In case of the United States ports it is a level of

MLLW (Mean Lower Low Water). For other ports

mentioned here, it represents levels (Datum)

oscillating between MLWS (Mean Low Water

Springs) or MLLW (Mean Lower Low Water) and

LAT (Lowest Astronomical Tide). Such a condition

creates the primary problem of the water heights

comparisons.

In a few cases-ports declared by tidal publications

as Primary Stations- tidal datum (being Chart Datum

at the same time) is clearly defined as MLLW,

MLWS (American ports) or LAT (Port of Spain).

The Chart Datum for Secondary/Subordinative

Stations remains unnamed (undesignated) among

tide tables used here.

Fig. 1. The most popular Tidal Levels [6]

6 UNIFYING THE TIDAL DATUM

In order to compare tidal datum and water heights in

consequence, the best solution would be to find

their (particular tidal datum) relation to fix Ordnance

Datum, an universal land levelling system for

instance. Unfortunately, such an universal system

does not exist, while majority of countries use their

own levelling systems incomparable to each other.

For that reason it is essential to find another

reference level, even such far from perfection as

MSL (Mean Sea Level).

Table 1 presents the results of Chart Datum

researches for the six ports mentioned here.

151

Table1. Datum of tide level predictions (heights)

Explanations: P-Primary Station, S-Secondary Station,

SOURCE

PLACE

BRITISH

ADMIRALTY

(m below MSL)

UNITED

STATES -NOAA

(m below MSL)

TRINIDADIAN

HYDROGR

UNIT

(m below MSL)

Port of

Spain

LAT (0,73m) P

[MLLW-0,3]

(0,725m) S

LAT (0,73) P

Scarborough

(0,69m) S

(0,425m) S

(0,70) P

Punta

Loyola

(6,2 m) P

[LAT+0,6

=MLWS-1,6]

(6,20m) P

Bahia San

Sebastian

(5,4 m) S

(5,40m) S

Charleston

MLWS

(0,9m) P

[LAT+0,4]

MLLW (0,85m)

Eastport

(3,85m -

0,9m*) S

MLLW (2,93m)

(*) Heights originally adjusted by 0.9m ([1] -‘Notes’- page 291) corrected here

to conform with British Datum

Mean Sea Level is to be treated as the average

level of the sea surface over a long period, preferably

18,6 years (USA: 19-year Metonic cycle [the

National Tidal Datum Epoch]). One must be aware

that MSL is not an equal level, a flat surface. It is

just the opposite actually! The MSL surface varies,

fluctuates geographically from place to place in

some extend. For that reason it is simply impossible

to unify and compare tidal/chart datum between

different places. The attempt may be successful (in

relation to MSL) for water heights within particular

port only!

Furthermore, Chart Datum level along the coast

forms very diversified and quite complicated area

(surface), difficult to reconstruct, characterize and

define mathematically.

Table 2. Chart Datum Corrections to apply in order to unify

local tide level (heights)

SOURCE

PLACE

BRITISH

ADMIRALTY

(+…m/--...m)

UNITED

STATES NOAA

(+…m/--…m )

TRINIDADIAN

HYDROGR UNIT

(+…m/--…m)

Port of

Spain

LAT [0,73]* (-0,01m) LAT [0,73]*

Scarborough (-0,01m) (-0,28m) [0,70]*

Punta

Loyola

[6,20]* [6,20]* -

Bahia San

Sebastian

[5,4]* [5,40]* -

Charleston (MLWS

+0,05m)

MLLW [0,85]* -

Eastport (+0,02m) MLLW [2,93]* -

(*) Values in brackets [ ] represent existing Chart Datum

depression [in metres] relating to local MSL.

Table 2 presents values needed to bring water

heights up to the same chart datum within each

port separately. The algorithm used here was

based on assumption that local data sources are

the most accurate and reliable among available

information.

7 MODIFIED TIDAL DATA

High and Low Water predictions used for

comparison [7] have been corrected according to

table 2. The results of the comparison are presented

in following paragraph:

8 FINAL RESULTS, CONCLUSIONS

The general method of comparison was based on

assumption that local data sources are the most

accurate and reliable among available information.

For that reason different reference sources

(Trinidadian for the Trinidad and Tobago ports,

NOAA for United States ports) were used. Lack of

such local data created a necessity to calculate mean

height values as a reference to other predictions.

Tidal height predictions [7] were compared (High

and Low Waters separately) to appropriate reference

data, then mean HW/LW values calculated for

particular ports and days. Figures 2-5 and Tables

3&4 illustrate their composition.

Both Tables 3&4 together with Fig. 2-5 illustrate

detailed results of the research based on

water heights corrected according to Tables 1&2

(Paragraph 6). Height Deviations of values 0,2m and

more are printed in italic, bold (0,5m-0,59m) and

underlined bold (over 0,59m).

Terms Springs and Neaps used here play a key

role to distinguish the biggest and smallest tides

within the lunar cycle. The days does not represent

strict spring and neap tides as their delay from

astronomical moments (Full/New Moon and It’s

First/Last Quarter) varies from port to port up to 4

days.

152

Table 3. High Water heights in relation to local or mean predictions [HW Height Deviations]

Table 4. Low Water heights in relation to local or mean predictions [LW Height Deviations]

153

Owing to very few statistic data used here

drawing any conclusions is simply unacceptable.

Some observations, however, seem to be

important and worth mentioning, such as:

− Notwithstanding tidal datum unification attempt

the differences between water heights are

considerable, quite often exceeding 0,1m,

sometimes even 0,5-0,6m(!)

− The highest values of Height Deviations show

South American ports: PUNTA LOYOLA and

Bahia San Sebastian (Argentina), relating to

Mean water heights(!). Supposedly it is caused

by two factors:

− * relatively much bigger tidal range than in other

compared ports;

− * lack of local tidal predictions sources;

− Neap tide heights presents uneven and more

diversified deviations from reference values than

Spring ones, which are slightly lower.

− In case of traditional British and American tidal

publications, accuracy of predictions seems to be

strictly connected with general classification of

ports distinguishing a group of Primary stations

(called STANDARD PORTS [UK] or

REFERENCE STATIONS [USA]) and

Secondary [UK] (Subordinate [USA]) stations

comprising all other places. Secondary stations

here show in most cases higher Deviations than

Primary ones.

− Electronic tidal information programs treat every

place equally due to harmonic method of

prediction usually used by them (SHM,

TOTALTIDE), being absolutely independent

from above mentioned division and comparison

results simply confirm that –similar Deviations

within every geographical pair [look into

Paragraph 3-PORT SELECTION].

− The TOTALTIDE predictions look very

inaccurate in South American ports (Argentina).

The reason must be insufficient harmonic data

entered manually from ATT vol.2 (CUSTOM

Port option) used here for all Totaltide

predictions, however, U.S. ports predictions were

based on set of 37 harmonic constituents

available on web site [6] and the result is much

better (no more than 0,15m!).

That is probably why despite using the same

source data (except for U.S. ports), predictions

within British sources differ.

HW Height Deviations (Neaps)

0,1

0,2

0,3

0,4

0,5

0,6

0,7

PORT OF

SPAIN

Scarborough PUNTA

LOYOLA

Bahia

S.Sebastian

CHARLESTON Eastport

[m]

Admiralty Tide Tables

American Tide Tables

SHM software

TOTALTIDE (software)

Fig. 2. High Water Height Deviations-Spring Tide

Low Water Height Deviation (Springs)

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

PORT OF

SPAIN

Scarborough PUNTA

LOYOLA

Bahia

S.Sebastian

CHARLESTON Eastport

[m]

Admiralty Tide Tables

American Tide Tables

SHM software

TOTALTIDE (software)

Fig. 3. High Water Height Deviations-Neap Tide

Low Water Height Deviation (Springs)

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

PORT OF

SPAIN

Scarborough PUNTA

LOYOLA

Bahia

S.Sebastian

CHARLESTON Eastport

[m]

Admiralty Tide Tables

American Tide Tables

SHM software

TOTALTIDE (software)

Fig. 4. Low Water Height Deviations-Spring Tide

Low Water Height Deviations (Neaps)

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

PORT OF SPAIN Scarborough PUNTA LOYOLA Bahia

S Sebastian

CHARLESTON Eastport

[m]

Admiralty Tide Tables

American Tide Tables

SHM software

TOTALTIDE (software)

Fig. 5. Low Water Height Deviations-Neap Tide

154

9 SUMMARY

Generally accuracy and reliability of tidal predictions

depends on many factors beginning with data

sources, geographical region till meteorological

ones. To rely upon them, navigator must bear in

mind their limitations and remember: the higher tide

range the bigger caution should be taken while using

tidal prediction.

It is the user’s (navigator’s) responsibility to take

into account as many factors, as possible including

present situation and local conditions to foresee

possible consequences of doubtful information and

assure safety to the ship itself, people and cargo on

board as well.

Presented here considerations are the second

stage of tidal comparison research touching Chart

Datum problem as a joint background for all tidal

heights and a key factor in either cartography or

practical navigation as well. Focusing on that

question seems to be an area worth investigating.

There are still no international norms or regulations

concerning Chart Datum and there is much to deal

with and explore for the future.

REFERENCES

[1] U.K. Hydrographic Office. 2004, NP202-04 Admiralty Tide

Tables vol. 2.

[2] International Marine (accepted by the U.S.Coast Guard).

2004 Tide Tables

[3] Trinidad and Tobago Hydrographic Unit. 2004, Tide

Tables.

[4] U.K. Hydrographic Office. 2004, SHM (Simplified

Harmonic Method of Tidal Prediction for Windows)

DP560-Edition 2.

[5] U.K. Hydrographic Office. 2004, ADMIRALTY TOTAL-

TIDE DP550, Version 5

[6] NOAA web sites: http://tidesandcurrents.noaa.gov/,

http://www.noaa.org/,

[7] Kabziński P. 2006, ‘Comparative analysis of available

information concerning tides, based on particular, selective

sources’

, XV-th International Scientific and Technical

Conference “The Role of Navigation in Support of Human

Activity on the Sea”,Gdynia, Poland, November15-17, 2006

Naval Academy of Gdynia.