Digital Tide Tables and Tidal Currents – Tide Predictions and Navigation Data

Introduction

Digital tide tables and tidal current publications are essential navigation resources for sailors who need accurate information about water levels, tidal ranges, current direction and current speed. Available in digital formats, these resources make it possible to consult, search and process tidal information quickly, whether for coastal navigation, passage planning, harbour approaches or professional maritime operations.

The Nautic Way Maritime Bookshop brings together digital tidal publications and datasets from major hydrographic authorities, including Shom and Admiralty. These resources cover the French coasts as well as the English Channel, North Sea, Atlantic, Mediterranean and many international sailing areas.

Digital tidal information can include tide tables, tidal current atlases, tidal current data, tidal range information and, depending on the product, structured digital data suitable for use with specialist software.

What are digital tide tables and tidal current publications?

Digital tide tables provide predicted times and heights of high and low water for selected ports or locations. Depending on the publication or dataset, they may also provide tidal coefficients, tidal ranges and other information required to interpret the tidal cycle.

Digital tidal current data provide information about the direction and speed of tidal currents at different stages of the tide. These data are particularly important in areas where tidal streams can have a significant influence on a vessel's speed over the ground and on the timing of a passage.

Unlike a traditional paper tide book, a digital resource can allow information to be searched rapidly, copied or processed electronically, depending on its format.

For example, Shom provides digital tidal predictions as well as digital tidal current data for French coastal areas. Some tidal-current datasets provide hourly information for typical tidal coefficients and include current speed and direction.

Digital tide tables from Shom

The Shom is the French national hydrographic and oceanographic service and provides a wide range of digital tidal information for mainland France and French overseas territories.

Digital tide tables provide predicted high and low water times and heights for ports and locations. Depending on the area, tidal coefficients may also be provided. Shom tide-table data can be supplied in several digital formats, including PDF, XML and TXT.

This information is particularly useful for planning navigation along the French Channel and Atlantic coasts, where tidal ranges can be substantial.

The relevant coastal areas include:

• Pas-de-Calais and the Bay of Somme

• Normandy and the Bay of Seine

• The Norman-Breton Gulf

• Northern Brittany

• The Iroise Sea and Finistère

• Brest Bay and Douarnenez Bay

• Southern Brittany

• The Loire estuary

• The Bay of Bourgneuf

• The Charente Pertuis

• The Bay of Biscay

Ports and navigation areas such as Dunkirk, Calais, Boulogne-sur-Mer, Dieppe, Cherbourg, Saint-Malo, Saint-Brieuc, Brest, Douarnenez, Audierne, Concarneau, Lorient, Quiberon, Le Croisic, Saint-Nazaire, La Rochelle and Royan can therefore be considered when selecting appropriate tidal information.

Digital tidal current data

Tide predictions provide information about the vertical movement of the sea, while tidal current data describe the horizontal movement of the water.

This distinction is fundamental for navigation.

A vessel may experience a current that considerably changes its actual speed over the ground. In areas such as the English Channel, the Iroise Sea, the Breton coast or the approaches to major ports, ignoring tidal streams can lead to significant differences between the planned and actual passage time.

Digital tidal-current data can provide information such as:

• Current direction

• Current speed

• Time-related changes in current

• Tidal current vectors

• Typical spring and neap tidal conditions

• Maximum current speeds

• Current information at different depths, depending on the dataset

Shom digital tidal-current datasets for the Channel and Atlantic can provide current components hour by hour for representative tidal coefficients. More specialised 3D datasets can provide current information at several depths, including surface, mid-water and near-seabed layers.

These digital resources are therefore particularly relevant to professional navigation, hydrographic applications, route planning and technical marine studies.

Examples of Shom digital tidal-current areas

Shom provides digital tidal-current information for several French coastal sectors.

In the English Channel, tidal currents are particularly important around Cherbourg, the Cotentin Peninsula, the Norman-Breton Gulf, the Channel Islands and the approaches to the French and English coasts.

The Iroise Sea and Brest Bay are also areas where tidal streams can be strong. Digital information can be used to analyse current direction and speed around Brest and surrounding coastal waters.

The Loire estuary and the approaches to Saint-Nazaire are another example where tidal currents have a significant influence on navigation.

Depending on the product, digital data may be available in formats such as PDF, GeoTIFF, Shapefile or other structured formats. The format therefore needs to be considered according to the intended application and the software used.

Admiralty digital tide information

The Admiralty range provides international tidal information covering a very large number of ports and navigation areas.

Traditional Admiralty Tide Tables cover standard and secondary ports in the United Kingdom, Ireland, Europe and numerous other regions around the world. Digital solutions extend this information through electronic tidal prediction systems.

One of the principal digital solutions is Admiralty TotalTide, which provides tidal predictions for thousands of ports and tidal-stream stations worldwide.

Such information is particularly relevant to sailors undertaking long-distance passages or navigating outside French waters.

Coverage can include:

• United Kingdom and Ireland

• English Channel

• North Sea

• Northern Europe

• Mediterranean

• Atlantic Ocean

• Indian Ocean

• South China Sea

• Southeast Asia

• Pacific Ocean

• Other international navigation areas

Ports such as Dover, Portsmouth, Southampton, Rotterdam, Amsterdam, Gibraltar, Lisbon, Colombo, Singapore and Sydney illustrate the international scope of digital Admiralty tidal information.

Standard ports and secondary ports

An important concept in tide prediction is the distinction between standard ports and secondary ports.

A standard port has detailed tidal predictions calculated specifically for that location. A secondary port is generally associated with a reference port, with corrections applied to the reference predictions.

These corrections may concern:

• Time of high water

• Time of low water

• Height of high water

• Height of low water

• Tidal range

Understanding this distinction is important when using digital tide information for precise passage planning.

The navigator should always verify that the selected port or location corresponds to the intended navigation area and that the appropriate reference information is being used.

Tidal coefficients, spring tides and neap tides

For French waters, tidal coefficients are commonly used to describe the amplitude of the tide.

The difference between spring tides and neap tides has a direct influence on both tidal range and tidal currents.

During stronger tidal conditions, tidal ranges generally increase and tidal currents can become stronger. During weaker tidal conditions, the tidal range and current strength are generally reduced.

Digital tidal-current datasets can therefore provide several representative conditions. Shom, for example, uses typical coefficients such as 45 and 95 for certain digital tidal-current products.

This information can be used to anticipate how the current may influence a planned route.

Using digital tidal information for navigation

Digital tide tables and tidal-current data can be used at several stages of a voyage.

Before departure, the navigator can determine:

• Suitable departure times

• Expected tidal windows

• Times of high and low water

• Expected tidal range

• Possible current direction

• Estimated current speed

• The best time to enter or leave a harbour

• The influence of the tide on a coastal passage

During a passage, tidal information can be compared with the vessel's actual speed over the ground.

This is particularly important when sailing through areas with strong tidal streams, narrow channels, estuaries or harbour entrances.

For example, a sailing yacht travelling along the Brittany coast may use tidal predictions for Brest, Saint-Malo or surrounding ports together with tidal-current information to determine when a coastal section should be sailed.

Tidal currents and route planning

The effect of current should be incorporated into passage planning rather than considered only after departure.

A vessel's speed through the water and its speed over the ground are not necessarily identical. A favourable current can increase the speed over the ground, while an opposing current can reduce it.

Over several hours, even a relatively moderate current can produce a significant difference in the vessel's position.

Digital tidal data are therefore particularly useful when planning:

• Coastal passages

• Offshore passages

• Harbour approaches

• Estuary crossings

• Narrow channels

• Sailing races

• Fishing operations

• Diving operations

• Commercial navigation

For longer passages, tidal information can be combined with weather forecasts, wind direction, sea state and vessel performance.

Digital formats and specialist applications

Digital tidal information is not limited to a single format.

Depending on the source and product, information may be supplied as:

• PDF documents

• Digital tables

• XML files

• TXT files

• GeoTIFF data

• Shapefiles

• Structured digital datasets

• Specialist navigation or prediction software

The appropriate format depends on the intended use.

A PDF may be particularly convenient for a navigator who simply wants to consult a tidal table or current atlas. Structured data may be more appropriate for professional applications, geographic information systems or specialist navigation software.

Shom's digital catalogue includes both tidal predictions and specialised tidal-current datasets, including two-dimensional and three-dimensional information.

Digital tide information for professional navigation

Tidal data are not limited to recreational sailing.

They are also important for:

• Merchant shipping

• Port operations

• Pilotage

• Commercial fishing

• Offshore operations

• Hydrographic surveys

• Dredging

• Marine construction

• Environmental studies

• Search and rescue

• Naval operations

For professional users, the ability to process digital information and combine it with other navigation data can be particularly valuable.

Digital tidal information can also be integrated into broader navigation workflows involving electronic charts, weather data, bathymetry and route-planning systems.

Tides, depth and under-keel clearance

Tidal information also has a direct relationship with available water depth.

In shallow areas, the difference between high and low water can determine whether a vessel can safely enter a harbour, cross a shoal or pass through a channel.

When planning a passage, the navigator can combine:

1. Charted depth

2. Chart datum

3. Predicted tidal height

4. Vessel draught

5. Safety margin

6. Atmospheric effects

7. Sea conditions

This approach is particularly important for vessels with significant draught and for navigation in estuaries, tidal harbours and shallow coastal waters.

Tidal information should therefore always be interpreted together with the appropriate nautical chart and other relevant navigation information.

Why choose digital tide tables and tidal current data?

The principal advantage of digital information is flexibility.

Digital resources allow the navigator to search information rapidly, consult numerous locations and, depending on the format, process data electronically.

They are particularly suitable for:

• Preparing a voyage at home

• Checking several ports before departure

• Planning a coastal passage

• Comparing tidal windows

• Analysing tidal currents

• Working with navigation software

• Professional marine operations

• Updating navigation information efficiently

Digital publications can complement paper nautical charts, sailing directions, almanacs and other navigation documents rather than replacing the entire navigation documentation system.

Nautic Way Maritime Bookshop: digital tidal navigation documentation

The Nautic Way Maritime Bookshop brings together nautical documentation covering tidal prediction and tidal-current navigation.

The range includes digital resources associated with major hydrographic authorities such as Shom and Admiralty, with coverage extending from the French coast to major international sailing areas.

The objective is to provide navigators with appropriate tidal information according to their sailing area, vessel, navigation method and technical requirements.

Whether the requirement concerns a tidal prediction for a French harbour, tidal currents in Brittany, the English Channel or international tidal information for a long-distance voyage, digital tidal publications provide an efficient technical complement to nautical charts and sailing directions.

Conclusion

Digital tide tables and tidal-current publications are fundamental navigation tools for understanding the constantly changing relationship between sea level, tidal range and tidal streams.

For navigation around France, Shom provides digital tidal predictions and detailed tidal-current datasets covering numerous sectors of the Channel and Atlantic coasts. Internationally, Admiralty digital tidal resources provide extensive coverage of ports and tidal-stream stations throughout Europe and around the world.

Used together with nautical charts, sailing directions, weather information and appropriate route-planning methods, digital tidal information helps the navigator anticipate changes in water level and current and make better-informed passage-planning decisions.

Frequently Asked Questions

What is a digital tide table?

A digital tide table provides predicted times and heights of high and low water for a selected port or location. Depending on the product, it can also include tidal coefficients and other information.

What is a digital tidal-current atlas?

A digital tidal-current atlas provides information about the direction and strength of tidal currents at different stages of the tidal cycle. It can be supplied as a PDF, geographic dataset or specialist digital format.

What is the difference between tide and tidal current?

The tide describes the periodic rise and fall of sea level. The tidal current describes the horizontal movement of water caused mainly by the tidal cycle.

Does Shom provide digital tide tables?

Yes. Shom provides digital tidal predictions for French coastal areas and French overseas territories. Depending on the product, the data can be available in PDF, XML or TXT format.

Does Shom provide digital tidal-current data?

Yes. Shom provides digital tidal-current datasets for French coastal areas, including the Channel and Atlantic coasts. Some products provide hourly current information and others provide three-dimensional data at several depths.

Are digital tide tables useful for sailing?

Yes. They are particularly useful for determining departure times, harbour access, tidal windows and the expected influence of currents on a coastal passage.

What is a secondary port?

A secondary port is a location for which tidal predictions are generally derived from a reference or standard port, with appropriate corrections for time and height.

Can tidal currents affect a vessel's speed over the ground?

Yes. A current flowing in the same direction as the vessel can increase its speed over the ground, while an opposing current can reduce it.

Can digital tidal data be used with navigation software?

Some digital tidal products are designed specifically for processing or integration into specialist software. The available formats and compatibility depend on the individual product.

Are digital tide tables sufficient on their own for navigation?

No. Tidal information should be used together with appropriate nautical charts, sailing directions, weather information and the other navigation documents required for the voyage.

Which areas are covered by digital tidal information?

Coverage ranges from French coastal waters, including the English Channel, Brittany and the Atlantic coast, to international areas covered by Admiralty tidal publications and digital prediction systems.

Are digital tidal-current data useful for professional navigation?

Yes. They are used for coastal and commercial navigation, port operations, offshore activities, hydrographic surveys, fishing, marine construction and other professional maritime applications.