Hydrography Manual

Understanding hydrographic surveys, bathymetry and marine charting

Hydrography is the science concerned with measuring, describing and representing the physical characteristics of maritime areas and navigable waters. It is one of the foundations of navigational safety: water depths, seabed characteristics, the position of hazards, water levels, currents, aids to navigation and data accuracy are all essential elements in understanding the marine environment.

The Hydrography Manual is intended for anyone wishing to understand the methods, principles and techniques used to carry out hydrographic surveys and transform observations made at sea into data that can be used for nautical charting.

This documentation is relevant to hydrographic and marine-charting professionals as well as students, engineers, surveyors, technicians, teachers and navigators wishing to develop a deeper understanding of nautical documentation.

Modern hydrography uses a wide range of technologies: GNSS positioning, single-beam and multibeam echosounders, inertial measurement systems, tide gauges, water-level observations, digital data processing and bathymetric models. The manual explains the logic linking these different operations.

What is hydrography?

Hydrography can be defined as the group of sciences and techniques used to acquire and process information relating to the physical characteristics of navigable waters.

It covers in particular:

• measurement of water depths;

• determination of seabed position;

• description of seabed characteristics;

• observation of water levels and tides;

• study of currents;

• positioning of submerged or exposed features;

• detection of hazards to navigation;

• quality control of measurements;

• processing and modelling of bathymetric data;

• production of nautical and cartographic documents.

Hydrographic data therefore form essential source material for national hydrographic offices and organisations responsible for producing nautical charts.

The International Hydrographic Organization, or IHO, plays a major role in standardising hydrographic methods and data. Its S-44 publication defines standards for hydrographic surveys according to their intended use.

The hydrographic survey: from measurement to data

A hydrographic survey consists of collecting measurements over a maritime or inland-water area in order to determine its characteristics to a defined level of accuracy.

A survey may, for example, aim to determine the depths of a harbour channel accurately, monitor an anchorage area, search for a submerged obstruction or update the data required for a nautical chart.

The quality of a survey depends on many parameters:

• horizontal positioning accuracy;

• vertical accuracy;

• performance of the sounding system;

• vertical reference used;

• knowledge of variations in water level;

• density and coverage of measurements;

• equipment calibration;

• data processing and quality control;

• meteorological and oceanographic conditions;

• qualifications and experience of operators.

The IHO specifies that hydrographic survey standards are defined according to the intended use of the data. S-44 is not simply a procedure requiring a particular type of equipment: it defines performance requirements that can be used to assess the quality achieved.

Bathymetry and depth measurement

Bathymetry is a fundamental component of hydrography. It consists of measuring and representing the depth of the seabed or riverbed.

Traditionally, depths were determined using relatively simple sounding methods. Modern techniques can now measure very large numbers of points and produce detailed bathymetric models.

A multibeam echosounder can cover a substantial swath of seabed beneath the vessel. It can provide a high density of measurements and help detect variations in underwater relief.

Single-beam echosounders are also still used for many applications, particularly when the characteristics of the area, the objectives of the survey or operational constraints make them appropriate.

A hydrography manual provides an understanding of the physical and methodological principles behind these different techniques without being limited to the description of a particular piece of equipment.

Positioning and geodetic reference

A depth measurement has hydrographic value only when its position is known with an accuracy appropriate to the purpose of the survey.

Modern positioning relies heavily on GNSS systems, but hydrography also requires an understanding of coordinate systems, geodetic datums and transformations between different reference systems.

Positioning can also be combined with inertial measurement units to account for vessel motion:

• roll;

• pitch;

• yaw;

• accelerations;

• heave.

These parameters are essential for correcting measurements made by echosounders.

Understanding horizontal and vertical datums, ellipsoids, geoids and reference systems is therefore an important part of understanding modern hydrographic data.

Tides, water levels and soundings

A depth measured by an echosounder is not necessarily directly comparable with the depth shown on a nautical chart.

Sea level varies under the influence of tides, storm surges, weather conditions, atmospheric pressure and other oceanographic phenomena.

Water-level observations therefore make it possible to relate bathymetric measurements to a defined vertical reference.

This issue is particularly important in areas with a large tidal range.

In the English Channel, for example, water-level variations can be considerable. Ports such as Saint-Malo, Granville, Cherbourg, Dieppe and Le Havre require a good understanding of the relationship between measured depth, water level and the chart datum.

In the Atlantic, hydrographic data must also take account of tidal characteristics in the Bay of Biscay and along the French, Spanish and Portuguese coasts.

In the Mediterranean, where the tidal range is generally smaller but weather conditions can significantly affect sea level, understanding the vertical reference system remains essential.

Quality, accuracy and uncertainty

One of the fundamental concepts in hydrography is the distinction between a measurement and a measurement qualified by its uncertainty.

Two surveys may produce very similar values while having different confidence levels depending on the methods used, the equipment employed and the conditions under which the data were acquired.

Quality control covers in particular:

• positional accuracy;

• vertical accuracy;

• measurement repeatability;

• seabed coverage;

• detection of anomalies;

• data consistency;

• treatment of outliers;

• traceability of operations.

IHO S-44 defines different levels of requirements for hydrographic surveys. Edition 6.2.0, published in October 2024, is the current edition listed by the IHO.

Understanding these requirements is particularly important for professionals who have to specify, carry out or validate a hydrographic survey.

From hydrography to the nautical chart

The data collected during a hydrographic survey do not directly constitute a nautical chart.

They must be checked, processed, structured and interpreted before they can be incorporated into a cartographic product.

The process may include:

• data acquisition;

• quality control;

• cleaning and validation;

• reduction and processing of soundings;

• incorporation of water-level observations;

• identification of hazards;

• generation of bathymetric models;

• selection of relevant information;

• cartographic compilation;

• final quality control.

This process explains why a nautical chart is much more than a simple graphical representation of measurements taken from the seabed.

Hydrographic data now contribute to both paper charts and electronic charts, as well as to geospatial databases.

Hydrography and electronic charts

The development of digital charting has profoundly changed the way hydrographic data are processed.

Electronic navigational charts, particularly ENCs used in ECDIS, are based on structured digital data. The IHO defines specific standards and specifications for ENC data, validation and display.

International hydrographic and cartographic standards therefore cover the entire chain, from the survey to the use of digital data on board. The IHO distinguishes, in particular, the S-44 standards for hydrographic surveys from specifications concerning ENC data and their display.

Understanding the hydrographic origin of digital chart data is therefore useful when assessing their quality and limitations.

Hydrography in ports and coastal areas

Hydrography has particularly numerous applications in port areas.

In a port such as Marseille, surveys can contribute to monitoring available depths in channels and harbour basins.

In Brest, Lorient, Saint-Nazaire or La Rochelle, hydrographic data contribute to understanding approaches, channels and port infrastructure.

In the North Sea, ports such as Rotterdam and Antwerp operate in environments where shallow water, sedimentation, heavy shipping traffic and dredging operations make detailed bathymetric knowledge particularly important.

Hydrographic data are also used for planning maritime works, dredging operations, offshore infrastructure and monitoring changes in seabed morphology.

Hydrography and major ocean areas

Hydrography is not limited to coastal waters.

Surveys may cover the Bay of Biscay, the Atlantic Ocean, the Mediterranean Sea, the North Sea, the Baltic Sea, the Indian Ocean, the Pacific Ocean and polar regions.

In deep-ocean areas, the objectives and techniques differ from those used in a harbour channel. Measurement density, operational constraints, water depth and the objectives of the survey determine the appropriate methods.

Hydrography also contributes to scientific knowledge of the seabed and to projects involving subsea infrastructure, marine renewable energy, geophysics and management of the marine environment.

The IHO also indicates that its survey standards can be used beyond navigational safety, including certain applications involving infrastructure, dredging, renewable energy, geophysics and geotechnical work.

Hydrographic offices and marine-chart publishers

Hydrographic data are used by national hydrographic offices and by different marine-chart producers.

In France, the SHOM – Service hydrographique et océanographique de la Marine plays a central role in producing and providing official nautical data and documentation.

For international navigation, hydrographic data and products are also associated with organisations such as the UK Hydrographic Office (Admiralty) and numerous other national hydrographic offices.

Private publishers such as NV Charts and Imray also use hydrographic and cartographic information adapted to their respective product ranges and navigation areas.

A hydrography manual therefore helps explain the technical chain that lies behind the nautical chart used by the navigator.

Who is a hydrography manual intended for?

This type of documentation can be useful to:

• hydrographers;

• hydrographic engineers and technicians;

• surveyors and geospatial specialists;

• cartographers;

• marine-science students;

• maritime schools and training organisations;

• hydrographic offices;

• marine-construction professionals;

• dredging specialists;

• maritime engineering consultancies;

• bathymetric survey operators;

• offshore professionals;

• navigators wishing to understand the production of nautical data.

It is also an excellent reference for understanding why the information shown on a nautical chart has a defined accuracy, source and set of limitations.

Why consult a hydrography manual?

Understanding hydrography makes it easier to interpret nautical documentation and understand the chain linking observation of the marine environment to cartographic representation.

A manual can provide a deeper understanding of:

• principles of hydrographic surveying;

• bathymetry;

• single-beam and multibeam sounding;

• GNSS positioning;

• reference systems;

• horizontal and vertical datums;

• tides and water levels;

• quality control;

• measurement uncertainty;

• data processing;

• bathymetric modelling;

• cartographic production;

• digital data and ENCs;

• international IHO standards.

The IHO C-13 Manual on Hydrography is specifically dedicated to these methodological and technical aspects. The IHO identifies C-13 as its Manual on Hydrography and maintains it alongside S-44 within the work of its Hydrographic Surveys Working Group.

Conclusion

The Hydrography Manual is a reference source for understanding how the physical characteristics of the marine environment are measured, controlled and transformed into hydrographic data used for navigation and charting.

From bathymetry to positioning, from tides to data processing, and from multibeam echosounders to digital terrain models, hydrography combines observation, metrology, geodesy, oceanography, computing and cartography.

This knowledge is particularly relevant to understanding SHOM nautical charts, Admiralty products and the various charting systems used in the Atlantic, English Channel, North Sea, Mediterranean and along major ocean routes.

For professionals, students and navigators wishing to deepen their nautical knowledge, a hydrography manual provides a better understanding of an essential aspect of maritime safety: accurately knowing the environment in which vessels operate and understanding what information can be derived from hydrographic data.

FAQ – Hydrography Manual

What is a hydrography manual?

A hydrography manual is technical documentation covering the methods used to measure, process and control the physical characteristics of navigable waters, including depths, water levels, positions and bathymetric data.

What is hydrography used for?

Hydrography is used in particular to produce and update the data required for marine charting, improve knowledge of the seabed and contribute to navigational safety.

What is the difference between hydrography and bathymetry?

Bathymetry mainly concerns the measurement and representation of water depths and seabed relief. Hydrography covers a broader field including positioning, water levels, currents, seabed characteristics, aids to navigation and data processing.

What is a hydrographic survey?

A hydrographic survey is a measurement campaign designed to determine the characteristics of a maritime or inland-water area to a defined level of accuracy. It may use echosounders, GNSS systems, inertial measurement units and water-level measurement systems.

What is the IHO S-44 standard?

S-44 is the International Hydrographic Organization standard for hydrographic surveys. It defines performance and quality requirements according to the intended use of the data. Edition 6.2.0 has been listed by the IHO since October 2024.

What is the difference between S-44 and the IHO C-13 Manual on Hydrography?

S-44 primarily defines the standards to be achieved for hydrographic surveys. C-13 provides more information about methods, equipment, survey operations and data processing. The two publications are therefore complementary.

Why are tides important in hydrography?

Tides change the height of the water column above the seabed. Water-level observations make it possible to relate measured depths to a defined vertical reference and make the data comparable with depths represented on nautical documentation.

What is the role of a multibeam echosounder?

A multibeam echosounder measures many points on the seabed simultaneously across a wide swath beneath the vessel. It is particularly suitable for surveys requiring detailed bathymetric coverage.

Is hydrography only concerned with nautical charts?

No. Hydrographic data can also be used for dredging, port works, offshore infrastructure, marine renewable energy, geophysics, geotechnical studies and certain environmental applications.

Why is it useful to understand hydrography when using a nautical chart?

Understanding the origin of the data helps users better interpret depths, vertical references, accuracy and the possible limitations of nautical documentation. It also provides a better understanding of how hydrographic observations are transformed into charted information.

Which hydrographic offices produce data used for marine charting?

Many countries have a national hydrographic office. In France, SHOM is the national reference organisation. Internationally, the UK Hydrographic Office, through Admiralty products, and many other national hydrographic offices contribute to the production and dissemination of nautical data.

Is a hydrography manual useful to recreational sailors?

Yes. It can be particularly useful to sailors wishing to understand how the data shown on nautical charts are produced, how depths are determined and why certain information has defined accuracy or coverage limitations.