1.1.  Introduction

This chapter describes the orders of safety of navigation surveys which are considered acceptable by hydrographic offices or authorities to generate navigational products and services that allow surface vessels to navigate safely. As requirements vary with water depth, geophysical properties, and expected shipping types, five different orders of survey are defined; each designed to cater to a range of needs.

The five orders are described below along with a description of the intended area(s) of usage. The minimum standards required to achieve each order (Table 1 and Table 2) along with a new tool for enhancing and customising these orders (Specification Matrix) is presented in Chapter 7.

The hydrographic offices or authorities responsible for acquiring surveys should select the order of survey that is most appropriate for the requirements for safety of navigation in the area. A single order may not be appropriate for the entire area to be surveyed and, in these cases, the different orders should be explicitly defined through the survey area. For example, in an area traversed by Very Large Crude Carriers (VLCCs) and expected to be deeper than 40 metres, an Order 1a survey may have been specified. However, if the surveyor discovers shoals of less than 40 metres depth, then it may be more appropriate to survey these shoals and surrounding areas to Special Order or even Exclusive Order in some limited circumstances.

To be fully compliant with an S-44 Order, a hydrographic survey must comply with all bathymetric and feature detection requirements (Table 1) for that order and with all the other requirements (Table 2) for the same order, where applicable. Additionally, the tables must be read in conjunction with the detailed text in the following chapters. The challenge presented by each order, in particular Special and Exclusive Orders, is establishing the appropriate survey methodology to achieve the specified standards.

To ensure surveys are systematic where bathymetric coverage is specified at less than 100%, the horizontal distance between registered positions of depths should be no greater than 3 times the depth or 25 metres, whichever is greater.

1.2.  Order 2

This is the least stringent order and is intended for areas where the depth of water is such that a general depiction of the bottom is considered adequate. As a minimum, an evenly distributed bathymetric coverage of 5% is required for the survey area. It is recommended that Order 2 surveys are conducted in areas which are deeper than 200 metres. Once the water depth exceeds 200 metres, the existence of features that are large enough to impact on surface navigation and yet still remain undetected by an Order 2 survey is considered to be unlikely.

1.3.  Order 1b

This order is intended for areas where the types of surface vessels expected to transit the area is such that a general depiction of the bottom is considered adequate. As a minimum, an evenly distributed bathymetric coverage of 5% is required for the survey area. This means some features will not be detected, although the distance between areas of bathymetric coverage will limit the size of those features. This order of survey is only recommended where underkeel clearance is considered not to be an issue. An example would be an area where the bottom characteristics are such that the likelihood of there being a feature on the bottom that will endanger the type of surface vessel expected to navigate the area is low.

1.4.  Order 1a

This order is intended for areas where features on the bottom may become a concern for the type of surface traffic expected to transit the area but where the underkeel clearance is considered not to be critical. A 100% feature search is required in order to detect features of a specified size. bathymetric coverage less than or equal to 100% is appropriate as long as the least depths over all significant features are obtained and the bathymetry provides an adequate depiction of the nature of the bottom topography. Underkeel clearance becomes less critical as depth increases, so the size of the feature to be detected increases with depth in areas where the water depth is greater than 40 metres. Examples of areas that may require Order 1a surveys are coastal waters, harbours, berthing areas, fairways and channels.

1.5.  Special Order

This order is intended for those areas where underkeel clearance is critical. Therefore, 100% feature search and 100% bathymetric coverage are required and the size of the features to be detected by this search is deliberately more demanding than for Order 1a. Examples of areas that may require Special Order surveys are: berthing areas, harbours, and critical areas of fairways and shipping channels.

1.6.  Exclusive Order

Exclusive Order hydrographic surveys are an extension of IHO Special Order with more stringent uncertainty and data coverage requirements. Their use is intended to be restricted to shallow water areas (harbours, berthing areas and critical areas of fairways and channels) where there is an exceptional and optimal use of the water column and where specific critical areas with minimum underkeel clearance and bottom characteristics are potentially hazardous to vessels. For this order, a 200% feature search and a 200% bathymetric coverage are required. The size of features to be detected is deliberately more demanding than for Special Order.

2.1.  Introduction

Positioning is a fundamental part for every survey operation. The hydrographer must consider the geodetic reference frame, horizontal and vertical reference systems, their connections to other systems in use (e.g. land survey datums), as well as the uncertainty inherent within associated measurements.

In this standard, position and its uncertainty refer to the horizontal component of the sounding or feature, while the depth and its uncertainty refers to the vertical component of the same sounding or feature.

2.2.  Geodetic Reference Frame

Positions should be referenced to a geodetic reference frame, which can be the realisation of either a global (e.g. ITRF2018, WGS84(G1762)) or a regional (e.g. ETRS89, NAD83) reference frame and their later iterations. As there are frequent updates to geodetic reference frames, it is essential that the epoch is recorded for surveys with low positioning uncertainty.

Since positions are most often referenced in a compound coordinate reference system/frame such as geodetic, geopotential, and height reference system/frame, they can be separated into horizontal and vertical components.

2.3.  Horizontal Reference System

If horizontal positions are referenced to a local datum, the name and epoch of the datum should be specified and the datum should be tied to a realisation of a global (e.g. ITRF2018, WGS84(G1762)) or a regional (e.g. ETRS89, NAD83) reference frame and their later iterations. Transformations between reference frames/epochs should be taken into account, especially for surveys with low uncertainty.

2.4.  Vertical Reference System

If the vertical component of the positions is referenced to a local vertical datum, the name and epoch of the datum should be specified. The vertical component of the positions (e.g. depths, drying heights) should be referenced to a vertical reference frame that is suitable for the data type and intended use. This vertical reference frame may be based on tidal observations (e.g. LAT, MWL, etc), on a physical model (i.e. geoid) or a reference ellipsoid.

2.5.  Chart and Land Survey Vertical Datum Connections

In order for bathymetric data to be correctly and fully utilised, chart and land survey vertical datum connections or relationships must be clearly determined and described. The IHO Resolution on Datums and Benchmarks, Resolution 3/1919, as amended, resolves practices which, where applicable, should be followed in the determination of these vertical datum connections.

This essential resolution 3/1919, as amended, is available in the IHO Publication M-3, Resolutions of the International Hydrographic Organization, which is downloadable from the IHO homepage www.iho.int.

2.6.  Uncertainties

This standard addresses total propagated uncertainty (TPU) by the two components; total horizontal uncertainty (THU) and total vertical uncertainty (TVU). The TVU and THU values must be understood as an interval of ± the stated value.

A statistical method, combining all uncertainty sources for determining both the horizontal and the vertical positioning uncertainty should be adopted to obtain THU and TVU respectively. The uncertainties at the 95% confidence level must be recorded with the survey data.

The ability of the survey system should be demonstrated by a priori uncertainty calculations (THU and TVU). These calculations are predictive and must be calculated for the survey system as a whole, including all instrument, measurement, and environmental uncertainty sources. This estimation should be updated during the survey to reflect changes from environmental conditions such as wind, waves, etc. in order to make appropriate changes to survey parameters.

Final uncertainty values for the survey may consist of an a priori and a posteriori calculation, explicitly empirical values (e.g. based on standard deviation of vertical depths alone), or some combination of the aforementioned values. The metadata should include a description of the uncertainty type and the uncertainty(s) achieved.

Within this standard, for ease of use, allowable horizontal uncertainty is assumed to be equal in both dimensions. Therefore, assuming a normal distribution of error, the position uncertainty is expressed as a single number.

2.7.  Confidence Level

In this standard the term confidence level is not the strict statistical definition, but is equivalent to the terms “level of confidence” or “coverage probability” as discussed in the Guide to the Expression of Uncertainty in Measurement, JCGM 100:2008, section 6.2.2.

It must be noted that confidence levels (e.g. 95%) depend on the assumed statistical distribution of the data and are calculated differently for one-dimensional (1D) and two-dimensional (2D) quantities. In the context of this standard, which assumes normal distribution of error, the 95% confidence level for 1D quantities (e.g. depth) is defined as 1.96 x standard deviation, and the 95% confidence level for 2D quantities (e.g. position) is defined as 2.45 x standard deviation.

3.1.  Introduction

The navigation of surface vessels requires accurate knowledge of depth and features. Where underkeel clearance is potentially an issue, bathymetric coverage must be at least 100%, feature detection must be appropriate, and depth uncertainties must be tightly controlled and understood.

For customisation or enhancement of safety of navigation survey orders or other applications, survey criteria may be specified by selecting required criteria values from the Matrix (See Section 7.5 and Annex A).

3.2.  Depth

3.3.  Feature Detection

Minimum standards for feature detection are specified in Table 1. A cubic feature is used as a basic shape reference for a system feature detection ability and implies a symmetrical 3-D shape of six equal square sides.

In assessing a survey system’s feature detection ability, the entire survey system, including equipment, methodologies, procedures, and personnel, must be assessed. It is the responsibility of the hydrographic office or authority that is gathering the data to assess the capability of any proposed survey systems to detect significant features.

Specified feature detection abilities are not implicit determinations of what constitutes a hazard to navigation. In some cases, significant features smaller than the defined sizes specified in Table 1 can be classified as hazards to navigation. It may therefore be deemed necessary by the hydrographic office or authority to detect smaller significant features in order to minimise the risk of undetected hazards to navigation. However, no single survey system can guarantee detection of all features. If there is a concern that hazards to navigation may exist within an area that may not be detected by the survey system, consideration should be given to use an alternative survey system.

3.4.  Feature Search

Minimum standards for Feature Search are specified in Table 1.

For Order 1a, a 100% feature search may be achieved with a survey system that does not measure depth. Under those circumstances, least depth measurements from an independent bathymetric system will be required for any detected significant feature. Whenever possible, it is recommended to conduct a 100% feature search in conjunction with 100% bathymetric coverage.

A feature search greater than or equal to 100% must be planned and conducted with the intent of detecting all features of the sizes specified in this standard. Where more than 100% feature search is required, including 200% for Exclusive Order, it may be accomplished by adequately overlapping collection or by acquiring more than one independent dataset within a survey.

3.5.  Bathymetric Coverage

The concept of bathymetric coverage was introduced in this edition of S-44, in order to make the standard technology independent. Acquisition of bathymetric coverage requires use of a sensor that measures and records depths. Table 1 specifies the minimum bathymetric coverage to be achieved by each survey order.

3.6.  Hazards to Navigation

Hydrographic offices and authorities must consider the expected local traffic (e.g. draught of vessels) as well as general configuration of depths when assessing hazards to navigation.

Sufficient data must be acquired over features that are potential hazards to navigation (e.g. wreck or other obstructions) to ensure the least depth and position are adequately determined by appropriate methods, while meeting the minimum requirements of the appropriate order in Table 1.

Given current ship specifications, features with least depths deeper than 40m would not likely constitute a hazard to surface navigation. However, this statement should be constantly re-evaluated based on local circumstances and their potential changes.

The hydrographic office, or authority, responsible for survey quality, may define a depth limit beyond which a detailed bottom investigation, and thus an examination of features, is not required.

3.7.  Charted Object Confirmation / Disproval

For an object which has previously been recorded/presented in a chart, document, electronic publication, or database, it is recommended to confirm or disprove the existence of those charted objects such as rocks, wrecks, obstructions, aids to navigation, and doubtful data. Findings should be addressed in the report of survey.

Doubtful data includes, but is not limited to, data which are usually denoted on charts by PA (Position Approximate), PD (Position Doubtful), ED (Existence Doubtful), SD (Sounding Doubtful), or as “reported danger”. Charted objects should be confirmed or disproved relative to their charted position.

No empirical formula for defining the search area can cover all situations. For object confirmation or disproval it is recommended that the search radius should be at least 3 times the estimated position uncertainty of the reported hazard. If a charted object is not located or indicated within the search radius, the charted object can then be recommended as disproved.

It is the responsibility of the hydrographic office or authority which is gathering the data to assess whether the charted object has been sufficiently disproved before removing it from the chart.

3.8.  Nature of the Bottom

The nature of the bottom should be determined in potential anchorage areas, other critical areas, and in areas where bottom conditions are suspected to have significant influence on required feature detection. Bottom Characterisation Methods include: physical sampling (PHY) with visual (VIS) and / or laboratory (LAB) analysis, inference technique (INF) from other sensors (e.g. backscatter or reflectivity), or inference technique with physical ground truth sampling (INF w/ GT) and visual (VIS) and / or laboratory (LAB) analysis.

Bottom Sampling Frequency may be at a spacing sufficient for the intended product (e.g. chart), seabed geology, and as required to ground truth any inference technique. Bottom sampling for ground truth of inference technique does not require a regular sampling pattern, or distances. An average value, or maximum value, for the distance between samples can be used. If bottom sampling has been performed in specific areas, such as anchorage or other areas of interest of the surveyed area, the limits for the sampling area should be recorded.

There are currently no IHO safety of navigation standards for bottom characterisation methods or bottom sample frequencies. However, the Matrix may be used to task and classify any such work performed. What is appropriate for these parameters varies greatly based on the nature and configuration of the bottom as well as the intended use of the area. The surveyor should exercise judgement in determining appropriate bottom characterisation methods and bottom sample frequency to adequately characterise the area.

4.1.  Introduction

In this chapter, water levels are considered in the context of supporting the vertical solution of depth measurements, rather than water level measurements as a discrete dataset to define tidal harmonics etc, which are covered within other IHO documents. Tides and other changes in water levels which impact the TVU of depth data must be considered for any hydrographic survey regardless of the technology used to conduct the survey. Flow observations will often be required to support safe navigation, and when specified in the survey requirement, those observations must meet the parameters presented in this standard.

For requirements to clearly determine chart and land survey vertical datum connections, or relationships, see Section 2.5.

4.2.  Water Level (Tidal) Predictions

Water level observations may be required to facilitate generation and maintenance of tidal prediction models and the production of Tide Tables. Water level observations should cover as long a period as possible and preferably not less than 30 days.

4.3.  Reductions for Water Level Observations

Whenever surveyed/predicted water levels or tides are used to reduce soundings to a datum, allowance shall be made in the TVU calculations for the uncertainty of the values. Observed values are preferred over predicted.

4.4.  Water Flow (Tidal Stream and Current) Observations

The speed and direction of water flows (tidal streams and currents) which may exceed 0.5 knots should be observed in key areas, if not already defined. For example, at the entrances to harbours and channels, at any change in direction of a channel, in anchorages, and adjacent to wharf areas. It is also recommended to measure coastal and offshore streams and currents when they are of sufficient strength to affect surface navigation.

The water flow (tidal stream and current) at each position should be measured at depths sufficient to meet the requirements of normal surface navigation in the survey area. In the case of tidal streams, simultaneous observations of tidal height and meteorological conditions should be made. It is recommended that the period of observation be at least 30 days.

The speed and direction of the water flow (tidal stream and current) must be measured at 95% confidence level as defined in Table 2. Where there is reason to believe that other factors (e.g. seasonal river discharge) influence the water flows, measurements should be made to cover the entire period of variability.

5.1.  Introduction

Surveys above the vertical datum are necessary for safe and efficient navigation and mooring. Topographic and geodetic measurements that are of specific importance for navigation are presented in the following sections. Their corresponding allowable uncertainties (THU and TVU as applicable) are defined in Table 2.

Additional information such as drawings or photographs of these features should be captured where possible to support the measurement.

For Chart and Land Survey Vertical Datum Connection requirements see Section 2.5.

5.2.  Fixed Aids and Topographic Features Significant to Navigation

Fixed aids to navigation include, but are not limited to: beacons, day marks, range markers, and lighthouses.

Topographic features significant to navigation are conspicuous features, landmarks, and objects which assist mooring, docking, and manoeuvring in confined spaces and / or provide some aid in navigation.

Conspicuous features which provide some aid in navigation without being a dedicated aid to navigation may include, but are not limited to, conspicuous natural features, cultural features, and landmarks such as: chimneys, flare stacks, hill or mountain tops, masts, monuments, towers, refineries, religious buildings, silos, single buildings, tanks, tank farms, towers, and windmills. Features of this type may be both significant to navigation and less significant to navigation (Section 5.5) depending on the feature’s individual characteristics and surroundings.

Essential harbour, mooring, and docking features include, but are not limited to: groins, moles, wharfs (quays), piers (jetties), mooring dolphins, piles, bollards, slipways, docks, lock gates, and breakwaters.

Allowable THU and TVU for the positioning of these fixed aids and features significant to navigation are presented in Table 2.

One may consider drying features (including rocks) which are positioned by topographic means to be topographic features significant to navigation. Regardless of the positioning methodology, maximum allowable uncertainties for drying features shall not exceed those specified in this standard for the adjacent permanently submerged features (unless a different order of survey has intentionally been specified by the commissioning authority).

5.3.  Floating Objects and Aids to Navigation

Floating objects and aids to navigation include, but are not limited to: buoys, articulated beacons, fish farms, and floating docks.

For floating objects, the surveyed position uncertainty should be significantly lower than the sway (object’s allowed movement). Sway due to currents, wind, and water level must be taken into account when computing the mean position of these objects.

Allowable THU for the positioning of these objects are presented in Table 2. Allowable TVU is not applicable to these measurements.

5.4.  Coastline

IHO S-32, IHO Hydrographic Dictionary, generally defines coastline or shoreline as the line where shore and water meet. IHO S-4, Regulations of the IHO for International (INT) Charts and Chart Specification of the IHO, describes it more specifically as high water mark, or the line of mean water level where there is no appreciable tide or change in water level. The coastline may also be defined as the low water line. Allowable THU for the positioning of these objects are presented in Table 2. Allowable TVU is not applied to these measurements within this standard.

5.5.  Features Less Significant to Navigation

Features less significant to navigation are non-conspicuous features which provide context and additional information, but are not likely to aid navigation. As stated in Section 5.2, topographic features of the same type may be both conspicuous / significant to navigation and less conspicuous / less significant to navigation depending on the feature’s individual characteristics and surroundings. Topographic features less significant to navigation may include, but are not limited to non-conspicuous landmarks such as: chimneys, flare stacks, hill or mountain tops, masts, monuments, towers, refineries, religious buildings, silos, single buildings, tanks, tank farms, and windmills.

Allowable THU and TVU for the positioning of these objects are presented in Table 2.

5.6.  Overhead Clearances, Range Line and Sector Lights Heights

Overhead obstructions such as bridges and cables may pose a hazard to navigation. Range line and sector light heights may be of use for determining distance from shore. Allowable THU and TVU for the positioning of overhead clearances (including associated horizontal clearances), range line and sector light heights are presented in Table 2.

5.7.  Angular Measurements

Angular measurements include, but are not limited to: limits of sectors and arcs of visibility of lights, alignments of leading lights and clearing lights, directions for passing off-lying dangers, and alignment of recommended tracks. Allowable THU for the measurement of these angles is presented in Table 2. Allowable TVU is not applicable to these measurements

6.1.  Introduction

Metadata is fundamental to ensure that survey data is correctly understood and utilised as required for chart production or other purposes. This Standard identifies the minimum metadata that is to be provided with hydrographic surveys conducted for safety of navigation. Where additional metadata is available this should be included to enhance the value of the survey data for other uses. Examples of metadata include overall quality, data set title, source, positional uncertainty, and ownership.

6.2.  Metadata Content

Metadata can be provided in any format such as in the Report of Survey, or embedded within a specific metadata file. The chosen format should support discovery, clarity of understanding, and software compatibility. Each hydrographic office or authority may adopt metadata requirements beyond that specified here and should develop and document a list of additional metadata used for their survey data. The table below should be seen as a schema, and not a final data model.

Metadata should be comprehensive, but should include, as a minimum, information on:

Metadata should preferably be an integral part of the digital survey record and conform to the “IHO S-100 Discovery Metadata Standard”, when this is adopted. Prior to the adoption of S-100, ISO 19115 can be used as a model for the Metadata. If this is not feasible, similar information should be included in the documentation of a survey.

7.1.  Introduction

As in previous editions, this edition of S-44 presents key elements of safety of navigation survey specifications in table format. This edition has two Tables (1 and 2) and provides a new specification Matrix for added flexibility for other types of hydrographic surveys carried out for purposes beyond safety of navigation. The new Matrix allows for customisation and enhancement of safety of navigation survey standards.

7.2.  Safety of Navigation Standards

Minimum bathymetry standards are defined in Table 1. Other minimum standards for positioning and water flow measurements are defined in Table 2. Both tables must be read in conjunction with the detailed text in this document.

As stated above, all standards defined in Table 1 and Table 2 are included in the specification Matrix within ranges of specification values which are available to enhance and customise safety of navigation surveys. Although the Matrix is available for this purpose, its usage will not reduce the minimum standards defined for safety of navigation survey orders. See Annex A for guidance on how to use the Specification Matrix.

7.3.  Minimum Bathymetry Standards for Safety of Navigation Hydrographic Surveys

To be read in conjunction with the full text set out in this document, m = metres, all uncertainties at 95% confidence level, * = Matrix Reference.

Table 1

7.4.  Other Minimum Standards for Safety of Navigation Surveys

To be read in conjunction with the full text set out in this document. Standards for Table 2 data types only apply where such measurements are required for the survey.

m = metres. All uncertainties at 95% confidence level. * = Matrix Reference.

Table 2

7.5.  Matrix Description

The Specification Matrix provides a range of selectable criteria for bathymetric parameters and other data types collected, reported, and delivered as part of a hydrographic survey. It is introduced to allow flexibility and customisation in the tasking and assessing of hydrographic surveys, accommodation of new and emerging technologies, and inclusion of hydrographic surveys conducted for purposes other than safety of navigation. By design, it is expandable and can evolve in future S-44 editions. The Matrix can be used both as a tool when specifying a survey, but also as a tool for classification of data after a completed survey.

It is important to note that the Matrix alone does not define any standards for hydrographic survey. Safety of navigation survey standards (as defined in Table 1 and Table 2) are referenced to the Matrix criteria and the Matrix can be used to customise and enhance these minimum standards. Standards for surveys conducted for purposes other than safety of navigation (e.g. geophysical, oil and gas, dredging, and geotechnical) are not currently defined in this document. However, the range of accuracies presented in the Matrix was designed to accommodate these surveys and to provide a common framework for tasking and assessing hydrographic surveys in general.

Additionally, with the emergence of new nautical products and associated specifications / data models (e.g. Electronic Nautical Charts (ENC) and S-101 ENC Product Specification), additional types of information will be available to the mariner. The Matrix can be used to help define and categorise the increasing variety of data that will be used in these evolving products.

See Annex A for guidance and additional information on how to use the Specification Matrix.

7.6.  MATRIX

Matrix for Hydrographic Surveys. To be read in conjunction with the full text set out in this document, m = metres, all uncertainties at 95% confidence level.