IHO Standards for Hydrographic Surveys
|Full name||S-44, Standards for Hydrographic Surveys|
|Languages||English, French, Spanish, Portuguese|
|Type of standard||IHO International Standard |
|Application||S-44 sets the minimum requirements for surveys conducted for the safety of surface navigation.|
|Conformance classes||None specified|
This publication is designed to provide a set of standards for the execution of hydrographical surveys for the collection of data, which will primarily be used to compile navigational charts to be used for the safety of surface navigation and the protection of the marine environment.
It must be realised that this publication only provides the minimum standards that are to be achieved. Where the bathymetry and expected shipping use requires it, hydrographical offices / organisations wishing to gather data may need to define more stringent standards. Also, this publication does not contain procedures for setting up the necessary equipment, for conducting the survey or for processing the resultant data. These procedures (which are a fundamental part of the complete survey system) must be developed by the hydrographical office/organisation wishing to gather data that is compliant with these Standards. Consideration must be made of the order of survey they wish to achieve, the equipment they have at their disposal and the type of topography that they intend to survey. Annexes A and B provide guidelines for Quality control and Data Processing and it is intended that these will be moved to the Manual on Hydrography (IHO Publication M-13) which provides further guidance on how to perform hydrographical surveys.
There is nothing to stop users adopting these Standards for other uses. Indeed, such a broadening of the use of these Standards is welcomed. However, users who wish to adopt these for other means must bear in mind the reason why they were written and therefore accept that not all parts may be suitable for their specific needs.
To be compliant with an S-44 Order a survey must be compliant with ALL specifications for that order included in these Standards.
It is also important to note that the adequacy of a survey is the end product of the entire survey system and processes used during its collection. The uncertainties quoted in the following chapters reflect the total propagated uncertainties of all parts of the system. Simply using a piece of equipment that is theoretically capable of meeting the required uncertainty is not necessarily sufficient to meet the requirements of these Standards. How the equipment is set up, used and how it interacts with the other components in the complete survey system must all be taken into consideration.
All components and their combination must be capable of providing data to the required standard. The hydrographical office / organisation needs to satisfy itself that this is so by, for example, conducting appropriate trials with the equipment to be used and by ensuring that adequate calibrations are performed prior to, as well as during and, if appropriate, after the survey being carried out. The surveyor is an essential component of the survey process and must possess sufficient knowledge and experience to be able to operate the system to the required standard. Measuring this can be difficult although surveying qualifications (e.g. having passed an IHO Cat A/B recognised hydrographical surveying course) may be of considerable benefit in making this assessment.
It should be noted that the issue of this new edition to the standard does not invalidate surveys, or the charts and nautical publications based on them, conducted in accordance with previous editions, but rather sets the standards for future data collection to better respond to user needs.
It should also be noted that where the sea floor is dynamic (e.g. sand waves), surveys conducted to any of the Orders in these Standards would quickly become out-dated. Such areas need to be resurveyed at regular intervals to ensure that the survey data remains valid. The intervals between these resurveys, which will depend on the local conditions, should be determined by national authorities.
The requirements set out in S-44 are of great importance to improve the safety of navigation. The data captured by hydrographical surveys are used to compile navigational charts. These navigational charts are used for surface navigation and the protection of the marine environment. S-44 defines four types of order of surveys and the measurements and observations required to ensure that navigational charts of the highest quality and standardized worldwide.
First to be addressed by the IHO standards for hydrographical surveys (S-44) is the Classification of surveys. This chapter specifies the orders of survey that are acceptable to allow hydrographical offices or organisations to produce navigational products. The navigational products will allow the expected shipping to navigate safely across the area surveyed. Due to varying requirements four types of orders of survey are defined:
- Special Order is the most rigorous and is only intended for areas where under-keel clearance is critical. Its type of order requires the full sea floor to be searched and the size of feature to be detected is kept small.
- Order 1a is intended for areas where the sea is sufficiently shallow to allow natural or man-made features on the seabed to be of concern to the type of surface shipping expected to transit. Under-keel clearance is not as critical. Order 1a also requires a full sea floor search, however the size of the feature to be detected are larger than for Special Order. Order 1a surveys may be limited to water shallower than 100 metres.
- Order 1b is meant for areas shallower than 100 metres where the general depiction of the seabed is considered adequate for the type of surface shipping expected to transit the area. Some features may be missed, since a full sea floor search is not required. This order of survey is only recommended where under-keel clearance is not required.
- Order 2 is intended for areas where the water depth is such that a general depiction of the seabed is considered adequate. A full sea floor search is not required. This order of survey is recommended for water deeper than 100 metres.
Positions should be referenced to a geocentric reference frame based on the International Terrestrial Reference System (ITRS), for example WGS84. Uncertainty of a position is the uncertainty at the position of the sounding or feature within the geodetic reference frame. S-44 further sets out the requirements for the horizontal uncertainty of features significant to navigation, the coastline and topographic features.
The navigation of vessels requires accurate knowledge of the water depth in order to exploit safely the maximum cargo carrying capacity, and the maximum available water for safe navigation. The measured depths and drying heights shall be referenced to a vertical datum that is compatible with the products to be made or updated from the survey. S-44 specifies the requirements for the following depth measurements: vertical uncertainty, water-level observations, depth measurement, feature detection, and sounding density.
Other measurements standardized by S-44 are seabed sampling, chart and land survey vertical datum connection, and tidal prediction. These measurements are not required for all order of surveys.
S-44 recommends the use of S-100 IHO Universal Hydrographic Data Model to perform a comprehensive assessment of the quality of the data. If a Bathymetric Model is required, the metadata should include the following: model resolution, computational model, underlying data density, uncertainty estimate, and a description of the underlying data. The Report of Survey shall provide a clear and comprehensive account of how the survey was performed, the results, the difficulties encountered, and the shortcomings. The report is the means by which the Surveyor in charge approves the survey record.
To improve the safety of navigation, it is desirable to eliminate doubtful data. This is achieved through carefully defining the area to be searched and subsequently surveying that area according to the standards outlined in S-44.
- ISO 19115:2003, Geographic information – Metadata
- S-100 IHO Universal Hydrographic Data Model
- S-44, Standards for Hydrographic Surveys
- ISO/IEC 98:1995 Guide to the expression of uncertainty in measurement
- ISO/IEC 99:2007 International Vocabulary of Metrology – Basic and general concepts and associated terms (VIM)