DUTCH STANDARDS FOR HYDROGRAPHIC SURVEYSpublicaties.minienm.nl/download-bijlage/92306/dutch... · For hydrographic surveys in the rest of the control region and hydrographic surveys

DUTCH STANDARDS FOR HYDROGRAPHIC SURVEYS

1st edition, July 2009

based on

IHO STANDARDS FOR HYDROGRAPHIC SURVEYS (S-44)

5th edition, February 2008

Rijkswaterstaat Ministry of Infrastructure and the Environment

Royal Netherlands Navy Netherlands Hydrographic Service

Dutch standards for hydrographic surveys Juli 2009

iii

DUTCH STANDARDS FOR HYDROGRAPHIC SURVEYS

1st edition, July 2009

based on

IHO STANDARDS FOR HYDROGRAPHIC SURVEYS (S-44)

5th edition, February 2008

Dutch standards for hydrographic surveys Juli 2009 v

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Colofon Published by: In corporation with:

Rijkswaterstaat Data-ICT-Dienst Royal Netherlands Navy Netherlands Hydrographic Service

Information: Telephone: E-mail:

Data-ICT-Dienst Netherlands Hydrographic Service +31(0)15-2757700 +31(0)70-3162800 [email protected] [email protected]

Made by: Periplus Consultancy b.v. Rijkswaterstaat, Data-ICT-Dienst

Layout: Rijkswaterstaat

Date: 3 July 2009

Status: Definite

Version number: 3.0

Dutch standards for hydrographic surveys Juli 2009 1

CONTENTS

Preface .................................................................................................................................. 3 Introduction ....................................................................................................................... 5 1 Classification of Hydrographic Surveys................................................ 9

1.1 Introduction.......................................................................................................... 9 1.2 NL Order A ............................................................................................................ 9 1.3 NL Order B ............................................................................................................ 9 1.4 IHO Special Order .............................................................................................. 9 1.5 IHO Order 1a ..................................................................................................... 10 1.6 IHO Order 1b ..................................................................................................... 10 1.7 IHO Order 2........................................................................................................ 10 1.8 Overview orders ............................................................................................... 11

2 Positioning.............................................................................................................. 13 2.1 Horizontal uncertainty .................................................................................... 13

3 Depths ...................................................................................................................... 15 3.1 Introduction........................................................................................................ 15 3.2 Vertical uncertainty ......................................................................................... 15 3.3 Reductions for Tides / Water-level Observations.................................. 16 3.4 Minimum depth ................................................................................................. 16 3.5 Feature detection ............................................................................................. 16 3.6 Sounding density / Line spacing ................................................................. 17

4 Other measurements...................................................................................... 19 4.1 Introduction........................................................................................................ 19 4.2 Seabed sampling .............................................................................................. 19 4.3 Chart and land survey vertical datums connection .............................. 19 4.4 Tidal predictions ............................................................................................... 19 4.5 Tidal stream and current observations..................................................... 19

5 Data attribution .................................................................................................. 21 5.1 Introduction........................................................................................................ 21 5.2 Metadata ............................................................................................................. 21 5.3 Point data attribution ...................................................................................... 21 5.4 Bathymetric model attribution..................................................................... 22 5.5 Report of survey ............................................................................................... 22

6 Elimination of doubtful data ...................................................................... 23 6.1 Introduction........................................................................................................ 23 6.2 Extent of Area to be Searched .................................................................... 23 6.3 Conducting the search.................................................................................... 23 6.4 Presentation of search results ..................................................................... 23

Abbreviations and glossary................................................................................... 25



Preface

Out of various responsibilities the Netherlands Hydrographic Service and Rijkswaterstaat carry out hydrographic surveys. It is therefore logical that our departments work together. Our activities are harmonized as much as possible and data is being exchanged between the two departments. In order to aid this cooperation, the Dutch Hydrographic Institute has been active for years. It takes initiatives such as the here-described Hydrographic standards. In order to be able to work together effectively, a common reference frame, such as the S-44-Standards of the International Hydrographic Organisation (IHO), is important. The 5th edition of these Standards was published in February 2008. The Dutch Hydrographic Standards build on this with some additional Standards for the Dutch situation. The Dutch Standards prescribe the minimum requirements regarding hydrographic surveys. They have influence in almost every hydrographic work process and end product, such as planning and execution of hydrographic surveys, the purchasing, maintenance and operational use of hydrographic equipment and the education and competences of the personnel involved. Rijkswaterstaat has declared the Dutch standards for hydrographic surveys applicable to all surveys performed by the Meet- and Information services and for surveys performed by external parties commissioned by Rijkswaterstaat. Further more the standards are applicable on all surveys performed by the Netherlands Hydrographic Service for their survey plan. For military-hydrographic reconnaissance and surveys these standards are applied if the operational task permits it We hope, that by using these standards, our organisations can have information with a clearly defined quality and that our cooperation will be strengthened by it. De Chef der Hydrografie Rijkswaterstaat Waterdienst F.P.J. de Haan Dr. R. Allewijn Kapitein ter zee. Directeur Water en Gebruik



Introduction

This publication has been prepared in accordance with the 5th edition of the IHO Special Publication S-44 “Standards for Hydrographic Surveys”. The layout and a part of the text are similar to it. The big difference is that this publication is dedicated to hydrographic work as performed by the Dutch government. It therefore contains some important additions and extensions. The purpose of this publication is to establish a set of standards for conducting and processing hydrographic surveys. The standards apply to all hydrographic surveys conducted by or on behalf of Rijkswaterstaat or the Netherlands Hydrographic Service. The S-44 standard is an international standard for hydrographic surveys for the maritime shipping. Nautical information gathered and published for the maritime shipping needs, in accordance with the International Convention for the Safety of Life at Sea (SOLAS), to follow these standards. In the Netherlands, the Hydrographic Service of the Navy Command is responsible for these activities. For hydrographic surveys in the rest of the control region and hydrographic surveys for purposes other than vessel traffic, Rijkswaterstaat in the past had formulated standards to complement those established by the IHO S-44 standard. These, for the Netherlands specific standards, are more strict due to the stringent requirements that apply to coastal protection, morphological research, environment and management and maintenance of inland waterways and because methods for inland and the coastal region may be more accurate. The Dutch standards are prepared in accordance with IHO standards, so they mutually well comparable. The result is six standards for hydrographic surveys. The strictest standard, NL Order A, corresponds to what is reasonably achievable with existing equipment and existing way of working in the relatively shallow Dutch waters. The least stringent standard, IHO Order 2, applies at sea at depths exceeding 100 meters. This standard does not apply for the Dutch continental waters, however so in the waters around the Netherlands Antilles and Aruba. Sections exceeding 200 meters need to comply with the GEBCO standards and are outside the scope of this document. The four IHO standards in this document are translated from the final version of the IHO S-44 standard, fifth edition, February 2008. To them NL Order A and B are added. The standards describe, among other things, to what conditions validated xyz data must comply and which objects / obstacles that should be able to be detected. It must be stressed that if a survey must comply with a specific standard from this document, it thus must comply demonstrably with all the requirements for this standard, from this document. One should also realize that this publication provides only the minimum requirements that must be fulfilled. Local circumstances may for example lead to the collection of data with more stringent conditions than described in this document. In occurring cases, the awarding authority should describe these more stringent requirements in accordance with the standards in this document and notify the contractor of the more stringent requirements. It is also important to note that the degree to which a survey meets these standards is the result of the whole measuring system and the processes applied thereto. The uncertainties listed in the following chapters therefore describe the total propagated uncertainty of all parts of the system. Using a certain tool that in theory is able to achieve the mentioned precision is not necessarily sufficient to meet the requirements of this document. The way the instrument was setup, used and the interaction with other parts of the whole measuring system should be taken into account. An online apriori THU / TVU calculation and an aposteriori analysis are needed to test the results against the required standards. Due to the great variety


of equipment, surveymethodes, local seabed conditions, etc., it is impracticable to lay down standard procedures for each standard. These procedures and instructions are the responsibility of the awarding authority and the contracting authorities. They must have such procedures and competent staff to be able to comply with the requirements of this standard. All individual components of the measuring system and the measuring system as a whole must be able to deliver data at the desired standard level. The awarding authority and the contracting authority should convince herself of this for example by carrying out appropriate tests with the system used and by convincing herself that the appropriate calibrations are performed prior to, during and, if applicable, after the survey. The hydrographic surveyor is an essential part of the hydrographic surveying process. This person must have sufficient proven knowledge and experience to perform hydrographic surveys in accordance with the standards required. IHO Manual M-5, “Standards of Competence for Hydrographic Surveyors” and IHO Special Publication S-47, “Training courses in hydrography and nautical cartography” offer the internationally accepted guidelines for this. The hydrographic competences of the staff involved, should be mentioned in the report of the hydrographic survey. At those locations where the seabed is dynamic as a result of for example erosion, sedimentation and sediment transport, surveys performed according to a particular standard will be a good representation of reality for a shorter period. Such areas must be re-surveyed periodically to ensure that the seabed in that area is in ongoing compliance with the managing requirements. The corresponding survey frequency is to be determined by the awarding authority that should be based on local morphological and climatic conditions, on risk defining factors, such as for instance traffic intensity and under keel clearance and on the desired maximum uncertainty in the presented results. Although the standards in this publication apply to hydrographic surveys conducted by or on behalf of the Dutch government, it should be stressed that they certainly don’t have the exclusive right of use. On the contrary, the use of these standards by other hydrographic agencies is strongly recommended. The hydrographic sector as a whole benefits from a general application of these standards. They provide clarity in the execution but also in the procurement of contracts for both the awarding and the contracting authorities. However, authorities should at all times check if the standards listed in this document are applicable to the concerning contract. To the 5th edition of S-44 two appendices are added. Annex A “Guidelines for Quality Control” and Annex B “Guidelines for Data Processing”. These annexes are not part of this publication. The IHO states that they are not an integral part of the S-44 standards and that they will be removed when the information contained in them is fully included in IHO Publication M-13, “Manual on hydrography”. Rijkswaterstaat and the Netherlands Hydrographic Service have, based on these guidelines, established coordinated data management and quality control procedures to fulfil these NL standards. A glossary of terms used in this publication is added, after Chapter 6. Italic words in the text are included in the glossary. In the electronic version there are hypertext links between the italicized words and their definition in the glossary. During the translation of the terms from the original English version there has been chosen to use the terms of Aquo-lex as much as possible as far as they are consistent with the meaning of the terms in the S-44. An important exception is the use of the


term “uncertainty”. In the Aquo-lex it is stated by the term “uncertainty” that the use of this term in relation to measurements is to be discouraged. Because of ambiguity in the original IHO version and the there used terms it was chosen to maintain the term “uncertainty”. The following “fundamental terms” of the glossary are considered essential for understanding the standards and are therefore listed here. Fundamental terms: Full sea floor search: A systematic method for examining the sea floor performed in such a way that most of the features and objects as specified in the applicable standard will be mapped. In practice it is impossible to achieve 100% coverage and the use of the term 100% bathymetric coverage is therefore discouraged. Object detection: The ability of a system to detect objects and / or obstacles of a specific size. The standards in this document define the minimum size of objects that must be able to be detected during the survey. Reduced z-value: Observed z-value plus any subsequent adjustments associated with the survey and processing with the result that the value is reduced to the appropriate datum. Total propagated uncertainty (TPU): The resultant propagation of the uncertainties when all known uncertainties, both random and systematic are considered. The TPU is due to the uncertainty in the measurements and is a 3-dimensional variable. Total horizontal uncertainty (THU): The component of the total propagated uncertainty (TPU) calculated in the horizontal plane. The THU is a 2-dimensional variable and listed as a single number. The assumption is that the horizontal uncertainty is isotropic, in other words, there is a negligible correlation between errors in x-and y-direction. Because of this a normal distribution becomes circularly symmetric, so a single number can be used to describe the distribution of the errors around the true value. Total vertical uncertainty (TVU): The component of the total propagated uncertainty (TPU) calculated in the vertical dimension. The TVU is a 1-dimensional variable. Z-value: This term indicates both the value for the depth as for the bottom height.



1 Classification of Hydrographic Surveys

1.1 Introduction This chapter describes the standards for hydrographic surveys that apply to various types of activities and various areas. As the requirements and possibilities vary strongly with depth, objective of the measurement, bottom conditions and the need for information in general, two extra standards (NL Order A, NL Order B) have been defined in addition to the four existing IHO standards (special, 1a, 1b, 2). For the six standards is documented individually for which type of survey and in what kind of area they are applicable. The specification of each standard is given in Table 1, which is provided with a detailed explanation of the different categories used in the table that must be read to be able to interpret the standard correctly. The awarding authority for a survey, will determine which standard applies in the regarding area based on the information given here. It should be noted that it is possible that multiple standards may apply within a single area. In that case, the awarding authority has to divide the area into zones in such a way that it is clear which standard is applicable in a given zone. The situation as encountered in the field may differ considerably from what was expected. After analyzing the data it can become known that a different standard should be used in the future, for example because shallows are less critical of even more critical than originally assumed. In those cases where it turns out that a stricter standard should have been chosen, for example with the occurrence of shoals, the awarding authority must assess the area again using this stricter standard.

1.2 NL Order A This standard was initially prepared by Rijkswaterstaat with the aim to establish more stringent standards than specified by the IHO Special Order. This standard is intended for specific projects such as morphologic research, examination of constructions, in particular for construction, management and maintenance, and dredging operations. This requires another survey regime than what is necessary for maritime shipping. This makes additional requirements necessary. In particular, the requirements regarding position and z-value have been strengthened with respect to the IHO Special Order. Besides this, a full sea floor search has to be carried out so that all objects and characteristics of the sea floor can be sufficiently captured.

1.3 NL Order B This standard was initially prepared by Rijkswaterstaat with the aim to establish a standard that applies to waterways outside the scope of NL Order A. It concerns mostly areas that are not part of navigation channels and harbours, and where no inspection of construction applies. A full sea floor search is not necessary in all cases, meaning that some objects shall not be detected, although the size of possibly undetected objects may be limited by the maximum line spacing. This standard is recommended only for those areas where under keel clearance is supposed to be not a problem. With this, this standard is an alternative for IHO Order 1b for the continental waters.

1.4 IHO Special Order This standard was specifically enacted with the aim to ensure the safe navigation in those areas where under-keel clearance is critical. It is therefore the most stringent standard in the S-44 standard and is only exceeded by the Netherlands-specific


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standard, NL Order A. Because under keel clearance is critical a full sea floor search is required and the size of the features to be detected by this search is deliberately small Kept. The nature of the bottom is also relevant. In rocky soil, this standard is more likely required then in case of for example sludge. Since under-keel clearance is critical it is considered unlikely that Special Order surveys will be conducted in waters deeper than 40 meters. Examples of areas that may warrant Special Order surveys are: berthing areas, harbours and critical areas of shipping channels.

1.5 IHO Order 1a This order is intended for those areas where the sea is sufficiently shallow to allow natural or man-made features on the seabed to be a concern to the type of surface shipping expected to transit the area but where the under-keel clearance is less critical than for Special Order above or NL Order A. Because man-made or natural features may exist that are of concern to surface shipping, a full sea floor search is required, however the size of the feature to be detected is larger than for Special Order and the NL Order A. Under-keel clearance becomes less critical as depth increases so the size of the feature to be detected by the full sea floor search is increased in areas where the water depth is greater than 40 metres. Order 1a surveys may be limited to water shallower than 100 metres.

1.6 IHO Order 1b This order is intended for areas shallower than 100 metres where a general depiction of the seabed is considered adequate for the type of surface shipping expected to transit the area. A full sea floor search is not required which means some features may be missed although the maximum permissible line spacing will limit the size of the features that are likely to remain undetected. This order of survey is only recommended where under-keel clearance is not considered to be an issue. An example would be an area where the seabed characteristics are such that the likelihood of there being a man-made or natural feature on the sea floor that will endanger the type of surface vessel expected to navigate the area is low.

1.7 IHO Order 2 This is the least stringent order and is intended for those areas where the depth of water is such that a general depiction of the seabed is considered adequate. A full sea floor search is not required. It is recommended that Order 2 surveys are limited to areas deeper than 100 metres. Parts deeper than 200 meter must comply with the GEBCO-orders and therefore don’t have to comply to this document. Once the water depth exceeds 100 metres the existence of man-made or natural 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. This order is not to be used in the Dutch inlandwaters or on the Dutch continental shelf. In the waters around the Dutch Antilla and Aruba however it does have to be used.


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1.8 Overview orders An overview of the above mentioned orders can be found in table 1. To be able to understand this table some items will be explained here. Notes on table 1: 1. The line spacing for IHO Orders 1b and 2 can be expanded if procedures for

ensuring an adequate sounding density are used. 2. These only apply where such measurements are required for the survey. 3. Recognising that there are both constant and depth dependent uncertainties

that affect the uncertainty of the depths, the formula below is to be used to compute, at the 95% confidence level, the maximum allowable TVU. The parameters “a” and “b” for each Order, as given in the Table, together with the depth “d” have to be introduced into the formula in order to calculate the maximum allowable TVU for a specific depth:

22 )( dba ×+

Where: a represents that portion of the uncertainty that does not vary with depth b is a coefficient which represents that portion of the uncertainty that

varies with depth d is the reduced depth b x d represents that portion of the uncertainty that varies with depth

4. For safety of navigation purposes, the use of an accurately specified mechanical

sweep to guarantee a minimum safe clearance depth throughout an area may be considered sufficient for Special Order and Order 1a surveys.

5. If for the Dutch Orders A and B full sea floor search is required, a maximum of

5% of the survey area may be left uncovered with bathymetric data. These uncovered areas may, individually, not be bigger then 0.5% of the total area. Using neighbouring values, an estimate must be made of the risk there is a critical bottom depth within the uncovered area. The intention is always to have full bottom coverage. Due to circumstances during the survey it is not always possible to meet the requirements in table 1 for the entire area

6. A cubic feature means a regular cube each side of which has the same length. It

should be noted that the NL Order A, IHO Special Order and Order 1a feature detection requirements, are minimum requirements. In certain circumstances it may be deemed necessary by the hydrographic offices / organizations to detect smaller features to minimise the risk of undetected hazards to surface navigation. For Order 1a the relaxing of feature detection criteria at 40 metres reflects the maximum expected draught of vessels.


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Table 1: Minimum Standards for Hydrographic Surveys

NL Order A NL Order B IHO Special Order IHO Order 1a IHO Order 1b IHO Order 2

Description of areas Surveys in harbours, shipping lanes, e.d. where under-keel clearance is critical and for inspections of underwater constructions

Areas outside shipping lanes, harbours inside the coastline and inspections of underwater constructions.

Areas where under-keel clearance is critical

Areas shallower than 100 m where under-keel clearance is less critical but features of concern to surface shipping may exist.

Areas shallower than 100 m where under-keel clearance is not considered to be an issue for the type of surface shipping expected to transit the area.

Areas generally deeper than 100 m where a general description of teh sea floor is considered adequate.

Line spacing (1) Not defined as full sea floor search is required.

Depending on the project but no greater then IHO Order 1b.

Not defined as full sea floor search is required.

Not defined as full sea floor search is required.

3 x average depth or 25 metres, whichever is greater. For bathymetric Lidar a spot spacing of 5x5 m.

4 x average depth

Positioning of fixed aids of navigation and topography significant to navigation (95% confidence level) (2)

2m 2m 2m 2m 2m 5m

Positioning of the coastline and topography less significant to navigation (95% confidence level) (2)

5m 5m 10m 20m 20m 20m

Mean position of floating aids to navigation (95% confidence level)

Preparation, planning,

surveying and

additional inform

ation

5m 5m 10m 10m 10m 20m

Correcting for errors during the survey Correct for all irregularities during survey and processing. Filtering off the data, including the removal of blunders and systematic errors. Applying tidal and other water level reductions. Validated data points should comply with the requirements described below.

Maximum allowable THU, 95% confidence level

0.40m 0.40m 2m 5m + 5% of depth 5m + 5% of depth 20m +10% of depth

a = 0.10 m a = 0.15 m a = 0.25 m a = 0.5 m a = 0.5 m a = 1.0 m Maximum allowable TVU for reduced depths, 95% confidence level (3)

Validation of

the m

easurements

b = 0.0075 b = 0.0075 b = 0.0075 b = 0.013 b = 0.013 b = 0.023

Sea floor search (4) Full sea floor search Not required Full sea floor search Full sea floor search Not required Not required

Coverage in survey line (5) Not defined as full sea floor search is required

One measurement a metre 95% of the survey line.

Not defined as full sea floor search is required

Not defined as full sea floor search is required

Not defined Not defined

Feature detection (6)

Check

on the

measurem

ents Cubic features > 1m Not applicable Cubic features > 1m Cubic features > 2m in depths up to 40 m; 10% of depth beyond 40 m

Not applicable Not applicable


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2 Positioning

2.1 Horizontal uncertainty The uncertainty of a position is the uncertainty at the position of the sounding or feature within the geodetic reference frame. Positions should be referenced to a geocentric reference frame based on the International Terrestrial Reference System (ITRS) e.g. WGS84 or ETRS89. Preferebly, reference to ETRS89 is used. If, exceptionally, positions are referenced to the local horizontal datum, such as RD, this datum should be tied to a geocentric reference frame based on ITRF. For RD this is accompliced by referring to the latest transformation procedure RDNAPTRANSTM. The uncertainty of a position is affected by many different parameters, e.g. errors in the positioning system, errors in the various sensors, errors in the geometry of the vessel, e.d. The contributions of all such parameters to the total horizontal uncertainty (THU) should be accounted for. A statistical method, combining all uncertainty sources, for determining positioning uncertainty should be adopted. The position uncertainty at the 95% confidence level should be recorded together with the survey data (see also 5.2). The capability of the survey system should be demonstrated by the THU calculation. The position of soundings, dangers, other significant submerged features, navaids (fixed and floating), features significant to navigation, the coastline and topographical features should be determined such that the horizontal uncertainty meets the requirements specified in Table1. This includes all uncertainty sources not just those associated with positioning equipment.


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3 Depths

3.1 Introduction 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. Where under-keel clearances are an issue the depth uncertainties must be more tightly controlled and better understood. In a similar way, the sizes of features that the survey will have or, more importantly, may not have detected, should also be defined and understood. A similar situation occurs during the inspection of objects and the monitoring of dredging activities. 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 e.g. chart datum. Ideally this sounding datum should also be a well-defined vertical datum such as, LAT, MSL/MV, a geocentric reference frame based on ITRS or a geodetic reference level (NAP).

3.2 Vertical uncertainty Vertical uncertainty is to be understood as the uncertainty of the reduced depths. In determining the vertical uncertainty the sources of individual uncertainties need to be quantified, e.g. errors in the positioning system, errors in the various sensors, errors in the geometry of the vessel, errors caused by tidal- and draft corrections, e.d. All uncertainties should be combined statistically to obtain a total vertical uncertainty (TVU). The maximum allowable vertical uncertainty for reduced depths as set out in Table 1 specifies the uncertainties to be achieved to meet each order of survey. Uncertainty related to the 95% confidence level refers to the estimation of error from the combined contribution of random errors and residuals from the correction of systematic errors. The capability of the survey system should be demonstrated by the TVU calculation. Recognising that there are both depth independent and depth dependent errors that affect the uncertainty of the depths, the formula below is to be used to compute, at the 95% confidence level, the maximum allowable TVU. The parameters “a” and “b” for each order, as given in Table 1, together with the depth “d” have to be introduced into the formula in order to calculate the maximum allowable TVU for a specific depth:

22 )( dba ×+

Where: a represents that portion of the uncertainty that does not vary with

depth b is a coefficient that represents that portion of the uncertainty that

varies with depth d is the depth b x d represents that portion of the uncertainty that varies with depth

The vertical uncertainty at the 95% confidence level should be recorded together with the survey data (see also 5.3).


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3.3 Reductions for Tides / Water-level Observations Observations sufficient to determine variations in the water level across the entire survey area must be taken for the duration of the survey for the reduction of soundings to the relevant sounding datum. These may be determined either by direct measurement of the water level (i.e. by using a gauge) and if necessary carried across the survey area by co-tidal corrections or by 3D positioning techniques linked to the required sounding datum by a suitable separation model. Tidal / water-level reductions need not be applied to depths greater than 200 metres if TVU is not significantly impacted by this approximation.

3.4 Minimum depth All anomalous features previously reported in the survey area and those detected during the survey should be examined in greater detail and, if confirmed, their position and least depth determined. If a previously reported anomalous feature is not detected refer to Chapter 6 for disproving requirements. The agency responsible for survey quality may define a depth limit beyond which a detailed sea floor investigation, and thus an examination of anomalous features, is not required. For wrecks and obstructions which may have less than 40 metres clearance above them and may be dangerous to normal surface navigation, their position and the least depth over them should be determined by the best available method while meeting the depth uncertainty standard of the appropriate order in Table 1. Side scan sonar should not be used for depth measurement but to define areas requiring more detailed and accurate investigation.

3.5 Feature detection When a full sea floor search is required, the equipment used to conduct the survey must be demonstrably capable of detecting features of the dimensions specified in Table 1. Additionally, the equipment must be considered as part of a system (includes survey / processing equipment, procedures and personnel) that will ensure there is a high probability that these features will be detected. It is the responsibility of the hydrographic office / organisation that is gathering the data to assess the capability of any proposed system and so satisfy themselves that it is able to detect a sufficiently high proportion of any such features. The NL Order A and the IHO Special Order and Order 1a feature detection requirements of 1 metre and 2 metre cubes respectively are minimum requirements. Features may exist that are smaller than the size mandated for a given order but which are a hazard to navigation or are for other reasons important enough to be detected/mapped. It may therefore be deemed necessary by the hydrographic office / organization to detect smaller features in order to minimise the risk of undetected hazards to surface navigation. This extra demand must be defined explicitly in the survey instructions. It should be noted that even when surveying with a suitable system 100% detection of features can never be guaranteed. Therefore a minimum coverage of 95% is requested in NL Order A, supplemented by criteria for the 5% that is not covered. In case of single beam surveys combined with side scan sonar not the entire area is covered with depth data, but the gaps regarding the object detection are filled with side-scan sonar observations. These measurements do not generate depths, but give an image of the sea floor and the presence of possible objects and the dimensions thereof.


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If there is concern that features may exist within an area that may not be detected by the Survey System being used, consideration should be given to the use of an alternative system (e.g. a mechanical sweep) to increase the confidence in the minimum safe clearance depth across the area.

3.6 Sounding density / Line spacing In planning the density of soundings, both the nature of the seabed in the area and the requirements of safe surface navigation have to be taken into account to ensure an adequate sea floor search. For NL Order A, IHO Special Order and IHO Order 1a surveys no recommended maximum line spacing is given, as there is an overriding requirement for full sea floor search. Full sea floor search is not required for NL Order B and IHO Orders 1b and 2. Table 1 recommends maximum line spacing (IHO Orders 1b and 2) and bathymetric LIDAR spot density (IHO Order 1b). The nature of the seabed needs to be assessed as early as possible in a survey in order to decide whether the line spacing / LIDAR spot density from Table 1 should be reduced or extended. For surveys where NL Order B is needed, no maximum line spacing is given. In these cases the line spacing is to be defined depending on the specific project and must be explicitly mentioned in the survey instructions.


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4 Other measurements

4.1 Introduction The following observations may not always be necessary but if specified in the survey requirement should meet the following standards. Additional requirements for these measurements may be specified in regulations issued by the awarding authority. If so, the awarding authority will refer to these documents and the requirements in these documents become part of the standard to be achieved.

4.2 Seabed sampling The nature of the seabed should be determined in potential anchorage areas, but may also be relevant for for instance dredging activities. It may be determined by physical sampling or inferred from other sensors (e.g. single beam echo sounders, side scan sonar, sub-bottom profiler, video, etc.). Physical samples should be gathered at a spacing dependent on the seabed geology and as required to ground truth any inference technique.

4.3 Chart and land survey vertical datums connection IHO Technical Resolution A2.5, as set out in IHO Publication M-3, “Resolutions of the International Hydrographic Organisation”, requires that the datum used for tidal predictions should be the same as that used for chart datum. In order for the bathymetric data to be fully exploited the vertical datum used for tidal observations should be connected to the general land survey datum (e.g. NAP) via prominent fixed marks in the vicinity of the tide gauge/station/observatory. Ellipsoidal height determinations of the vertical reference marks used for tidal observations should be made relative to a geocentric reference frame based on ITRS, preferably WGS84 or ETRS89, or to an appropriate geodetic reference level.

4.4 Tidal predictions Tidal data may be required for analysis for the future prediction of tidal heights and the production of Tide Tables in which case observations should cover as long a period of time as possible and preferably not less than 30 days.

4.5 Tidal stream and current observations The speed and direction of tidal streams and currents which may exceed 0.5 knots should be observed 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 desirable to measure coastal and offshore streams and currents when they are of sufficient strength to affect surface navigation. The 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 and the period of observation should ideally be 30 days with a measurement interval not bigger then 1 hour.


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The speed and direction of the tidal stream and current should be measured to 0.1 knots ( 0.05 m/s) and the nearest 10º respectively, at 95% confidence level. Where there is reason to believe that seasonal river discharge influences the tidal streams and currents, measurements should be made to cover the entire period of variability. For current measurements other then for navigational purposes, extra requirements may be defined.


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5 Data attribution

5.1 Introduction To allow a comprehensive assessment of the quality of survey data it is necessary to record or document certain information together with the survey data. Such information is important to allow exploitation of survey data by a variety of users with different requirements, especially as requirements may not be known when the survey data is collected.

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

- the survey in general e.g. purpose, date, area, equipment used, name of survey platform;

- the geodetic reference system used, i.e. horizontal and vertical datum including ties to a geodetic reference frame based on ITRS (e.g. WGS84) if a local datum is used;

- calibration procedures and results; - sound speed correction method; - tidal datum and reduction; - uncertainties achieved and the respective confidence levels; - any special or exceptional circumstances; - rules and mechanisms employed for data thinning.

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. The metadata must also comply with the Dutch Metadata Standard for Geography as prepared by Geonovum. This national standard is a further specification of the international ISO 19115 standard for metadata and also meets the European INSPIRE directive. To support the above requirements both Rijkswaterstaat and the Netherlands Hydrographic Service have, for their activities, prepared lists with the required and optional metadata elements that every dataset must contain. The contracting authority must see to this as regular part of their hydrographic reports.

5.3 Point data attribution All data should be attributed with its uncertainty estimate at the 95% confidence level for both position and, if relevant, depth. The computed or assumed scale factor applied to the standard deviation in order to determine the uncertainty at the 95% confidence level, and/or the assumed statistical distribution of errors should be recorded in the survey’s metadata. (For example, assuming a Normal distribution for a 1 Dimensional quantity, such as depth, the scale factor is 1.96 for 95% confidence. A statement such as “Uncertainties have been computed at 95% confidence assuming a standard deviation scale factor of 1.96 (1D) or 2.45 (2D), corresponding to the assumption of a Normal distribution of errors,” would be adequate in the metadata.) For soundings this should preferably be done for each individual sounding; however a single uncertainty estimate may be recorded for a number of soundings or even for an area, provided the difference between the individual uncertainty estimates and the collectively assigned uncertainty estimate is negligible. The attribution should, as a minimum, be sufficient to demonstrate that the requirements of these Standards have been met.


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5.4 Bathymetric model attribution If a Bathymetric Model is required, metadata should include: the model resolution; the computation method; the underlying data density; uncertainty estimate/uncertainty surface for the model; and a description of the underlying data.

5.5 Report of survey The Report of Survey is the principal means by which the Surveyor in Charge approves the contents of all survey records. It must give a clear and comprehensive account of how the survey was carried out, the results achieved, the difficulties encountered and the shortcomings. Emphasis should be placed on the analysis of achieved accuracies and whether the survey specifications have been met.


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6 Elimination of doubtful data

6.1 Introduction To improve the safety of navigation it is desirable to eliminate doubtful data, i.e. data which are usually denoted on charts by PA (Position Approximate), PD (Position Doubtful), ED (Existence Doubtful), SD (Sounding Doubtful) or as "reported danger". To confirm or disprove the existence of such data it is necessary to carefully define the area to be searched and subsequently survey that area according to the standards outlined in this publication.

6.2 Extent of Area to be Searched No empirical formula for defining the search area can cover all situations. For this reason, it is recommended that the search radius should be at least 3 times the estimated position uncertainty of the reported hazard at the 95% confidence level as determined by a thorough investigation of the report on the doubtful data by a qualified hydrographic surveyor. If such report is incomplete or does not exist at all, the position uncertainty must be estimated by other means as, for example, a more general assessment of positioning and depth measurement uncertainties during the era when the data in question was collected.

6.3 Conducting the search The methodology for conducting the search should be based on the nature of the feature, the area in which the doubtful data is reported and the estimated danger of the potential hazard to surface navigation. Once this has been established, the search procedure should be that of conducting a hydrographic survey of the extent defined in 6.2, to the standards established in this publication.

6.4 Presentation of search results Doubtful data shall be replaced with actual data collected during the search if the hazard has been detected. If not detected, the agency responsible for the survey quality shall decide whether to retain the hazard as charted or to delete it.


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Abbreviations and glossary

Abbreviations: 1D 1-Dimensional 2D 2-Dimensional 3D 3-Dimensional dGPS Differential Global Positioning System ED Existence Doubtful ETRS89 European Terrestrial Reference System 1989 GEBCO General Bathymetric Chart of the Oceans GNSS Global Navigation Satellite System IHO International Hydrographic Organisation ITRS International Terrestrial Reference System LAT Lowest Astronomical Tide LIDAR Laser Imaging Detection And Ranging MSL Mean Sea Level MV Middenstandsvlak NAP Normaal Amsterdams Peil PA Position Approximate PD Position Doubtful RD Rijksdriehoeksmeting REP Reported but not confirmed RTK Real Time Kinematic SD Sounding Doubtful SOLAS Safety Of Lives At Sea THU Total Horizontal Uncertainty TPU Total Propagated Uncertainty TVU Total Vertical Uncertainty WGS84 World Geodetic System 1984 Glossary: Note: The terms defined below, are the most relevant for this publication. A much larger selection of terms are defined in IHO Special Publication S-32 “Hydrographic Dictionary” and in the by the Information Desk Standards Water (Aquo-lex) maintained dictionary. If a certain term is not included here, the here mentioned publications should be consulted. If a term as shown below has a different definition with respect to the mentioned publications, then the definition given below, should be used for this publication. While making the Dutch version of this document it was chosen to use the glossary of the Aquo-lex as much as possible whenever their meaning is consistent with that of the glossary from the original S-44. The hydrographic glossary in the Aquo-lex, used the glossary of the Hydrographic Working Group of Rijkswaterstaat, edition 2006, for input. Accuracy: The degree to which a measured or calculated value corresponds to the assumed or accepted value. (See: uncertainty, error). Bathymetric model: A digital representation of the topography (bathymetry) of the seafloor with coordinates and z-values. Blunder: Sole errors that occur through negligence or a mistake. These errors are often large and, by definition, unpredictable. They are usually caused by sudden changes in environmental conditions, system failures or user errors. Blunders are easily detected by performing more measurements than strictly necessary.


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Confidence interval: A measure of uncertainty. Confidence Level: The probability that a given confidence interval contains an unknown. It should be noted that the confidence level (often set at 95%) depends on the assumed statistical distribution of the data and the calculation is different for 1-dimensional (1D) than for 2-dimensional (2D) quantities. In this publication, based on the normal distribution of the error, the 95% confidence level for 1D quantities (z-value) are defined as 1.96 x standard deviation and the 95% level for 2D quantities (XY coordinates) as 2.45 x standard deviation. Correction: The best estimate that can be made of the difference between the actual and the measured value of a parameter. The sign is such that the correction added to the observation yields the measurement. Datum: The vertical reference plane to which z-values during a survey are determined. Error: The difference between an observed or computed value of a quantity and the true value of that quantity. (NB The true value can never be known, therefore the true error can never be known. It is legitimate to talk about error sources, but the values obtained from what has become known as an error budget, and from an analysis of residuals, are uncertainty estimates, not errors. See uncertainty) Feature detection: The ability of a system to detect objects and / or obstacles of a specific size. The standards in this document define the minimum size of objects that must be able to be detected during the survey. Feature: under this standard, an object, artificial or not, which protrudes above the bottom and yields a potential danger to shipping or should be examined. Full sea floor search: A systematic method for exploring the sea floor performed in such a way that most of the features and objects as specified in the applicable standard will be mapped. In practice it is impossible to achieve 100% coverage and the use of the term 100% bathymetric coverage is therefore discouraged. Integrity monitoring: This is the ability of a system to provide timely warnings to users when the system should not be used. Integrity monitor: equipment consisting of a GNSS receiver and radio transmitter set up over a known survey point that is used to monitor the quality of a Differential GNSS (DGNSS) signal/system. Positional discrepancies are continuously monitored and timely warnings are transmitted to users indicating when the system should not be used. Line spacing: The distance between two adjacent survey lines. Metadata: Data about data. Or the information on the characteristics of the data, such as the uncertainty of the survey, the overall quality, data set title, source, uncertainty of positioning and copyrights. Reduced z-value: Observed z-value including all corrections related to the survey and post processing and reduction to the used vertical datum. Survey line: An imaginary line across water and/or land, set out for conducting surveys, measurements, sampling, etc.


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Total horizontal uncertainty (THU): The component of the total propagated uncertainty (TPU) calculated in the horizontal plane. Although THU is quoted as a single figure, THU is a 2-dimensional quantity. The assumption has been made that the horizontal uncertainty is isotropic (i.e. there is a negligible correlation between errors in x- and y-direction). This makes a Normal distribution circularly symmetric allowing a single number to describe the radial distribution of the errors about the true value. Total propagated uncertainty (TPU): The resultant propagation of the uncertainties when all known uncertainties, both random and systematic are considered. The TPU is due to the uncertainty in the measurements and is a 3-dimensional variable. Total vertical uncertainty (TVU): The component of the total propagated uncertainty (TPU) calculated in the vertical dimension. The TVU is a 1-dimensional variable. Uncertainty: The distribution of a stochastic variable around the given value. The corresponding confidence level and the assumed probability distribution should be given thereto. Uncertainty surface: a model, typically grid based, which describes the depth uncertainty of the product of a survey over a contiguous area of the skin of the earth. The uncertainty surface should retain sufficient metadata to describe unambiguously the nature of the uncertainty being described. Under-Keel Clearance: The distance between the lowest point of the hull of the ship, usually a point on the keel, and the bottom Z-value: This term indicates both the value for the depth as for the bottom height. The Royal Netherlands Navy uses only depth. At Rijkswaterstaat the determination of the bottom height is also used.


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Documents

DUTCH STANDARDS FOR HYDROGRAPHIC SURVEYSpublicaties.minienm.nl/download-bijlage/92306/dutch... · For hydrographic surveys in the rest of the control region and hydrographic surveys