INTERNATIONAL HYDROGRAPHIC ORGANIZATION

S-67

MARINERS’ GUIDE TO ACCURACY OF DEPTH INFORMATION IN

ELECTRONIC NAVIGATIONAL CHARTS (ENC)

Edition 0.97

September July 20198

© Copyright International Hydrographic Organization 2018

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S-67

MARINERS’ GUIDE TO ACCURACY OF DEPTH INFORMATION IN

ELECTRONIC NAVIGATIONAL CHARTS (ENC)

Edition 0.97

September July 20198

Published by:

The International Hydrographic Organization4b, quai Antoine 1er

B.P. 445Monaco, MC 98011 Cedex

MONACO<info@iho.int><www.iho.int>

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Foreword

This publication is intended to illuminate the accuracy and reliability of the bathymetry in an Electronic Navigational Charts (ENC) through an understanding of the Data Quality indicators in an ENC. The intended audience for the is publication are mariners and those wishing to improve their knowledge of how to determine which parts of the bathymetry in an ENC are accurate and reliable, and which parts require caution.

Contents

Section

1

ContentPrefaceIntroduction

Page

21 Accuracy of depth information in paper chartsNautical Charts 5

32 Accuracy of depth information in Electronic Navigational ChartsZones of Confidence

5

3.13 Generalized informationENC ZOC Symbols 6

3.24 Safety contourThe components of an assessment 7

3.3…4.1 Depth accuracy in relation to safety contoursTypical survey characteristics

8

3.4…4.2 Quality descriptions of individual objects dangerous to safe navigationSeafloor coverage

9

3.5…4.3 Survey reliabilityPosition accuracy 10

4…4.4 Zones of Confidence symbolsDepth accuracy 12

5 Assessment of the quality of a survey into a Zone of Confidence by the HOImpact of ZOC categories upon mariners

12

…5.1 Assessment examplesAn alternative way to understand ZOC (using the star symbols)

13

…5.2 Position accuracy of a surveyZOC A1 (6 stars) 14

6…5.3 Impact of ZOC categories upon marinersZOC A2 (5 stars) 15

6.1 …5.4

Effect of overzoomingZOC B (4 stars) 16

…5.5 ZOC C (3 stars) 17

…5.6 ZOC D (2 stars) 18

…5.7 ZOC U (U) 19

7 6 Summary and recommendations 19

Annex A Zones Of Confidence Categories 20

Annex B Dangerous effects of over-scale ECDIS display near ‘Isolated dangers’

22

4

Preface

IHO publication S-67 “Mariners Guide to Accuracy of Depth Information in an ENC” is a guide to navigators, and those planning ‘navigational operations’, on the degree of confi-dence they should have in the adequacy and accuracy of charted depths and their positions in an Electronic Navigational Chart.

This document is laid out, as far as possible, along the lines of the IHO publication “Regula-tions of the IHO for International (INT) Charts and Chart Specifications of the IHO – S-4”, “IHO Transfer Standard for Digital Hydrographic Data – S-57” and “Specifications for Chart Content and Display Aspects of ECDIS – S-52.”

The intended readers for this document are navigators on coastal or international voyages, organizations training navigators for coastal and/or international voyages.

This document is supplementary to already existing IHO Standards, to provide a more in-depth knowledge how a navigator should interpret the depth information presented to him/her by an Electronic Chart Display and Information System (ECDIS).

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1. Introduction

The primary purpose of nautical charts is to provide the information required to enable the mariner to plan and execute safe navigation.1 The mariner has a need for appropriate, rele-vant, accurate and unambiguous information.

Most Hydrographic Offices have an obligation to provide nautical chart cover of their national waters to such an extent, and on such scales, as to permit safe navigation for all classes of vessel, from the smallest to the largest, throughout coastal waters, including major ports vis-ited by the largest vessels, and minor arms of the sea of purely local interest. In this, the best-known sense, nautical charts are navigational tools.2

National nautical chart series are usually the largest scale publications available showing the detailed configuration of the seabed offshore. In this respect, hydrographic offices have a de facto responsibility for their national waters similar to that of topographic mapping agencies for land areas. Such information about the shape of the seabed is required by a variety of na-tional users other than navigators: construction engineers concerned with offshore develop-ments, dredging contractors, oceanographers, defense departments, coastal zone managers and so on.3

The combined effect of the two requirements has caused national chart series to cover na-tional waters in great detail, reflected by medium scale charts to provide an overview, gen-eral picture, coastal image and large scale charts to provide information for an approach, harbor and berthing. Hydrographic Offices supply Electronic Charts in agreed scales, so-called Usage Bands. Their values are:

1. Overview2. General3. Coastal4. Approach5. Harbor6. Berthing

The mariner requires charts to be consistent throughout the scales, at least for essential data content; this is called ‘vertical consistency’. At smaller scales, details must be generalized, with only a selection of the available source data (including soundings) being portrayed, so that the information which is selected is clearly present. Any sounding on the smallest scale chart will also be present on the largest scale.4

A chart presents an image of the real world outside to the mariner. The depth information in a chart is compiled from various sources, each having their own adequacy and accuracy. How can the mariner distinguish, when using an ECDIS, what the adequacy and accuracy of the depth information is for the planned and executed voyage?

1 S-4 B-100.42 S-4 A-102.13 S-4 A-102.24 S-4 B-100.5

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12. Accuracy of Nautical depth information in paper Charts

Paper charts provide information to guide navigators, and those planning ‘navigationaloperations’ (including the planning of new routes and official routeing measures), on the degree of confidence they should have in the adequacy and accuracy of charted depths and their positions. This is portrayed in a so-called Source Diagram. A Source Diagram provides details of sources – date and scale. It gives an indication of:

The adequacy of the equipment used The thoroughness of examinations of dangers at particular depths (based on the

maximum draught of vessel afloat at that date) The likelihood of changes in depths, particularly in areas of mobile or unstable

seabed or coral growth.

The date of the edition of a published paper chart can be misleading (as the source data may be much older) but may have some value. Year dates only should normally be used.5

The type of survey should be stated on conventional Source Diagrams (the terms being translated as necessary):

‘ Survey’ implies a regular, controlled or systematic hydrographic survey of any date ‘Sketch survey’ or ‘Reconnaissance survey’ implies that there is a significant risk of

undetected dangers, even if the ‘survey’ is of recent date ‘Passage soundings’ implies soundings acquired on an uncoordinated basis over a

period of years Qualifying comments, for example: ‘(leadline)’; ‘(no sonar)’ and ‘(multibeam)’, may be

added after the type of survey where the date does not give sufficient indication of the survey methods

Where a charted survey is supplemented by occasional soundings from older or later sources, only the main survey should normally be listed.6

Areas of continual and rapid change occur in many tidal rivers and estuaries, for example HugliRiver (India) and Bahia Buenaventura (Colombia); over bars in the approaches to some ports, for example Esbjerg (Denmark) and Karachi (Pakistan); and over some off-lying banks, for example The Goodwin Sands (UK) and The Eastern Approaches to Nantucket Sound (USA).7

In most areas which have not been wire-swept or fully insonified, there is a possibility that depths somewhat shoaler than those charted may exist. Navigators normally allow for that and other uncertainties by allowing safety margins. Inadequately surveyed areas may be defined as those where bathymetry is based on older leadlinesurveys or other surveys which are either open in nature (for example reconnaissance surveys), or are not hydrographic surveys (for example seismic surveys). These types of surveys are inadequate for identifying all shoals that may exist between lines of soundings, or may not be ‘shoal-biased’ in their selection of recorded depths.8

All charts, whether paper or electronic, contain bathymetric data which varies in quality due to the age, accuracy and completeness of the individual surveys that were used to develop the charted depths. A chart can be considered as a jigsaw of individual surveys pieced together to form a single image. Most charts contain a mixture of surveys of differing quality.

In general, remote areas away from shipping routes tend to be surveyed less frequently, less 5 S-4 B-294.16 S-4 B295.27 S-4 B-4168 S-4 B-417

7

accurately, and with lower confidence that all features have been detected. Areas of high commercial traffic are re-surveyed frequently to very high levels of accuracy and completeness, particularly where under-keel clearances are small. However, the vast majority of coastal and international shipping routes fall somewhere between these two examples, where risks in the form of dangers to navigation are less well defined.

To assess these risks, mariners have traditionally relied upon familiar but often ambiguous indicators used on paper charts, usually in a source diagram. The details and interpretations of published Source Diagrams often varied widely between nations., like establishing how old a survey was, rather than how good. The variations in method, detail and interpretation render this type of quality information unsuitable for use in an electronic system such as ECDIS, as it prevents use of automated checking routines to look along a planned route to confirm suitability.

When making the transition from paper chart to the Electronic Navigational Chart, the International Hydrographic Organization developed and published Zones of Confidence areas in their “IHO Transfer Standard for Digital Hydrographic Data – S-57”.

To address this, the International Hydrographic Organization developed and published the international system to be used by all nations within their S-57 Electronic Navigational Charts (ENC). This is the “Zones Of Confidence” system, often referred to as “ZOC”. The degree of reliance which can be placed in the depth information within an ENC can be consistently determined by understanding the Zone of Confidence assessment for an area, then applying a common-sense approach.

23. Zones of ConfidenceAccuracy of depth information in Electronic Navigational Charts

Depth Accuracy in Electronic Navigational Charts is described in two ways:

1. Generalized information in Zones of Confidence (ZOC) diagrams 2. Quality descriptions of individual objects dangerous to safe navigation3. Reliability of a survey

3.1 Generalized information:The quality of the hydrographic source data is assessed according to six categories (CATZOC): five quality categories for assessed data (A1, A2, B, C and D) and a sixth category (U) for data which has not been assessed.9 The assessment of hydrographic data quality and classification into zones is based on a combination of:

a. seafloor coverageb. depth accuracyc. position accuracy

The higher ZOC categories, A1 and A2, demand full seafloor ensonification or sweep and require very high accuracy standards which have only been achievable with the technology available since about 1980. Therefore many sea lanes which have hitherto been regarded as adequately surveyed may carry a ZOC B classification. Modern surveys of critical areas can be expected to carry ZOC A2 classification whilst ZOC A1 will cover only those areas

9 S-4 B297.4

8

surveyed under exceptionally stringent conditions for very special reasons.10

All S-57 ENCs use the Zones Of Confidence (ZOC) system. There may be several different ZOC areas within each individual ENC. These assessments enable mariners to consider the limitation of the bathymetric data from which the ENC was compiled, and to assess the associated level of risk to navigate in a particular area.

The ZOC system only applies to the bathymetry (depths, contours, submerged rocks and reefs, etc) – it does not apply to the accuracy of charting the high water line, wharves, navigation aids, pipelines and so on.

There are five basic levels within the ZOC system. Each differing level of quality is referred to as a ‘category’ within the overall ZOC system. Each category is therefore labeled as ‘CATZOC’ when queried within the ENC. The categories range from ‘very high confidence’ to ‘very low confidence’. There is an additional category for ‘Unassessed’. The impact upon mariners of the various categories is discussed in Section 5.

ZOC Categories:

ZOC Position accuracy Depth accuracy Seafloor coverageA1 ± 5m + 5% depth 0.50 m + 1% depth Full area search undertaken. Significant

seafloor features detected and depths measured.

A2 ± 20m 1.00 m + 2% depth Full area search undertaken. Significant seafloor features detected and depths measured.

B ± 50m 1.00 m + 2% depth Full area search not achieved; uncharted features hazardous surface navigation are not expected but may exist.

C ± 500m 2.00 + 5% depth Full area search not achieved, depth anomalies may be expected.

D Worse than ZOC C

Worse than ZOC C Full area search not achieved, large depth anomalies may be expected.

U Unassessed – The quality of the depth data has yet to be assessed.

The full version of this table can be found in Annex – A

As a Quick Reference Guide, the table above can be interpreted as follows:1. Good Quality Area (ZOC A1 and A2)2. Medium Quality Area (ZOC B)3. Poor Quality Area (ZOC C, D and U)

Good Quality AreaThe depth of this area has been measured by a collection of regular, controlled or systematic hydrographic surveys. Significant seafloor features dangerous to the safety of navigation (rocks, coral reefs, wrecks, submerged obstructions) have been identified, accurately positioned and their least depth value has been accurately determined. The mariner is advised not to navigate closer than 20 meter to these features unless they are so deep that the mariner is confident to sail over them.

Medium Quality AreaThere is a chance that significant seafloor features dangerous to the safety of navigation

10 S-4 B297.6

9

(rocks, coral reefs, wrecks, submerged obstructions) have not been identified, and do not appear in the chart. Those features that are present have a horizontal accuracy of ± 50 meter. The mariner is advised not to navigate closer than 50 meter to these features unless they are so deep that the mariner is confident to sail over them.

Poor Quality AreaThe mariner should take appropriate caution when navigating through this area. Charted depths may in reality be significantly shallower. It is very likely that some significant seafloor features dangerous to the safety of navigation (rocks, coral reefs, wrecks, submerged obstructions) have not been identified, and do not appear in the chart. Those features that are present have a horizontal accuracy of ± 500 meter. The mariner is advised not to navigate closer than 500 meter to these features unless they are so deep that the mariner is confident to sail over them.

The figure below shows this where a charted shoal may be out of position in areas of CATZOC C, D or U. The difference between the charted and true position of the shoal is much greater than the difference between the GNSS measured ship’s position and the ship’s true position. The mariner is advised to take appropriate caution.

A planned route should allow for both chart accuracy and ship’s positioning accuracy, as well as other factors.

3.2 Safety contourIn an ECDIS the default setting for a safety contour is the 30 meter depth contour. If the mariner is using the default settings of an ECDIS, depth areas deeper than 30 meters will be presented in white and areas shallower than 30 meters will be presented in blue. If the mariner enters a safety depth into ECDIS, the system will search for the nearest deeper depth contour and assign this as the safety contour to be used. White and blue colors will be adjusted accordingly.

In an ENC, the following contour lines are standard available:0, 2m, 5m, 10m, 20m, 30m, 50m, 100m, 200m, 300m, 400m, 500m, 1000m, 2000m, 3000m, 4000m.

Optionally the ENC may contain the following depth contours:3m, 8m, 15m, 25m, 40m, 75m, 600m, 700m, 800m, 900m.

For surface navigation the 0 to 100m range is of importance. For submarine navigation and deep water fishing the 0 to 800m is relevant.

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3.3 Depth accuracy in relation to safety contours

CATZOC provides a general impression of the quality of the source data that is used to create depth areas bounded by depth contours. A depth area is an area where the charted depths are bounded by a minimum and (possibly) maximum value. A depth contour is default displayed as a solid line, a boundary between deeper and shallower water. The Hydrographic Office may have provided additional information that the contour line is approximate, it will then be displayed as a dashed line.Several different depth areas may have the same CATZOC value. On the other hand, within one depth area more than one CATZOC value may be present. Zones of Confidence never overlap and have no gaps inbetween.

The mariner should take note of the accuracy of the depth areas the vessel is planning to transit and take appropriate caution:

CATZOCdept

hA1 A2 B C D U

0 0.5m 1.0m 1.0m 2.0m >2.0m unknown10 0.6m 1.2m 1.2m 2.5m >2.5m unknown20 0.7m 1.4m 1.4m 3.0m >3.0m unknown30 0.8m 1.6m 1.6m 3.5m >3.5m unknown40 0.9m 1.8m 1.8m 4.0m >4.0m unknown50 1.0m 2.0m 2.0m 4.5m >4.5m unknown75 1.3m 2.5m 2.5m 5.8m >5.8m unknown100 1.5m 3.0m 3.0m 7.0m >7.0m unknown

3.4 Quality descriptions of individual objects dangerous to safe navigation

In “IHO Transfer Standard for Digital Hydrographic Data – S-57”. the following (subsurface) items are considered to be hazardous to safe navigation:

O bstructions R ocks S oundings W recks

These items may contain additional quality information only applicable to this item. The structure of the ENC allows Hydrographic Offices to add this information, however it is not mandatory for the Hydrographic Offices to do so.

Obstructions:The following items are considered to be an obstruction11:

snags stumps wellheads diffusers cribs fish havens foul areas foul grounds

11 S-57 Appendix B.1 Annex A UOC Edition 4.1.0

11

booms ice booms sites of cleared platforms ground tackle

Individual obstructions, rocks, soundings, wrecks may have the following quality information:

Object Additional information OptionsObstructionRockSoundingWreck

Exposition of sounding 1. within the range of depth of surrounding depth area

2. shoaler than the range of depth of surrounding depth area

3. deeper than the range of depth of surrounding depth area

Quality of sounding 1. depth known2. depth unknown3. doubtful sounding4. unreliable sounding5. no bottom found at value shown6. least depth unknown7. least depth unknown, safe clearance at

value shown8. value reported, not surveyed9. value reported, not confirmed10. maintained depth11. not regularly maintained

Sounding Accuracy Value in meters

Technique of sounding measurement

1. found by echosounder2. found by side-scan sonar3. found by multi-beam4. found by diver5. found by lead-line6. swept by wire drag7. found by laser8. swept by vertical acoustic system9. found by electromagnetic sensor10. photogrammetry11. satellite imagery12. found by leveling (not applicable)13. swept by side-scan sonar14. computer generated

For obstructions, note the difference between a foul area and a foul ground. A foul area is defined as an area of numerous uncharted dangers to navigation. If the Hydrographic Office creates a foul area in an ENC, it will show in an ECDIS “base display” as an obstruction to navigation, with all associated alarms to indicate that it is unsafe for vessels to enter or transit the area.

Foul ground is defined as an area over which it is safe to navigate but which should be avoided for anchoring, taking the ground or fishing.

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The mariner can execute a “pick report” in the ECDIS to show the underlying information of an obstruction, rock sounding or wreck.

Note that the horizontal position accuracy for individual objects is not available in S-57. The value of the overlaying CATZOC applies to the horizontal accuracies of individual obstructions, rocks, soundings and wrecks.

3.5 Survey reliability

The Hydrographic Office may provide additional quality information on individual surveys. The information can be viewed by executing a pick report on the area. The components of the information are12:

Attribute Allowable values DefinitionsQuality of Position 1: surveyed the position(s) was(were) determined by the

operation of making measurementsfor determining the relative position of points on, above or beneath the earth’ssurface. Survey implies a regular, controlled survey of any date. (adapted from IHO Dictionary, S-32, 5195, & IHO Chart Specifications, M-4, 175.2)

2: unsurveyed survey data is does not exist or is very poor. (adapted from IHO Dictionary, S-32, 5732)

3: inadequately surveyed position data is of a very poor quality. (adapted from IHO Dictionary, S-32, 5732)

4: approximate a position that is considered to be less than third-order accuracy, but is generallyconsidered to be within 30.5 meters of its correct geographic location. Also may apply to an object whose position does not remain fixed. (adapted from IHO Dictionary, S-32, 213, 3967, & IHO Specifications, M-4, 424.1)

5: position doubtful an object whose position has been reported but which is considered to be doubtful.

6: unreliable an object’s position obtained from questionable or unreliable data.

7: reported (not surveyed) an object whose position has been reported and its position confirmed by somemeans other than a formal survey such as an independent report of the sameobject.

8: reported (not confirmed) an object whose position has been reported and its position has not beenconfirmed.

9: estimated the most probable position of an object determined from incomplete data or dataof questionable accuracy. (adapted from IHO Dictionary, S-32, 3960)

10: precisely known a position that is of a known value, such as the position of an anchor berth or other defined object.

11: calculated a position that is computed from data.

12 S-57 Appendix A, Chapter 2 - Attributes

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Quality of sounding measurement

1: depth known the depth from chart datum to the bottom is a known value.

2: depth unknown the depth from chart datum to the bottom is unknown.

3: doubtful sounding a depth that may be less than indicated. (adapted from IHO Dictionary, S-32, 5thEdition, 4840)

4: unreliable sounding a depth that is considered to be an unreliable value.

5: no bottom found at value shown

upon investigation the bottom was not found at this depth. (adapted from IHO Dictionary, S-32, 5th Edition, 4848)

6: least depth known the shoalest depth over a feature is of known value. (adapted from IHO Dictionary, S-32, 5th Edition, 2705)

7: least depth unknown, safe clearance at value shown

the least depth over a feature is unknown, but there is considered to be safe clearance at this depth.

8: value reported (not surveyed)

depth value obtained from a report, but not fully surveyed.

9: value reported (not confirmed)

depth value obtained from a report, which it has not been possible to confirm.

10: maintained depth the depth at which a channel is kept by human influence, usually by dredging.(IHO Dictionary, S-32, 5th Edition, 3057)

11: not regularly maintained depths may be altered by human influence, but will not be routinely maintained.

Scale value one numerical value(25000 -> scale 1:25 000)

The largest scale for the range of survey scale as used in source diagram information.

Scale value two Numerical value(250000 -> scale 1:250 000)

The smallest scale for the range of survey scale as used in source diagram information.

Sounding distance - minimum

numerical value(50 for 50 meters or feet)

The minimum spacing of the principal sounding lines of a survey.

Sounding distance - maximum

numerical value(150 for 150 meters or feet)

The maximum spacing of the principal sounding lines of a survey.

Survey authority name of the source survey

authorityThe authority which was responsible for the survey.

Survey end date CCYYMMDD

CCYYMMCCYY

The ‘survey date, end’ should be encoded using 4 digits for the calendar year (CCYY), 2 digits for the month (MM) (e.g. April = 04) and 2 digits for the day (DD). When no specific month and/or day is required/known, indication of the month and/or the day is omitted. This conforms to ISO 8601: 1988.

Survey start date CCYYMMDDCCYYMMCCYY

The ‘survey date, start’ should be encoded using 4 digits for the calendar year (CCYY), 2 digits for the month (MM) (e.g. April = 04) and 2 digits for the day (DD). When no specific month and/or day is required/known,

14

indication of the month and/or the day is omitted. This conforms to ISO 8601: 1988.

Survey type 1: reconnaissance/sketch

surveya survey made to a lower degree of accuracy and detail than the chosen scalewould normally indicate. (IHO Dictionary, S-32, 5th Edition, 5219)

2: controlled survey a thorough survey usually conducted with reference to guidelines.

3: unsurveyed – no longer in use

unsurveyed should now be encoded using the object unsurveyed area.

4: examination survey a survey principally aimed at the investigation of underwater obstructions and dangers.

5: passage survey a survey where soundings are acquired by vessels on passage

6: remotely sensed a survey where features have been positioned and delimited using remote sensing techniques.

INFORM text textual information about the objectNINFORM text textual information in national language

characters

15

The various ZOC categories are:

Category Confidence level General description - survey characteristicsA1 Significant seafloor features

detected and depths measuredHigh position and depth accuracy achieved using DGPS and a multi-beam, channel or mechanical sweep system.

A2 Significant seafloor features detected and depths measured.

Position and depth accuracy less than ZOC A1, achieved using a modern survey echo-sounder and a sonar or mechanical sweep system.

B Uncharted features, hazardous to surface navigation are not expected but may exist.

Similar depth accuracy as ZOC A2 but lesser position accuracy than ZOC A2 (generally pre-dating DGPS), using a modern survey echo-sounder, but no sonar or mechanical sweep system.

C Depth anomalies may be expected.

Low accuracy survey, data collected on an opportunity basis such as soundings on passage or low accuracy given due the passage of time.

D Large depth anomalies may be expected.

Poor quality data or unsurveyed.

U Unassessed. The quality of the bathymetric data has yet to be assessed. (Mariners should assume poor data quality until the area has been assessed).

The full version of this Table may be found in Section 7 at the end of this publication.

34. ENC ZOCZones of Confidence sSymbols

There are two different types of symbols for Zones of Confidence in an ENC:

Downward triangle Horizontal bar

There are two validations of Zones of Confidence:

Assessed Unassessed

Areas that have been assessed are symbolized by the number of stars inside the triangle or bar. Areas which have not been assessed are symbolized by the letter U inside the bar.

The number of stars inside the triangle or bar is an indication of the CATZOC value:6 stars = A15 stars = A24 stars = B3 stars = C2 stars = D

In the ENC the different ZOC quality levels are denoted by a series of symbols containing a varying number of stars, enclosed within a triangle or ellipse. This symbol is repeated throughout each area of equal quality. The symbols can be made visible or be hidden on the ECDIS screen depending upon the mariner’s needs at any particular time. The various categories range from six stars down to two stars. There is an additional category for areas which are ‘Unassessed’.

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Zones Of Confidence symbols, categories and depiction on an ENC.

The triangles are placed horizontally across the screen with a fixed separation of between two symbols. The bars are also placed horizontally across the screen with a fixed separation. The CATZOC value at the gravity point of each symbol is used the depict the value. This means that occasionally a CATZOC symbol may be pictured over two adjacent CATZOC areas with different values whereas only one value can be depicted. This type of symbology was developed in the 1990’s when computer monitors had less sophisticated capabilities than today’s technology.

To view the Zones of Confidence symbology, the mariner has to activate the “information on chart display layer” or a similar setting, depending on the type of ECDIS used.

This kind of symbology tends to clutter the screen. During execution of a voyage mariners will most likely de-activate this setting. However, when planning a new route of changing an existing route whilst enroute, mariners are recommended to active this and use the information before accepting the new route in the ECDIS system.

Quick Reference: 5 stars of more = good quality area 4 stars = medium quality area 3 stars of less = poor quality area U = unassessed, take appropriate caution

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45. The components of an assessmentAssessment of the quality of a survey into a Zone of Confidence by the HO

Hydrographic Offices receive different kinds of data collected by different technologies. Some data may be more than 50 years old whereas other data is collected with the latest technology. Some data may be collected using a leadline from a ship, other data may be measured by satellite from space. All this data is compiled to provide an image of the seabed and objects above the seabed. Some data is collected by the Hydrographic Office, other data may come from port authorities, scientific research institutes and through private ship-owners. The Hydrographic Office has the task to evaluate the quality of the data received and decide if and how this data should be made available to update the ENC. :

As a general guideline, the following choices are made by the Hydrographic Office: data from ports are assigned CATZOC B, some port authorities provide CATZOC A1 satellite data is assigned CATZOC C laser date by plane is assigned CATZOC B, sometimes A2 private ship-owner data is assigned CATZOC D data before 1980 is assigned CATZOC B, C or D. In general, the older the data, the

lower the value.

On a case-by-case basis, the Hydrographic Office may deviate from these general guidelines as they seem fit taking into account local knowledge of the area, intended shipping routes etc.

Assessments are made based upon four criteria, following which a single ZOC rating is derived for each area of differing quality. The lowest rating for any individual component within that area determines which ZOC category is assigned.

Individual assessment criteria are:

typical survey characteristics;

seafloor coverage (this relates to the possibility that something may have been missed and is therefore not on the chart);

position accuracy;

depth accuracy (this relates only to what has been detected and is therefore charted, not what might remain undetected).

The first criteria considered is the typical survey characteristics. The planning of the survey, the techniques used and the datum used are considered initially when assigning ZOC. A well planned and conducted survey has much greater potential than a poor one.

The most important criteria for mariners is seafloor coverage. For over 95% of the world’s coastal waters this determines the sensible clearance that should be maintained between a ship’s keel and the seabed and where any additional precautions may be needed. In the majority of coastal waters, and many oceanic areas, the potential size of an undetected seabed anomaly lying between the charted depths may be equal to or larger than any uncertainty indicating how good the charted depths may be.

The next most important criteria is position accuracy. As there will always be shoals and other features either too shallow or too risky to steam over, the sensible approach is to avoid them. The position accuracy for each category gives some idea of how far away from a hazard a ship should remain.

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The least important criteria is depth accuracy, simply because it is the controlling factor in only a small proportion of the world’s coastal waters. It only assumes greater relevance where full seabed coverage has been achieved, such as within or near ports, or in certain channels. In areas where full seabed coverage has not been achieved, the safety margin allowing for the possibility of uncharted hazards is much larger than the allowance for the accuracy of charted depths.

Mariners should not require a detailed understanding of survey characteristics, as long as they understand the implications for shipping explained within each different ZOC category. These four major contributing factors are discussed further in the following paragraphs, with the implications for shipping in each ZOC area discussed in Section 5.

One limitation of the ZOC system is the lack of information about when a survey was conducted, or whether the seabed is stable. While the date can be provided in an additional data field within an ENC, this is rarely done, doesn’t form part of the chart ’image’, and may be difficult to find. In areas where the seabed is subject to change, national hydrographic offices should be downgrading the assigned ZOC category, restoring it only once a replacement survey is incorporated. However, this isn’t always done, so it’s wise to note areas of sand-waves, dates within dredged channels, and any other notes advising that channels may have changed.

4.15.1 Typical survey characteristicsAssessment examples

Typical survey characteristics are the first considerations to make an assessment of seafloor coverage, depth accuracy and position accuracy. and generally overlap with the other three assessment criteria. The completeness of the survey with regard to the known nature of the seafloor is the first factor for assessment. Are there any holes in area of the survey that may have been important to survey? Are the techniques used for positioning and depth measurement modern? Finally, the systematic nature of the survey; does the survey comprise planned survey lines on a known geodetic datum that can be accurately transformed to WGS 84. How accurate are the transformation parameters when converting an old survey (before 1980) to the WGS84 datum used in the ENC? The Hydrographic Office should take this into consideration and downgrade the CATZOC areas accordingly.

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In this example, a single beam survey conducted in 1963 is very complete. Developments (more survey lines) were made around the shoal areas and crosslines were conducted to see if any shoals existed between survey lines. Due to this completeness of this survey no uncharted features, hazardous to surface navigation are expected. The resulting charted depth data would be given CATZOC of B. The area could not be given a CATZOC of A1 or A2 because full seafloor coverage was not achieved.

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4.2 Seafloor coverage

This is the most important factor in assessing and categorizing a survey. What is in the gaps between charted depths? Did the surveyor miss anything? Was it potentially small or very large? The question of whether there are any undetected dangers in an area affects the majority of the world’s coastal and oceanic waters – it is only when there is confidence that nothing has been missed (and therefore nothing left off the chart) that the question of how close a ship can pass to the charted seabed becomes relevant.

The possibility of dangers being missed typically arises from older surveys, which simply were not as effective as using modern systems.

In this example, the older handwritten survey was completed in 1899. It was done by leadline measurements (recorded in fathoms)*. These measurements are actually quite accurate. However, they are only isolated measurements, with no guarantee of finding any hazard between one leadline depth and the next. This old survey only includes hazards seen by the surveyors at or above the sea surface. It was assessed as ZOC C – depth anomalies may be expected.

In contrast, depths taken from the modern metric survey shows a significant 2.1 metremeter shoal not found during the original survey. It proves that the 1899 survey, if it was the only survey in this area, could not be trusted, and that precautions should be taken.

(* 1 fathom equals 1.8 metresmeters.)

It is only in ZOC areas A1 and A2 where full seafloor coverage has been achieved. It is therefore only in these areas that the accuracy of the charted depths directly defines where a ship can go, and how deep the draft of that ship can be.

Even then, according to the ZOC system, there is a very small possibility that small features may remain undetected (less than a maximum size of 2 cubic metres for depths less than 40 metres). More information is available in the ZOC A1 and A2 sections of this publication.

ZOC B, C and D areas result from surveys that were progressively less detailed. In these areas there is an increasing possibility of undetected features absent from the chart (ranging from a small rock or shoal through to a submerged reef).

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In a ZOC B area there is unlikely to be anything undetected affecting surface navigation , though it remains possible. The hydrographic office responsible for the chart will have (or should have) made their assessment based upon the quality of the survey, the depth of water and the size of vessels using the area. More information is available in the ZOC B section of this publication.

In a ZOC C area there is a strong possibility of undetected features (or charted features significantly out of position). These areas can be considered inadequately surveyed. More information is available in the ZOC C section of this publication.

In a ZOC D area there is a very strong likelihood of large undetected features absent from the chart (or charted features even further out of position). As these areas either have no systematic survey, or are completely unsurveyed, these features may well be as large as an entire submerged reef rising to just below the surface. If contemplating entering a ZOC D area, extensive precautions should be taken, in order to ensure there is sufficient time to react to dangers as they are revealed. More information is available in the ZOC D section of this publication.

4.3 Position accuracy of a survey

The next important factor in most circumstances is accuracy of position of the bathymetry. This includes depth contours, charted depths, reef edges or other charted seabed features. Positioning accuracy of a survey is typically determined by the positioning systems used during the hydrographic survey. The accuracy to position a ship anywhere on the globe has significantly improved over the last 100 years. , as well as any loss of accuracy transferring older data from the survey to the chart, or between older datums and WGS84.

Since 1978 the US government has provided a space-based radio navigation system, operated by US Air Force. This service, Global Positioning System (GPS), is available to an unlimited number of users with a GPS receiver. The user can determine accurate time and location, in any weather, day or night, anywhere around the globe. Reception in Artic areas is less due to the fact that the satellites do not overpass these areas. Other countries have provided a similar service, GLONASS (Russian), Beidou (Chinese) and Galileo (EU). A user with a Global Navigations Satellite System (GNSS) receiver can now use all these services at the same time, thus improving the horizontal and vertical accuracy of its position.

The accuracy of a GPS receiver in the 1980’s was approximately 30 meters. For hydrographic surveys, a land-based correction signal was supplied to correct for errors introduced by US Air Force for military purposes and for signal loss between satellites and receiver. The initial accuracy of 30 meters was initially brought down to 2 meters and eventually to 0.10 meters. The accuracy for a standard GNSS receiver is nowadays in the range of 5 meters. With the full service of Galileo soon, the accuracy of a standalone GNSS receiver will become 0.20 meters. This means that the position of the ship will become (far) more accurate than the surveys previously collected and charted.

Most ships using modern satellite based navigation systems can be navigated with much greater accuracy than most of the surveys still used in charts. While some parts of a chart will be based upon modern surveys, away from the most critical areas most charts still rely upon surveys done with progressively older survey systems. While they were on the forefront of technology and science for their time, few of these systems were as good as Global Navigation Satellite System (GNSS) of today.

It is only since the 1990s that satellite based navigation systems for survey ships have been widely available. These give a positioning accuracy of seabed features somewhere in the range of 2 to 20 metres. (Determining the position of a feature on the seabed can be much

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harder than just positioning the survey ship itself.)

From the late 1940s to the 1990s survey ships depended upon shore-based electronic positioning systems transmitting their signal over short or medium ranges, giving accuracy of around 20 to 100 metersres. In coastal areas, this means that anything the ship found could be up to 100 metersres from where it was thought to be. Much of this depended upon how accurately the transmitter ashore was positioned, as well as the accuracy of the transmitted ranges to generate the ‘fix’.

Prior to this, survey ships used sextants to measure angles between a system of prominent marks, or flag poles built on towers established ashore, with surveyors ‘angling’ for hours at a time. A second row of towers could be built in shallow water or on reefs to extend the network further offshore, but with a further reduction in accuracy. Depending upon how accurately the towers were placed, accuracy of 50 to 500 metersres was possible for the survey ship. So again, when something was found, particularly offshore, the true position could quite easily be up to 500 metersres from where it was surveyed to be.

Up to early 1940s: Survey flag on an offshore reef to extend horizontal sextant control further offshore could achieve accuracy typically between 50 – 500 metersres compared to GNSS.

Late 1940s to mid 1990smid-1990s: Shore based electronic position fixing systems could achieve accuracy typically between 20 – 100 metersres compared to GNSS.

Further offshore, where information was collected by ships relying entirely upon celestial navigation, positions could be considerably less accurate, typically no better than 1 to 2NM, and frequently worse.

While modern satellite imagery can be used to correct the position of many isolated visible offshore features, such as islands, reefs or perhaps shoals breaking in rough weather, anything more than a few metersres below the surface is likely to remain unseen, and therefore possibly well out of its true position.

However, the quoted positional accuracy of a charted features is only part of assessing how

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far to stay clear of a potential danger.

A planned route should allow for both chart accuracy and ship’s positioning accuracy, as well as other factors.

Using the example of a shoal surveyed to a 50 metre positional accuracy in a coastal area, the Master also needs to consider the size of the vessel, the accuracy of the ship’s navigation system, and possibly other vessel parameters. The more correct safe distance in this example is likely to be over 80 metres:

50m Positional accuracy of charted seabed feature, plus15m GNSS accuracy, plus 15m half ship’s beam, plus 5m ship orientation / motion =85m Total safe distance to clear hazard

4.4 Depth accuracy

Depth accuracy refers to how well the depth of a known feature has been measured below chart datum – it clearly does not refer to the accuracy of something which remains undetected. The margin allowed by a ship’s Master for the possibility that something remains undetected within a survey is a separate concern influenced by the seafloor coverage.

The three biggest factors affecting depth accuracy in relatively shallow coastal waters are the accuracy of the tidal observations, the motion of the survey ship and the setup of the echo sounder. Old leadline surveys actually contain very accurately measured depths, however they have a high risk of not detecting shoaler depths nearby. In contrast, a modern multibeam echo sounder misses very little, but requires careful setup and use to deliver accurate results.

5. Impact of ZOC categories upon mariners

Put in simple terms, mariners should be able to navigate with confidence in areas with ZOC A1 and A2 classifications. It is possible, but also unlikely that an uncharted danger affecting surface navigation exists in ZOC B areas. In ZOC C areas mariners should exercise caution since hazardous uncharted features may be expected, particularly in or near reef and rocky areas. A very high degree of caution is required for areas assessed as ZOC D, as these contain either very sparse data or may not have been surveyed at all. Finally, it is good

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practice to treat ZOC U areas with the same degree of caution as ZOC D areas.

To put this in perspective, the following table is an overall analysis of over 14 million square kilometers of coastal ENC 13 from 32 nations:

Category % area of English Channel

% area of Singapore & Malacca Straits

% area of world’s coastal ENC (32 nations)

Confidence

A1 (6 stars) 3.6% 1.4% 0.7% Very GoodA2 (5 stars) 9.4% 0.2% 1.0% Very GoodB (4 stars) 62.9% 2.5% 30.5% GoodMediumC (3 stars) 21.3% 76.2% 21.8% FairPoorD (2 stars) 2.8% 1.1% 20.5% LowPoorUnassessed (U) 0.0% 18.5% 25.4% LowPoor

5.1 An alternative way to understand ZOC (using the star symbols)

An alternative way to understand the basic concept of confidence levels might be to think about the number of stars symbolizing each area. Even if the specifics are not considered, most people understand that if something is given more stars in an assessment than another, the one with more stars is considered to be ‘better’.

A good example that works similarly to ZOCs might be choosing a hotel from listings on a website. Everyone knows that a six star hotel is better than a three star hotel. Equally, when a listed hotel hasn’t been assessed, most people are unsure so tend to assume it isn’t very good.

Zones Of Confidence work exactly the same way. Wherever possible ships should be kept in those areas rated with a higher number of stars (preferably four or more, and three stars only with caution). Those areas with only two stars, or unassessed, should be considered very carefully and avoided if circumstances permit.

13 From Navigation Purpose 3 and 4 ENC in 2015, covering 14,218,244 SQ KM. The analysis did not include ports.

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5.2 ZOC A1 (6 stars)

Surveys within this category have met the requirements for full seabed search. ZOC A1 is only achievable with recent technology.

ZOC A1 surveys generally only cover those areas of minimal under-keel clearance in harbours and shallow channels. The likelihood of any remaining undetected features is extremely low, and is most likely to be the result of undetected silting, or a channel which moves as a result of storms or seasonal changes. A very high degree of confidence can be had that there are no uncharted features between the charted depths or other features already shown on the chart.

The positions of charted seabed features should be better than 5 metres.

In practical terms, mariners should only require a relatively small allowance for an under-keel clearance in a ZOC A1 area. For a 10-20m draft ship this would be an allowance of at least 0.6 to 0.8m for the accuracy of charted depths, plus allowances for squat, settlement, ship motion and the accuracy of tidal predictions (if real-time tides are unavailable).

ZOC A1 – Full area search undertaken. Significant seafloor features detected (Annex A, note 4) and depths measured.

Position accuracy (uncertainty) better than 5 metres. Depth accuracy (uncertainty) approximately 0.6 – 0.8m (Annex A, table).

If the Master considers that there is the possibility of undetected features, such as in an area where depths may have recently changed, it may be wise to allow another 2m safety margin.

Conversely, the Harbour Master or pilot may advise that a smaller under-keel margin is possible. This will be the result of what is known as a ‘Special Order’ survey. While still within the overall ZOC A1 category, these surveys have achieved vertical accuracy better than +/- 0.25m. Under-keel margins this small should only be considered on the specific advice of the Harbour Master or Pilot, supported by real-time tidal observations, or with the benefit of excellent, and very recent, local knowledge. Without this knowledge or advice, under-keel margins as small as these should not be considered.

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5.3 ZOC A2 (5 stars)

Surveys within this category have also met the requirements for full seabed search. They have the same level of confidence as ZOC A1 that there are no uncharted features lying between the charted depths or other features already shown on the chart. However, the safety margins the Master should allow in a ZOC A2 area are larger than those in a ZOC A1 area.

Surveys meeting ZOC A2 requirements are generally undertaken in those port areas used by smaller vessels (such as outside the dredged channel), as well as port approach areas and coastal shipping routes. ZOC A2 areas may be subject to a program of periodic re-survey but this is likely to be less frequent.

The positions of charted seabed features should be better than 20 metres.

In practical terms, mariners should only require a relatively small allowance for an under-keel clearance in a ZOC A2 area, approximately twice as much as for ZOC A1. For a 10-20m draft ship this would be an allowance of at least 1.2 to 1.4m in a ZOC A2 area, plus allowances for squat, settlement, ship motion and the accuracy of tidal predictions (if real-time tides are unavailable).

ZOC A2 – Full area search undertaken. Significant seafloor features detected (Annex A, note 4) and depths measured.

Position accuracy (uncertainty) better than 20 metres. Depth accuracy (uncertainty) approximately 1.2 – 1.4m (Annex A, table).

If the Master considers that there is the possibility of undetected features, such as in an area where depths may change due to silting, it may be wise to allow an additional 2m safety margin.

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5.4 ZOC B (4 stars)

ZOC B typically includes well conducted coastal surveys prior to the late 1990s. Many sea lanes regarded as adequately surveyed carry a ZOC B classification, and have proven entirely suitable over time. Some surveys are still conducted to this standard away from shipping routes.

While the vertical accuracy of charted depths (the ‘known’ depths) is the same as for ZOC A2, the difference is in the size of seabed objects which may not have been detected and therefore are not charted. The size of possible undetected features is not specified.

However, in assigning ZOC B to an area, the national hydrographic office responsible for the ENC has assessed that ‘undetected features hazardous to surface navigation are unlikely but may exist’. In making that assessment, they are likely to have considered ‘surface navigation’ as shipping that draws no more than 15 metres when underway (even though vessels with greater draft now exist). This draft estimate is likely to vary from one hydrographic office to another.

The positions of charted seabed features should be better than 50 metres.

As a general recommendation, it would be prudent to allow at least an additional 5 metres under-keel margin in ZOC B areas covering well used shipping routes, and more in other ZOC B areas. These margins should be increased in or near reef or rocky areas or in areas subject to change (such as a sand-wave area).

ZOC B – Full area search not achieved; uncharted features, hazardous to surface navigation are not expected but may exist.

Position accuracy (uncertainty) better than 50 metres. Depth accuracy (uncertainty) of surveyed features approximately 1.0m (Annex A, table).

However, the potential size of undetected features is not specified.

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5.5 ZOC C (3 stars)

It is the expectation that uncharted features hazardous to surface navigation are likely to exist that is the key difference between this category and ZOC B. ZOC C covers a broad range of surveys, including:

relatively modern surveys which may be very thorough but just fail to meet the higher position accuracy ZOC B standard;

older systematic surveys best described as ‘historic’ and likely to have either missed shoals, or did not fully investigate shoals that have become significant over time as the size of ships has increased;

passage sounding (as long as they are not just isolated tracks).

The positions of charted seabed features should be better than 500 metres.

A typical ZOC C area is unlikely to have included a comprehensive sonar sweep to cover the gap between adjacent survey lines. As the distance between survey lines may be from 250m to as much as a kilometre (0.6NM), it is highly likely that seabed features may remain undetected between those lines. A ZOC C survey may be considered ‘inadequately surveyed’, particularly for depths of 30 to 40 metres or less.

Caution is therefore advised when navigating close to shore or adjacent reefs or rocky areas, where depths may rise rapidly from the sea floor, or where the seabed appears subject to change.

ZOC C – Full area search not achieved, depth anomalies may be expected. Position accuracy (uncertainty) better than 500 metres. Depth accuracy (uncertainty) of surveyed features approximately 2.0m (Annex A, table).

However, the potential size of undetected features is not specified and may be significantly larger than the accuracy of known features.

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5.6 ZOC D (2 stars)

Soundings in ZOC D areas are similarly sourced from historic surveys, but in this case those conducted with large distances between adjacent survey lines, or simply soundings collected on an opportunity basis by ships undertaking routine passage.

Large depth anomalies may include:

Uncharted features rising from the seafloor to the surface in coastal areas – 20 and 50 metre high pillars rising to a metre below the surface have been found in former ZOC D areas;

Uncharted seamounts or very poorly positioned coral atolls in oceanic areas;

Uncharted shoals rising several hundred metres from the seafloor, with gradients too steep for a vessel at sea speed to stop or turn away in time.

The positions of charted seabed features are likely to be worse than 500 metres.

Although many ZOC D areas will appear blank (unsurveyed), some may show a few broken depth contours or isolated depths. These should not be considered an invitation to disregard the ZOC D assessment.

In attempting to navigate a ZOC D area, while following a line of depths on the chart may be better than navigating the white space between, it remains a very poor precaution. Passage soundings typically come from low accuracy and often very old sources – the earlier ship may have passed close to, but not detected, a significant shoal. Equally, the true track followed by the original ship may be well over 500 metres from where they reported it to be.

ZOC D – Full area search not achieved, large depth anomalies may be expected.Position accuracy (uncertainty) worse than 500 metres.No quoted Depth accuracy (uncertainty). Undetected features may potentially rise to or near the sea surface.

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5.7 ZOC Unassessed (U)

This category is used to indicate areas where survey information included has not been assessed for accuracy. This may occur when:

newly received information has been included, as an urgent precaution, prior to the data being fully assessed;

the national hydrographic office has only limited resources, so has initially published a large number of their first-generation ENCs faster than their survey assessment teams can complete assessments;

the area depicted is on a small scale ENC, (smaller than 1:500,000 – a carry-over from some nations’ paper charts), though the same area may be covered by one of the other categories within an overlapping larger scale ENC. In these cases mariners should refer to the larger scale ENC for precise detail.

6.1 Effect of overzooming

ENC’s are compiled at a certain scale. IHO S-57 Appendix B.1 recommends that the compilation scales for ENC’s are based upon standard radar ranges:

Selectable range Standard scale (rounded)200 NM 1:3.000.00096 NM 1:1.500.00048 NM 1:700.00024 NM 1:350.00012 NM 1:180.0006 NM 1:90.0003 NM 1:450001.5 NM 1:22.0000.75 NM 1:12.0000.5 NM 1:80000.25 NM 1:4000

A large scale chart covers a small area with high level of details. The associated Zones of Confidence therefore also provide a high level of detail. When using a lower scale chart, at some point two adjacent Zones of Confidence will merge into one. At that point only the lesser value of the two Zones will be available for safety reasons. Shipping accidents have occurred when mariners did not have the largest scale chart in their ECDIS available, they overzoomed using a medium scale chart, and ran aground by passing to close to isolated underwater dangers.

Further details and examples are provided in Annex B of S-67

6. Summary and recommendations

Accuracy of Depth Information in an ENC can be visualized by showing the Zones of Confidence areas. A Zone of Confidence area is a generalized picture of the quality of depth information for that area. The quality of the hydrographic source data is assessed according to six categories (CATZOC): five quality categories for assessed data (A1, A2, B, C and D) and a sixth category (U) for data which has not been assessed. The assessment of hydrographic data quality and classification into zones done by Hydrographic Offices is based on a combination of:

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a. seafloor coverageb. depth accuracyc. position accuracy

As a Quick Reference Guide, this can be interpreted as follows:1. Good Quality Areas (ZOC A1 and A2), shown as 5 stars or more2. Medium Quality Areas (ZOC B), shown as 4 stars3. Poor Quality Areas (ZOC C, D and U), shown as 3 stars or less or letter U

Zones of Confidence can be visualized in an ECDIS by activating the information on chart display layer. (or some other setting, depending on the type of ECDIS). The following recommendations are made to the mariner:

when planning a new voyage, Zones of Confidence should be visualized as an overall check of the quality of the area the vessel is going to transit;

when changing the planned route whilst enroute, the Zones of Confidence should be visualized as an overall check of the quality of the area the vessel is going to transit;

in ZOC A1 and A2, the vessel should stay clear from isolated dangers by 20 meters, unless the mariner is confident to sail over them;

in ZOC B, the vessel should stay clear from isolated dangers by 50 meters, unless the mariner is confident to sail over them;

in ZOC C, D and U, the vessel should stay clear from isolated dangers by 500 meters, unless the mariner is confident to sail over them;

The mariner should take note of the accuracy of the depth areas the vessel is planning to transit and take appropriate caution, especially in situations where under keel clearance is critical.

In ZOC D and U the mariner is advised to take caution as charted depths may in reality be significantly shallower. It is very likely that some significant seafloor features dangerous to the safety of navigation (rocks, coral reefs, wrecks, submerged obstructions) have not been identified, and do not appear in the chart

By using a Pick Report in ECDIS, the mariner can read additional quality information on isolated dangers to the safety of navigation, if these have been included in the ENC. Otherwise the mariner should assume that the isolated danger may in reality be out of position and/or be shallower as indicated by the Zone of Confidence.

The mariner should make sure to have the full portfolio of ENC’s available at the appropriate chart scales of the areas the vessel is transiting through. ECDIS does allow the mariner to overzoom hence giving a false sense of security of the accuracy of isolated dangers if Zones of Confidence are not checked.

Put in simple terms, mariners should be able to navigate with confidence in areas with ZOC A1 and A2 classifications. It is also unlikely that an uncharted danger affecting surface navigation exists in ZOC B areas. In ZOC C areas mariners should exercise caution since hazardous uncharted features may be expected, particularly in or near reef and rocky areas, or areas of mobile seabed. A very high degree of caution is required for areas assessed as ZOC D, as these contain either very sparse data or may not have been surveyed at all. Finally, it is good practice for mariners to treat ZOC U areas with the same degree of caution as ZOC D areas.

Within ports, the Pilot or HarbourHarbor Master may advise that higher accuracy surveys have been conducted that allow for smaller under-keel clearances (subject to tides, weather, speed, and maneuvering margins). In the absence of this advice, smaller under-keel safety margins should not be assumed.

In coastal shipping areas the most common assessments likely to be encountered are:

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ZOC B – around 30% of the world’s coastal waters,

ZOC C – around 20% of the world’s coastal waters,

ZOC D – around 20% of the world’s coastal waters, and

ZOC U – around 25% of the world’s coastal waters.

While these percentages may vary from place to place, the key point to note is that the standards of surveying in port are only very rarely encountered outside those ports. Ships are therefore at greaterst risk away from ports, even though depths may be deeper. In areas deeper than 100 meters, the risks will decrease, with increasing under keel clearance. Areas deeper than 200 meters are generally considered safe for surface navigation. An understanding of how much confidence can be placed in the data depth information within in an ENC is therefore most important.

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Annex A to S-67

Zones Of Confidence Categories

ZOC Category (note 1)

Position Accuracy (note 2)

Depth Accuracy(note 3)

Seafloor Coverage Typical Survey Characteristics (note 5)

A1 ± 5 m + 5% depth

=0.50 + 1%d Full area search undertaken. Significant seafloor features detected (note 4) and depths measured.

Controlled, systematic survey (note 6) high position and depth accuracy achieved using DGPS and a multi-beam, channel or mechanical sweep system.

Depth (m) 10 30 100 1000

Accuracy (m)± 0.6 ± 0.8 ± 1.5 ± 10.5

A2 ± 20 m = 1.00 + 2%d Full area search undertaken. Significant seafloor features detected (note 4) and depths measured.

Controlled, systematic survey (note 6) achieving position and depth accuracy less than ZOC A1 and using a modern survey echo-sounder (note 7) and a sonar or mechanical sweep system.

Depth (m)10 30 100 1000

Accuracy (m)± 1.2 ± 1.6 ± 3.0 ± 21.0

B ± 50 m = 1.00 + 2%d Full area search not achieved; uncharted features, hazardous to surface navigation are not expected but may exist.

Controlled, systematic survey (note 6) achieving similar depth but lesser position accuracies than ZOCA2, using a modern survey echo-sounder (note 5), but no sonar or mechanical sweep system.

Depth (m)10 30 100 1000

Accuracy (m)± 1.2 ± 1.6 ± 3.0 ± 21.0

C ± 500 m = 2.00 + 5%d Full area search not achieved, depth anomalies may be expected.

Low accuracy survey or data collected on an opportunity basis such as soundings on passage. Depth (m)

10 30 100 1000

Accuracy (m)± 2.5 ± 3.5 ± 7.0 ± 52.0

D Worse than ZOC C

Worse than ZOC C Full area search not achieved, large depth anomalies may be expected.

Poor quality data or data that cannot be quality assessed due to lack of information.

U Unassessed - The quality of the bathymetric data has yet to be assessed

Column: 1 2 3 4 5Source: IHO S-57 Ed3.1 Supp 3 (Jun 2014), pp 13-14

Remarks:

To decide on a ZOC Category, all conditions outlined in columns 2 to 4 of the table must be met.

Explanatory notes quoted in the table:

Note 1. The allocation of a ZOC indicates that particular data meets minimum criteria for position and depth accuracy and seafloor coverage defined in this Table. ZOC categories reflect a charting standard and not just a hydrographic survey standard. Depth and position accuracies specified for each ZOC category refer to the errors of the final depicted soundings

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and include not only survey errors but also other errors introduced in the chart production process.

Note 2. Position accuracy of depicted soundings at 95% CI (2.45 sigma) with respect to the given datum. It is the cumulative error and includes survey, transformation and digitizing errors etc. Position accuracy need not be rigorously computed for ZOCs B, C and D but may be estimated based on type of equipment, calibration regime, historical accuracy etc.

Note 3. Depth accuracy of depicted soundings = a + (b*d)/100 at 95% CI (2.00 sigma), where d = depth in metresmeters at the critical depth. Depth accuracy need not be rigorously computed for ZOCs B, C and D but may be estimated based on type of equipment, calibration regime, historical accuracy etc.

Note 4. Significant seafloor features are defined as those rising above depicted depths by more than:

Depth Significant Feature a. <40m: 2 m b. >40m: 10% depth

A full seafloor search indicates that a systematic survey was conducted using detection systems, depth measurement systems, procedures, and trained personnel designed to detect and measure depths on significant seafloor features. Significant features are included on the chart as scale allows. It is impossible to guarantee that no significant feature could remain undetected, and significant features may have become present in the area since the time of the survey.

Note 5. Typical Survey Characteristics - These descriptions should be seen as indicative examples only.

Note 6. Controlled, systematic surveys (ZOC A1, A2 and B) - surveys comprising planned survey lines, on a geodetic datum that can be transformed to WGS 84.

Note 7. Modern survey echo-sounder - a high precision single beam depth measuring equipment, generally including all survey echo-sounders designed post 1970.

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Annex B to S-67

Dangerous effects of over-scale ECDIS display near ‘Isolated dangers’

Use of over-scale display of an ENC may dangerous in certain circumstances. There is a mistaken belief that zooming in allows for greater accuracy – however, this is not the case. In reality, zooming in beyond the intended maximum display scale may be misleading and dangerous, particularly for ‘Isolated dangers of depth less than the safety depth’.

Every ENC is compiled at an intended maximum scale. At this scale the maximum level of detail is revealed, while zooming out will progressively reduce the level of detail. None of this affects the accuracy of the chart. Zooming in may reveal a new, larger scale ENC, but this too has limits, and a point will be reached where there is no point zooming in further.

At the intended maximum compilation scale, details which are too small to chart, but which still present a hazard to navigation, are typically replaced by a symbol larger than the charted size of the feature (such as a very small reef). Zooming in to over-scale destroys the relationship between the size of the (now larger) hazard and the size of the symbol.

When the ENC is displayed correctly, the danger to a ship close to an isolated danger is clear.

However, when displayed at over-scale, a ship the same unsafe distance from the isolated danger incorrectly appears to be safe because the isolated danger symbol is still the same size.

This is not more accurate, and is definitely not safe.

Remember, the positioning accuracy of the isolated danger may be worse than 500 metresmeters. Routes should be planned to clear these dangers by at least as far as the ZOC category immediately around the danger dictates.

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