Understanding a Nautical Chart E-Book

Paul Boissier

• Herausgeber: Fernhurst Books Limited
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 250

Veröffentlichungsjahr: 2018

Beschreibung

We all rely on charts to navigate at sea – but are we missing essential information? A mass of data is included on each chart and deciphering the many symbols and abbreviations can be complicated. The accuracy of some charted depths can be trusted entirely while others should be treated with caution. This book will tell you where to find and how to understand this vital knowledge. This updated second edition explains how charts are compiled before guiding you through the elements that make up these vital navigational tools. Improve you understanding of charts and you will navigate with increased safety and confidence. Understanding a Nautical Chart offers superb value as, in addition to a wealth of practical advice, there is a key to all the recently updated chart symbols and abbreviations from Symbols and Abbreviations Used on Admiralty Charts (5011). It includes information on electronic charts, explains how to update a chart and how to establish the accuracy of each chart. It is ideal for professional mariners and leisure sailors.

Leseprobe

Understanding a Nautical Chart

Understanding a Nautical Chart

A Practical Guide to Safe Navigation

Paul Boissier

Second edition published in 2018 by Fernhurst Books Limited

The Windmill, Mill Lane, Harbury, Leamington Spa, Warwickshire. CV33 9HP. UK

Tel: [phone] | www.fernhurstbooks.com

First edition published in 2011 by John Wiley & Sons Ltd

Copyright © 2018 Paul Boissier

The right of Paul Boissier to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise except under the terms of the UK Copyright, Designs and Patents Act 1988.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book.

This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher accepts no responsibilty for any errors or omissions, or for any accidents or mishaps which may arise from the use of this publication.

The fact that an organisation or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organisation or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read.

This product has been derived in part from material obtained from the UK Hydrographic Office with the permission of the UK Hydrographic Office, Her Majesty’s Stationery Office.

© British Crown Copyright, 2018. All rights reserved.

NOTICE: The UK Hydrographic Office (UKHO) and its licensors make no warranties or representations, express or implied, with respect to this product. The UKHO and its licensors have not verified the information within this product or quality assured it.

THIS PRODUCT IS NOT TO BE USED FOR NAVIGATION

Front cover photograph: © Adam Burton, Alamy

A catalogue record for this book is available from the British Library

ISBN 978-1-912177-07-3

eISBN 978-1-912177-71-4

Mobi ISBN 978-1-912177-72-1

ePDF ISBN 978-1-912177-73-8

Contents

1 What is a Chart?

2 A Chart is Never 100% Accurate

3 What the Chart-Maker Does for Us

4 Other Relevant Documentation

5 How to Use a Chart

6 Orientation

7 The Basics (and Where to Find Them)

8 Depth and Elevations

9 Landmarks, Lights and Coastal Features

10 Dangers: Wrecks, Rocks and Obstructions

11 Navigation Restrictions and Limits

12 Radar and Radio

13 Tides, Tidal Streams and Currents

14 Small Craft Features

15 Other Charting Authorities

16 GPS

17 Digital Charting

Symbols and Abbreviations used on Admiralty Paper Charts

To Susie

and

to all the wonderful people at the UK Hydrographic Office, who do so much to keep us all safe.

1What is a Chart?

I have been lost at sea on a number of occasions.

Admittedly, it was before the advent of GPS, and the fault was all mine. I was never very good at astronavigation. But the fact remains that getting lost at sea is quite scary. One bit of the sea is just like another: whichever way you look, it is flat and wet, leading to an inaccessible horizon, with no clear indication of depth and, without a compass or the sun, no sense of direction. When you are lost and out of sight of land, you naturally become a little paranoid, like a traveller in a Siberian snowstorm, wondering whether you will run right into a cliff or a rocky shoal the moment the sun goes down. You can only admire the early explorers, for whom seafaring was a constant battle against the elements, the unknown world and, for all they knew, sirens, sea monsters and mermaids.

But the courage and determination of these early mariners, who opened up the globe and spent years on end surveying its waters and coastline was not wasted. Their work, coupled with advancing technology and developments in the science of hydrography, has made it much easier for the mariner of today. We really should no longer worry about getting lost. But even if you know where you are, your safety still relies heavily on your ability to read a chart and to interpret the data which the chart and its associated publications give you. And it is my purpose in this book to help you stay safe by focusing on how you find and use that information.

So, what then is a chart? It is a remarkably high-quality document, made out of special paper that is designed to hold its shape when wet. It provides, in easily accessible form, a representation of some of the most important data that a mariner needs in that part of the world. In short, it is the special ingredient which turns this (empty sea and an awful lot of sky):

… into the chart shown above.

Without a high-quality chart, how else would you know that just two miles to the south-south-east of your position the depth reduces rapidly and becomes dangerously rocky? Or that, if you threaded your way through the rocks and beacons just out of the photograph to the right, you could work your way into an inlet behind the headland? Or, indeed, that just one mile to the south there is an explosives dumping ground where you would be ill-advised to anchor? That's what a chart does for you. It gives you a clear visual representation of your surroundings; it tells you how to get from A to B and, properly used and understood, it helps you to keep out of trouble.

In short, a chart will tell you:

where you are

what is around you, both under water and on the coast

where your destination is in relation to your position

and how to get there as safely as possible.

A chart is a very different beast from a land map. A huge amount of effort goes into the recovery of hydrographic information, but the sea is nothing like as well surveyed as the land. And sometimes (we will come to this later in the book), data is deliberately left out when it does not add to the mariner’s appreciations of his surroundings in order to improve the clarity of the chart. So you don’t have precise little contour lines around every last feature on the seabed. And, of course, on most charts the scale changes from the top of the chart to the bottom – so even reading linear distance needs a little bit of thought.

There are many people who use the seas responsibly – but a fair number too who don't. And right up there on my list of people who could so easily try harder are the ones who set out to sea without an appropriate chart, or with a chart that they haven't bothered to update. Crackers! Our knowledge of the seabed is at best incomplete, and in any case the bottom topography is changing the whole time, through seismic activity, the movement of sediment, human activity and erosion. The least you and I can do is keep up to date with the bits that the hydrographer does know about.

You quite simply cannot stay safe, and look after the safety of people who come to sea with you, unless you have (and are using) a good, accurate and up-to-date chart. And even then there are risks. Good charts may be expensive but they are a lot cheaper than the vessel that you are going to sea in and the expense is far less important than the safety of your crew. So charts really aren’t optional. Digital or paper charts, preferably both.

Digital charts are becoming increasingly commonplace in all vessels from a small yacht to a large container ship. I will deal with digital charts in Chapter 17, but in the early part of this book I will talk about paper charts; they are still carried in most boats’ and ships’ navigation suites, and the great majority of the information – and the way that we interpret that information – is identical.

How to Treat a Paper Chart

When you get your hands on a paper chart, inspect it carefully. It is a well-designed document, the product of years of development, research and painstaking cartography, and it has been printed with immense accuracy. You should always treat it with care.1 There is an often-told story about a ship one dark night taking a 50-mile detour around a coffee stain on the chart which the watch officer had mistakenly thought to be a reef.

Admiralty charts are designed to take a lot of punishment, but even they have their limits. I can remember keeping a watch on one of the Royal Navy's last open bridge warships in a North Sea storm when the chart became so wet and waterlogged that it was completely impossible to write on it – but even under these conditions it still kept its shape.

What Information Can You Expect a Maritime Chart to Contain?

The extraordinary thing about a chart, when you come to look at it in detail, is just how much diverse information it does contain. Different charts of the same stretch of water, drawn to various scales, will carry subtly different information and I would most strongly recommend that, each time you take out a chart to navigate on, you spend a few minutes studying it as a whole, reading the notes and absorbing the detail. There is a lot to take in.

In general, then, you would expect a chart to contain:

a distance scale, latitude and longitude references

a north reference, both magnetic and true, together with one or more compass roses

the coastline in serious detail and the hinterland in rather less detail, focusing largely on the features most likely to be of interest to the navigator

depth information with relevant contours and intelligently selected soundings

anchorages

underwater dangers, including rocks, wrecks, overfalls and obstructions

outline tidal height information

lights and navigation marks, sound signals, buoys, transits and shipping lanes

fishing areas, energy platforms, separation lanes, international boundaries

survey information showing the date and the thoroughness of surveys of each section of the chart

a list of applied corrections and their dates

and an awful lot more that we will come to later in the book.

Colour Convention of Charts

The colour convention of charts is important as well. From 1800 until 1968, all UK Hydrographic Office (UKHO) charts were published in fathoms and feet, but since 1968 there has been a gradual conversion of UK charts to metric data. The great majority of charts published by the UKHO are now in metric format, that is to say that depths and heights are shown in metres. These charts are printed in the colours that you will be most familiar with: the land shaded a kind of buff yellow and the sea and shoreline variously green, blue and white according to depth. There remain a number of charts, however, where depth and height are measured in imperial units, specifically charts published by the US Hydrographer and republished in the United Kingdom, and a number of less-used charts, generally of waters remote to the United Kingdom, which have yet to be converted to metric scales. These charts are printed in black and white with a number of coloured additions to identify specific features, like magenta flashes to highlight navigation buoys. More recent charts also use blue shading to indicate shoal water.

Digital Charts

I will be going into digital charts in more depth in Chapter 17, but it is important to recognise that they are pretty much the same as paper charts, drawn from the same data and using the same conventions. Digital charts are, however, produced in two separate and distinct formats, which it is important to both understand and recognise:

Raster Charts: Published by the UKHO under the title of Admiralty Raster Chart Service, or ARCS for short. These are straightforward electronic copies of the equivalent paper chart, directly scanned onto a CD. There is nothing added or taken away. As such, they are less versatile than the multi-layered Vector chart, but safer too because you cannot ‘lose’ information by having a layer of information inadvertently switched off. The Raster chart is quite simply the digital equivalent of a paper chart.

Imperial Chart of Plymouth Sound Published in the 1960s

Contemporary Metric Chart of Plymouth Sound

Vector Charts: These, too, are published by the UKHO as Electronic Navigational Charts, or ENCs, with an expanding worldwide coverage. They are also produced by a number of other charting authorities, to varying specifications. A vector chart is essentially a blank screen onto which a number of ‘layers’ of information are added: the coastline, land features, soundings, titles, currents, tides and tidal heights, etc. In fact, there is no limit to the number of layers that you can add, providing the mariner with AIS (automatic identification system), radar, harbour entry information, aerial views and much more as he or she requires. But the danger of this is that it all adds ‘clutter’ to the screen so, at any one time, a number of the layers need to be switched off in order to make the chart usable. So you never quite know what you are missing. Many mariners like to use a paper chart (or its raster equivalent) alongside a vector product so that they can see the general picture in a familiar format on the chart, with all the information in its allotted space, before looking more closely at the details on the vector display.

1See Chapter 5.

2A Chart is Never 100% Accurate

You should treat any chart with a degree of healthy suspicion.

I don’t know about you, but my first instinct when someone tells me something more or less credible with sufficient conviction is to believe them, particularly if that person is someone I trust. And it’s just the same when you see something on a chart, especially an Admiralty chart, which is after all a sort of archbishop among charts in terms of its pedigree and credibility. We all naturally tend to believe what we see on a chart without questioning its reliability … and that’s a dangerous thing to do.

Don’t get me wrong: in my opinion Admiralty charts are still the best, most accurate and most reliable charts available, but even the ‘archbishop’ is, by his own admission1, less than 100% accurate – and as a mariner you need to understand how and when to make allowance for these inaccuracies.

There are a lot of reasons for this: many charts contain areas of old surveys, sometimes even centuries old, because more modern data is just not available. There are two problems with old survey data: firstly, navigation techniques were not as good in the past as they are today, so the positioning of features on a chart may be suspect; secondly, the technology of surveying was less advanced, and old surveys inevitably left gaps in the survey coverage in which even quite sizeable dangers could be lurking. New and more thorough surveys regularly reveal shallow patches, previously unknown, which reach up to within a few metres of the surface.

But there are other reasons to question the accuracy of information that you are getting from your chart and its associated publications: the seabed may have shifted since the most recent survey; the chart may not be up to date; and you may from time to time experience tidal surges caused by strong winds and atmospheric pressure variations, which have the power to change the depth of water dramatically. There are also places where the seabed is just too mobile – with strong tides and a soft seabed – and the hydrographer simply does not bother charting the area at all. There are some examples of this very close to home. Take a look at Chart 1346 (opposite) of the Solway Firth, close to Carlisle on the western end of the border between Scotland and England. There is a section, about 10 x 15 nautical miles, where the seabed is so unstable that the hydrographer does not bother to chart it at all – and that area of the chart is left white.

Of course, it’s not as bad as it sounds. Many old surveys are surprisingly accurate, and quite good enough for the shipping that passes that way. Matthew Flinders’ survey of South Australia conducted in 1801–1802, for instance, was so accurate that it is still the basis of Admiralty charts of the area. In any case, the chart compiler generally tends to err on the side of caution to give you and me that extra margin of safety to keep us from hitting the bottom. But even so, it pays to treat the information very carefully.

Chart 1346 – Solway Firth

On 7 August 1992, the QE2 grounded on a shoal off the coast of Martha’s Vineyard on the eastern seaboard of the United States. Her draught at the time was 32 ft 4 in, the height of tide was about 1 ft 6 in, and the charted depth of water at the point where she grounded was 39 ft. By straightforward calculations she would have had about 8 ft of water under her keel. At the time she grounded, she was travelling at a speed of 25 knots. It transpired that the most recent survey of the area had been conducted in 1939 using an echo sounder. A post-event side scan survey, however, showed that the shallowest depth of water in the area was 31 ft and that the depth of water at the precise point where she grounded was 33 ft below Chart Datum, 6 ft less than the charted depth. It is likely that this patch of shallow water had quite simply fallen into one of the gaps left by the earlier echo sounder survey. A ship moving fast in any shallow water is certain to squat2 a foot or so … and suddenly, in the blink of an eye, you have reduced a confidently predicted clearance of 8 ft under the keel into an agonising incident with an awful lot of paperwork to fill in.

As a mariner, you need to know how to look at a chart in order to assess the risks and inaccuracies that it contains, so that you use it intelligently, recognising its limitations and identifying where you have to navigate with more caution. The information is often right there on the chart, paper or digital. It’s just a matter of knowing where to look, how to interpret the information and how much of a margin of error to apply.

Survey Data

Within the United Kingdom, serious offshore surveying started in about the middle of the eighteenth century with the voyages of Captain Cook, and has continued ever since, progressively improving the quality, coverage and accuracy of charts. The majority of surveys undertaken before 1935 were conducted with visual fixing and a leadline. From the mid-l930s, a series of electronic navigation aids (LORAN, Decca, satnav, etc.) made offshore position-keeping increasingly accurate, leading to the advent of GPS and Differential GPS (DGPS) in about 1980. The creation of portable computers followed by an exponential growth in computer power and the development of ever more advanced survey techniques have all served to massively improve the quality and reliability of our charts. This means that surveys which have taken place over the last quarter of a century are a quantum leap ahead of anything that has gone before, with greater reliability and fewer gaps in the coverage. Even so, it is a painstakingly slow process to get an accurate representation of the seafloor, and the great majority of the oceans remains thinly surveyed.

According to Dr Jon Copley of Southampton University:

The entire ocean floor has now been mapped at up to ~5 km resolution, which means we can see most features larger than ~5 km across in those maps..… Multi-beam sonar systems aboard ships can map the ocean floor at ~100 m resolution, but only in a track below the ship. Those more detailed maps now cover about 10 to 15 percent of the oceans, which is an area roughly equivalent to Africa in size.

This means that, along with older, single-beam echosounder data from ships crossing the ocean we probably have ‘soundings’ for a total of something like 20% of the ocean floor. These percentages don’t change much from year to year. Around the coast of the United Kingdom, it is surprising how much of the seabed has not been thoroughly surveyed – even now.

Take Lundy Island in the Bristol Channel, for instance. Lying only about 15 miles offshore, close to the English and Welsh coasts, this is not a particularly remote, inaccessible or difficult area to survey. But have a look at these three charts:

The first is a simple scan from the largest-scale Admiralty chart of the north-west corner of Lundy Island, showing an area of uneven bottom and overfalls just off the coast. This data was plotted from a leadline survey made in 1879, and this is the most accurate chart of the area that you could have bought up to about the summer of 2009.

During the early part of 2009, however, the UKHO commissioned a new survey of the area, using multi-beam sonar, which revealed a number of areas of shallow water that had never previously been identified. A detail from the new survey is shown in the second diagram. You will also see how very much more data is gathered by a modern survey; even this is only a small percentage of the topographical data that was recovered, but it gives the hydrographer a much more accurate representation of the shape of the seabed from which to draw the chart, and much more confidence that he has picked up all of the potential dangers.

The problem with using old data is best illustrated by superimposing the new survey onto the old chart, and this is shown, in a magnified format, in the third diagram. You will see how, in the most north-easterly circle, the original survey shows a depth of 31 metres, which is accurate enough with regard to the general depth of water in the area. However, the original leadline survey completely missed a pinnacle of 3.6 metres situated almost directly below this sounding point. The other two points that I have circled show depths of 5.5 metres and 6.2 metres respectively in an area that, according to the chart, you would expect something between 20 and 30 metres. If you look closely, you will see a number of other inconsistencies.3

I will probably mention it a dozen times over the course of this book, but this is a graphic illustration of why it is absolutely vital: you must keep your charts up to date. You could have decided to take your ship for a close pass of Lundy Island in the summer of 2010 using uncorrected charts, blissfully unaware of the new data which was there in the Notices to Mariners, sitting in your chart room drawer just waiting to be drawn in …

And just to show you what it really looks like, below is an example of the way in which this massive richness of multi-beam survey data can be displayed, showing the seabed in extraordinary clarity and accuracy. This is exactly the same stretch of water, to the north of Lundy Island, seen from the northwest in a three-dimensional view. The tip of the island is shown in the top left-hand corner of the picture, with the seabed in the foreground, all displayed in glorious Technicolor. The shallowest patches are coloured red and the deepest are shown in blue, and you will see just how many pinnacles there actually are in this stretch of water.

It is really no surprise that the early surveyors, equipped only with a leadline, failed to pick up anything but the most superficial detail from this complex bit of the seabed.

There are still a number of parts of the British coast, let alone other parts of the world, that remain poorly surveyed. This next chart Q6090, dated March 2017 shows just how much of the United Kingdom’s waters have yet to be thoroughly surveyed.

Only the dark green patches have been surveyed to the latest standards of accuracy by swathe bathymetry, which provides the most detailed picture of the sea floor. The yellow patches have been surveyed with a singlebeam echo sounder, which only identifies the depth (and obstructions) along the ship’s track. The red areas, of which there are many close inshore, have only been surveyed with lead line, or may even be unsurveyed. Yachtsmen in particular may like to check out the survey dates on their charts of the Channel Islands: much of the area was surveyed well over 100 years ago.

There are undoubtedly more surprises out there around the coast that are just waiting to be discovered and, if you venture into waters that might be less well used, or poorly surveyed, you must be aware that your chart might not be as reliable as you would like to believe.

It’s easy to overemphasise the dangers of chart inaccuracy. Of course, the navigator needs to be constantly aware of the limitations of the chart in use, but the danger is likely to be very much less when operating in areas of frequent and regular maritime activity, for the simple reason that other people will have trodden your path before you. When you venture off-piste, however, you need to be more careful.

Chart Coverage and Consistency

Before you use a chart, you should check the accuracy of the source data used by the chart compiler when drawing up the chart. And the place to look is on the chart itself in the margins, where you will nearly always find a ‘SOURCE DATA’ diagram and table.4 The diagram takes the form of a scaled replica of the chart divided into discrete areas, each of which represents an individual packet of survey data.

Since 2017, the UKHO has described the accuracy of a chart survey by Zones of Confidence, or ‘ZOC’, which describes the accuracy and coverage of the sea floor information that is shown on the chart. This is the system the Hydrographic Office uses on its digital charts, so it makes sense to use the same system their paper charts. I will discuss ZOCs further in Chapter 17, but you can see that here, on Chart 30 – Plymouth Sound and Approaches – some of the survey data around Drakes Island at the top of the Sound, and on the upper reaches of the River Yealm, is rather less than reliable. Most of the Sound, however, is regularly used by a variety of shipping, and it has been surveyed to a very high standard.

It is important to realise that the ‘ZOC’ codes refer solely to the accuracy of the survey; in areas where the bottom is unstable, the survey may be accurate, but it is possible that some changes in depth may have occurred since the survey was conducted – so you still need to treat the information with caution.

Charts published before the summer of 2017 have the older-style source data diagrams, like the illustration below:

This example of the older style source data table is taken from a pre-2017 chart of the Channel Islands. The first thing that will strike you is that much of the data originates from surveys that took place in the mid-nineteenth century, and that only a relatively small area has been surveyed by anything other than leadline. This does not necessarily make the data less accurate; much of the early surveying work is surprisingly accurate, but there are almost certainly gaps in the coverage and, as you navigate around this area, you may wish to ask yourself whether, in the interests of responsible seamanship, you may want to increase your margins of error a fraction … just in case. If you look at the source data diagram, you will see a ‘scale’ column, containing entries like ‘1:25 000’. Not a lot of people really understand what this means. This column allows you to get a feel for the survey line spacing. The assumption is that a surveyor will draw his survey lines on a working chart with a spacing of 5 mm. So, a 1:20 000 survey will be derived from a survey where the lines are 20 000 x 5 mm (or 100 metres) apart. This is the separation between adjacent tracks of the survey vessel which, in the days before sidescan sonar, was the maximum size of an uncharted obstacle that the surveyor could have missed. The area marked f on this chart has a scale of 1:520 000, which indicates that, in the deeper water to the west of the Casquets, some of the leadline survey runs were separated by 2 600 metres – substantially more than a mile. You will note, moreover, that this particular survey was conducted in the latter half of the nineteenth century.

The most recent surveys, carried out with multi-beam (or swathe) survey systems quite simply do not leave gaps and the term ‘full seafloor coverage’ is used instead of a scale.