Mastering Navigation at Sea E-Book

Paul Boissier

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

Veröffentlichungsjahr: 2020

Beschreibung

A lot of people are drawn to the sea, and for good reason – it's the world's last wild and largely unspoilt wilderness. But to properly enjoy the sea, and to do so safely, you must have the skills, knowledge and confidence to plan thoroughly and stay one step ahead of the game. This book is thoughtfully written to help yachtsmen do just that. It's not another RYA Course Handbook; it's written by a mariner for other mariners. It's well-informed, easy to read and honest about the author's triumphs and disasters over a lifetime's navigating. He has a unique perspective having navigated in many parts of the world from high up on the bridge of a warship, close to the water in a cruising yacht and at depth in a submarine. After his navy career he was Chief Executive of the Royal National Lifeboat Institution (RNLI), often dealing with the consequences of poor navigation. The author brings the subject to life in a book that is designed to help yachtsmen refresh their knowledge of, and their enthusiasm for, the timeless skills of navigation. It is packed with hundreds of illustrations – colour photographs, charts, diagrams and tables – making the text easy to understand. The book is part of Fernhurst Books' Skipper's Library series of practical books for the cruising sailor.

Leseprobe

Twenty years from now you will be more disappointed by the things you didn’t do than by the ones you did. So throw off the bowlines. Sail away from the safe harbour. Catch the Tradewinds in your sails. Explore. Dream. Discover.

Mark Twain

CONTENTS

INTRODUCTION

PART 1 – THE THEORY

  1 THE WORLD, & HOW IT’S PORTRAYED

  2 CHARTS

  3 POSITIONS, ANGLES, DISTANCE & SPEED

  4 MOVEMENT OF WATER: THE HEIGHT OF TIDE

  5 MOVEMENT OF WATER: TIDAL STREAMS & CURRENTS

  6 THE FIX

  7 VISUAL AIDS TO NAVIGATION

  8 RADAR & RADIO AIDS TO NAVIGATION

  9 NAVIGATION EQUIPMENT

10 REFERENCE BOOKS

11 A FEW THOUGHTS ABOUT SAFE NAVIGATION

12 A FEW TRICKS OF THE TRADE

PART 2 – TYPES OF NAVIGATION

13 PILOTAGE

14 BLIND PILOTAGE

15 ANCHORING

16 COASTAL NAVIGATION

17 OFFSHORE NAVIGATION

PART 3 – IN PRACTICE

18 PASSAGE PLANNING

19 LYMINGTON TO HURST CASTLE

20 HURST CASTLE TO THE NEEDLES

21 THE NEEDLES TO THE CASQUETS

22 THE CASQUETS TO THE LITTLE RUSSEL

23 THE LITTLE RUSSEL INTO ST PETER PORT

24 SOME CAUTIONARY TALES

PAUL BOISSIER

PROFESSIONAL NAVIGATOR

Paul Boissier has spent much of his professional career working on the sea, or in support of the people who go to sea, and in his leisure time he is an avid yachtsman.

In the Navy he commanded and navigated warships and submarines in many parts of the world, ending his career as a senior Admiral. He then spent 10 years as Chief Executive of the Royal National Lifeboat Institution, the charity that saves lives at sea and operates over 340 lifeboats around the UK and Republic of Ireland.

As a cruising yachtsman Paul has sailed his own boat around the UK, northern Europe and in the Mediterranean. He has also spent time afloat in other parts of the world as far apart as western Canada, South America and Australia.

This lifetime’s experience has given him a unique perspective on navigation, from the bridge of a warship, to the cockpit of a cruising yacht and the control room of a submarine. And, of course, at the RNLI, his work often involved dealing with the consequences of navigational miscalculations by other water users.

Paul brings the subject to life in this book that is designed to help yachtsmen refresh their knowledge of, and their enthusiasm for, the timeless skills of navigation.

He loves the sea and has had a lifelong fascination with charts and navigation, as illustrated by his other two books written for Fernhurst Books:

Learn the Nautical Rules of the Road

Understanding a Nautical Chart

INTRODUCTION

You only have to look at a globe, or a map of the world, to realise just how much of the surface of this planet is covered by water, and it doesn’t take much thought to recognise the importance the oceans have to our everyday life and well-being. As sources of food, in the way they dictate our weather, their role as global trade highways, for their mineral deposits, recreation and the almost unbelievable diversity of life that they support

There’s an extraordinary world out there, which sadly very few of us are lucky enough to witness at first hand. The sea is the world’s last great wilderness: wild, untamed and more powerful than you can possibly imagine. There are no roads crossing it, and even today we actually understand very little about it. In 2018 the US National Oceanography and Atmospheric Administration (NOAA), estimated that:

“Currently, less than ten percent of the global ocean is mapped using modern sonar technology.”

So, when you set out on a sea voyage, you really are going into the unknown. And you have to be prepared, because, for all its beauty, the sea doesn’t even notice that you’re there, and it’s completely unforgiving of any weakness that it finds, in you or your vessel.

One of the greatest skills of seafaring is, and always has been, the art of navigation. Finding your way across a featureless seascape by day and by night is not simple, and it can be quite daunting until you build up a bit of experience.

I remember looking out at the sea when I first joined the Navy, wondering how on earth I was supposed to navigate a ship across that great, empty expanse. But in the end it’s worth all those long hours of study – at home, and at sea – learning the timeless skills of navigation, because navigation unlocks the potential to explore the seas, and seafaring has a deep, visceral attraction for many of us.

We are fortunate today because, as navigators, we stand on the shoulders of the giants who went before us, and we have at our disposal the great array of skills, instruments and techniques that they developed. It wasn’t always this simple. Two thousand years ago, seafaring generally meant adopting the ‘point and shoot’ method of navigation: head towards the setting sun, and with luck America will eventually appear in front of you. This was an imprecise and risky form of navigation, but it got the Vikings across the Atlantic and it allowed the Romans and the Phoenicians to open up the seas around Europe for maritime trade and travel.

Then came the astonishing invention of the magnetic compass, first used by the Chinese for maritime navigation about a thousand years ago, which allowed seafarers to steer a moderately accurate and consistent course across the water without relying on daymarks, luck or heavenly bodies. It didn’t tell seafarers where they were, but it did make for more accurate voyages.

From the 15th century onwards, maritime charts became more accurate and more widespread, and people started to systematically record the position of heavenly bodies. This led, in the 16th-18th centuries, to a succession of increasingly accurate navigation instruments, all designed to measure the altitude of a celestial body – most commonly the sun or the pole star – which gave mariners the opportunity to calculate their latitude with a fair degree of accuracy. These instruments – backstaff, quadrant, octant and finally sextant – supplemented the simple directional reference provided by the compass and, with increasingly accurate charts, gave mariners the ability to estimate their position when out of sight of land. Returning to the English Channel from the Cape of Good Hope, you would go a good distance west into the South Atlantic, then head north, checking the midday altitude of the sun, and turn right when the sun told you that you had reached 50° of northern latitude. With a fair wind, clear skies and a little luck, the Lizard Lighthouse on the south coast of Cornwall would suddenly pop up over the horizon one day. That was the theory, although the history of shipwrecks around the Cornish peninsula shows that this was, quite literally, a hit-and-miss operation.

A sextant can give you a very accurate read-out of latitude, but to use it for longitude, you must take account of the earth’s rotation – and for that you need an accurate time reference. In 1714, the growth of an increasingly ambitious global navy led the British parliament to offer a massive prize of £20,0001 to anyone who could find a reliable way to determine longitude at sea. This famously encouraged John Harrison, the English clockmaker and carpenter, to develop a series of accurate maritime chronometers that allowed mariners, using some relatively simple spherical geometry2, to get an accurate position, both latitude and longitude, from the observation of heavenly bodies.

Until recently, astronavigation was the main way of fixing a ship’s position during long sea voyages. It’s only in the last few decades that the introduction of long-range radio navigation aids – Loran, Decca, the Transit satellite navigation system and now GNSS3 – have given everyone access to their precise location.

Even so, it would be a mistake to think of GPS as infallible. It’s good seamanship to ask yourself what you would do if you suddenly lost access to your satellite navigation system? Would you know where you are, and would you be able to complete the voyage safely? Or would you join that august band of navigators who have, at some point in their career, found themselves ‘temporarily uncertain of their position’?

There have certainly been times in my life when I would have found it difficult to say precisely where we were. And when this happens, you suddenly realise just how lonely the oceans are. That’s when you need all of the knowledge and experience that you have accumulated over the years to slowly re-establish an accurate position.

I once navigated a dived nuclear submarine back from the Atlantic into the North Channel, the small gap between Northern Ireland and Scotland, at night in a growing westerly gale. This was before satnav became widely available. The conditions were hideous; I hadn’t plotted a fix for 12 hours and we weren’t picking up any useful signals from the radio navigation aids. The echo sounder wasn’t telling me much either. I was on the point of thinking that it was just getting too difficult and that we would have to wait until daybreak when, like a tiny beacon of hope, I caught a glimpse of the loom of the Malin Head Lighthouse on the north coast of Ireland, and I took a bearing. Over the next hour or so I was able to plot a series of running fixes (Chapter 6) that got us home safely.

Good navigation is meticulous, but in all honesty it’s not that difficult, and it is a skill that improves with practice. Like so many things in life, you start to enjoy it more as you get better at it…. And that’s what this book is about: trying to de-mystify navigation and make it easier for people to understand and to practise, so that you are better able to enjoy your time on the water, in safety.

As a navigator, you should have the skills to:

■ Know where you are at any given time and understand how accurate your position is

■ Plan your voyage in a way that will keep your vessel and her crew safe at all times

■ Predict and understand the present and future movement of the water around you

■ Calculate the course to steer to get to your destination

■ Estimate when you will arrive

■ And be honest with yourself when things don’t feel right

You must be able to do all this, reliably, at any time of the day or night, and in any visibility, recognising that the water around you is constantly moving, and that the depth of water in coastal areas can vary by over 10 metres in a 6-hour period. You’re doing it, moreover, while you, your boat and chart table are being thrown around randomly by elements. That’s quite a tall order, but people have been navigating for thousands of years and there is a tremendous bank of well-tried knowledge, best practice and data to draw on.

If I leave you with just one message from this book, the single most important thing that you can do as a navigator is to prepare thoroughly in advance, for even the simplest passages. The better prepared you are, the easier you will find it to concentrate on where you are going, to anticipate problems, avoid obstacles and, most important of all, to keep your crew and your boat safe.

I’m not a natural worrier, but as both navigator and captain of big ships, submarines, and yachts, I’ve always paid close attention to the vessel’s navigation before leaving harbour, and all the time that we’re at sea.

This attention to detail will give precision to your navigation and, if your brain is anything like mine, it brings enormous satisfaction as well. The satisfaction of using your skill, your knowledge and your learning to get safely from A to B is a great feeling, whether you’re crossing the Atlantic or the Solent; whether you’re navigating a nuclear-powered submarine or a small yacht. And when you arrive at the other end, find the time to enjoy the landfall, because passage making is one of the great arts of seamanship, as old and mysterious as seafaring itself.

My first offshore voyage as a newly promoted submarine Captain was a surface passage from Rosyth, just outside Edinburgh, all the way round the top of Scotland to the naval base at Faslane, on the River Clyde. By any standards, this was not a complicated trip for a warship – and for the first 90 minutes it all went incredibly well. It was when we got to the mouth of the Firth of Forth that I started thinking that something was not quite right. I walked back to the control room where the chart table was situated and took a quick look at the chart. The officer of the watch, who was very inexperienced, had completely forgotten to turn left and head up the Scottish coast… so we were happily heading across the North Sea towards Denmark. The problem was quickly resolved, and we turned north with no harm done – but it taught me a valuable lesson about being a Captain, and the importance of worrying, and of acting on your instinct when something doesn’t feel quite right.

This is not a textbook to get you through the RYA Yachtmaster® syllabus. There are already a lot of very impressive books around to do that. I have written this book for people who are already sailors, people who may own their own boat, or charter regularly. It’s written as a sort of aide-memoire to help you navigate with confidence, because navigation can be daunting. There’s a lot to think about, and a lot that can go wrong. As someone said to me when I was training to become a submarine Captain:

“One day, when it all starts to go wrong, you’ll find everyone silently looking at you, wondering how you’ll get them out of this mess. It’s your job to do exactly that.”

The book consists of three parts:

■ The first part talks about the tools that you have at your disposal as a navigator – the instruments, the information and the techniques that are available to you, and how to use them

■ The second addresses the different types of navigation, from pilotage near the coast to offshore navigation

■ And in the third I describe a simple passage from Lymington to St Peter Port in the Channel Islands in a small yacht, showing how I would plan and make the passage

The book ends with a single chapter in which I have described some of the times when I, or others, haven’t got it right, with some thoughts on what we can learn from these incidents. I have done this to show that we all make mistakes: there’s no shame in it, and the best thing we can do afterwards is to learn from the lessons and avoid finding ourselves in the same situation sometime in the future. For that reason, if you have a cautionary tale to tell, send it to me at [email protected] and I will include a selection of the most valuable in subsequent editions of this book.

Throughout the book I have tried to describe the ‘right’ way to do things, but there are places where I have gone a little off-piste in demonstrating practical alternatives to the orthodox methods. These are practices and techniques that may or may not be in the textbooks, but which I have learnt to use and to rely on over the years. I’ve shared them in this book so that you can try them out for yourself.

At the end of the day, navigation is something very special. It’s half science and half art, with a dash of excitement thrown in by the unexpected… because something unusual always happens on a sea voyage. For me, navigation is a challenging, timeless and absorbing discipline that is endlessly rewarding – it’s a ticket to explore the other side of the horizon, and plot your own course over the sea – which is a freedom like no other.

I hope that this book will give you the confidence to consider yourself a navigator, and to enjoy this great art.

Paul BoissierSeptember 2020

1 This was a staggering sum of money – equivalent to over £3m in 2020 currency.

2 The words ‘simple’ and ‘spherical geometry’ may, to some, be something of an oxymoron. I agree wholeheartedly!

3 GNSS, or Global Navigation Satellite System, encompasses all the global satellite positioning systems. Most of us use the US Global Positioning System (GPS). Because it’s the most commonly used system, I will refer to GPS in this book.

PART 1THE THEORY

© Imray Laurie Norie & Wilson LTD

1THE WORLD, & HOW IT’S PORTRAYED

Life would be so much easier if the world was flat.

And even if we accept that it isn’t flat, it would be quite a lot easier if it was a perfect sphere. In either case, you could create a beautiful mathematical model for charting the world which would be both simple and accurate.

But life’s not like that.

The earth is very nearly a sphere… but not quite. It’s properly described as an ‘oblate spheroid’, a fact that is handy if you’re ever setting the questions in a pub quiz, but otherwise almost entirely useless.

4.543 billion years of constant rotation has given the earth a bit of a middle-aged spread: it’s rather shorter than a proper sphere, and broader round the equator – to the extent that the diameter across the equator is about 42.7 kilometres greater than the diameter through the poles.

CHART PROJECTIONS

The job of the chart makers is to find a way to accurately represent this irregular, almost-but-not-quite spherical object on a 2-dimensional piece of paper (or screen). And they have done well. There are getting on for 100 different ‘chart projections’4 listed in Wikipedia, and doubtless many more that have not been listed there. But there are just 3 of these, which are used extensively for maritime navigation, that you and I should be aware of:

Chart title showing the projection used

■ Mercator Projection

■ Transverse Mercator Projection

■ Gnomonic Projection

The projection in use is always marked in the title block of the chart. In the above case a chart of the southern North Sea has been drawn using the Mercator Projection.

MERCATOR PROJECTION

In many ways it’s quite impressive that the most common chart projection in use today was developed almost 450 years ago, by a Flemish cartographer called Gerardus Mercator.

Imagine the world floating in space. And imagine wrapping an enormous cylinder of white chart paper round it, touching the surface of the earth only at the equator.

Then, if this is not pushing your imagination too far, imagine a big light in the centre of the earth shining outwards, and projecting the shadow of the land masses onto this cylinder of paper. That’s the idea behind the Mercator Projection.

As you can imagine, at the equator there is very little difference between the size and shape of the land and its portrayal on the paper. But the charted scale of latitude extends as you get closer to the poles, and the north-south distortion increases. The poles don’t appear on the paper at all. You end up with a chart of the world that looks something like this.

The Mercator Projection

Flat map of the world showing huge distortion near the poles (Daniel R. Strebe, 2011)

You can see the extent of the distortion that occurs towards the poles by comparing the images of Greenland and Australia on the chart. It is perhaps surprising to realise that, in terms of land area, Greenland is actually just 28% of the size of Australia – which is not how it looks on this map.

There are nevertheless a number of attributes of the Mercator Projection that are incredibly valuable to the navigator:

■ All lines of latitude and longitude cross at right angles

■ Angles on the earth’s surface are the same as the equivalent angles on a chart

■ Lines of longitude are straight, and evenly spaced across the chart

■ Lines of latitude are also straight, but have variable spacing, becoming less compressed as they move away from the equator

■ And finally, a straight line on the chart from, say, Plymouth to Barbados will cross all lines of latitude and longitude at the same angle, giving you a constant bearing to steer from departure to destination – this ‘rhumb line’, as it is called, may not be the shortest distance between the two points on a big trans-Atlantic voyage5, but it is incredibly convenient for shorter journeys

As a navigator, the great majority of the charts that are used for everyday navigation are drawn with the Mercator Projection, or its younger brother, the Transverse Mercator Projection (see below).

BUT – and this is an important but – there is one big thing which you must bear in mind when using a Mercator chart. The latitude scale – and hence your reference for distance (see Chapter 3) – varies from the top of the chart to the bottom, so that when you measure distance using the latitude scale on the chart, you must use the scale adjacent to the part of the chart that you are working on.

Take a look at this relatively small scale, high latitude chart: the chart of Tierra del Fuego6 at the southern tip of South America. This is a Mercator chart.

If, on this chart, you set your dividers to measure 30nm (or 30 minutes of latitude) at Cape Horn (about 56°S), and then move them to the top of the chart (about 51°S), the distance covered by the dividers at this latitude is actually closer to 34nm.

This effect is more noticeable at high latitudes, and on small scale (large area) charts.

And that’s why you must always measure distance using the latitude scale closest to the part of the chart you’re working on.

TRANSVERSE MERCATOR PROJECTION

Even at high latitudes, you can work perfectly happily on a Mercator chart as long as you are in open water. But it’s understandably quite difficult working in pilotage or restricted waters if the difference in latitude and longitude scales gives you too much distortion. As a result, the chart maker often uses a Transverse Mercator Projection for mapping small harbours and bays around the coast.

The Transverse Mercator Projection is quite an ingenious extension of the simple Mercator Projection. Instead of wrapping the cylinder of white paper around the equator, why not wrap it around the poles, touching the surface of the earth along the line of longitude that you are particularly interested in?

Transverse Mercator Projection

That way, no matter what your latitude, so long as you’re working close to this line of contact, there will be little or no distortion.

In practice, Transverse Mercator Projections are only ever used on large scale (small area)7 charts because, as you move away from the line of contact, the increasing distortion would make the chart incredibly confusing.

You can see the result of this extreme distortion on this chart of the world, drawn with the Transverse Mercator Projection. There is next-to-no distortion along the two meridians, which pass through the UK, France and Spain. But I really would not like to use the chart for a passage from the UK to the Caribbean.

Transverse Mercator chart (Peter Mercator, 2010)

This is clearly not a projection to use over large sea areas, but it is perfect for small charts and plans. As long as you’re working in a small area, a Transverse Mercator chart will look and feel exactly the same as its older brother, the tried-and-tested Mercator Projection.

The UKHO’s Plans of Harbours and Creeks in The East Solent Area are all drawn using the Transverse Mercator Projection: they provide an accurate, distortion-free image of a small area, perfect for inshore navigation

GNOMONIC PROJECTION

Gnomonic charts (pronounced ‘no-monic’) are very different from Mercator charts in both construction and appearance. In this case, you have to imagine your big sheet of white chart paper resting flat on a single point of the earth’s surface, with the shadows projected onto it from a single, very bright light at the centre of the earth.

So, for instance, if you lay the paper on the North Pole, all the lines of longitude will be shown as straight radial spokes running away from the pole, and the lines of latitude will be concentric circles, centred on the pole8.

As you would expect, gnomonic charts produce increasing levels of distortion as you move away from the point of contact, but of course you can use any point on the surface of the earth as your reference point: you don’t have to use the North Pole.

Gnomonic charts have three specific uses in navigation:

1. Like the Transverse Mercator Projection, Gnomonic charts have the ability to portray small, localised areas close to the point of contact with minimal distortion. So, you will sometimes see small harbour chartlets drawn using Gnomonic Projection.

2. Secondly, they are invaluable when you are working at very high latitudes, where Mercator charts start to lose their relevance. Nuclear submarines heading north under the ice cap often use Gnomonic charts for their navigation.

3. And thirdly, a straight line drawn on a Gnomonic chart shows the Great Circle9, or the shortest route between the two points. These charts are therefore immensely useful for long-distance ocean route planning.

Gnomonic chart projection

A quick glance at a gnomonic chart will explain why, when flying from London to Vancouver, two cities on almost exactly the same latitude, you end up passing over Reykjavik and central Greenland, both of them a good deal further north than the start and destination cities.

If you’re ever planning a long ocean passage, it’s worth buying one of the Gnomonic Planning Charts produced by the UKHO. On these charts, the great circle track between 2 points is represented by a straight line, so the shortest distance between the SW Approaches and Bermuda, for instance, starts off by going almost directly west, and ends up heading into Bermuda on a south-westerly course. Plotted on a Mercator chart, this route would appear to be a long, lazy curve and quite a divergence from the straight rhumb line.

Of course, in a small boat, or even a big ship, the shortest distance between 2 points may not always be the quickest, and you should also consult the Admiralty Routing Charts, which are produced for every month of the year, and which show the most likely winds and currents to expect on your voyage.

The Great Circle route between London and Vancouver (Daniel R. Strebe, 2011)

UKHO’s Gnomonic planning chart for an ocean passage

Map of North Pole using Gnomonic chart projection (Daniel R. Strebe, 2011)

HORIZONTAL DATUMS

Chart projections are the cartographer’s way of portraying the 3-dimensional surface of the earth on a 2-dimensional sheet of paper (or screen), allowing us to make sense of the world, and navigate across its oceans. But they only solve one of the chart maker’s problems. The other big problem is how to assign meaningful positions to places on the earth’s surface when it is not exactly spherical.

This sounds like a slightly arcane problem, but increasingly you and I are asking our GPS sets and our phones to consistently navigate down to a few metres of accuracy on the irregular surface of an object that is roughly 7,900 miles in diameter. So, the way that the earth is modelled inside your GPS set or your mobile phone is critical to the accuracy of the system.

The models used to define position on the earth’s surface are called Horizontal Datums.

A number of countries have defined their own horizontal datums, including:

■OSGB36: Ordnance Survey Datum, used for land mapping of the UK

■ED50: European Datum, developed after the Second World War to properly map international borders

■GDA94: Geodetic Datum of Australia10

■NAD83: North American Datum, which ensures consistency of position across the United States, Canada, Mexico and Central America

Most of these are fairly localised datums, designed to provide consistency of position in a single country or a group of countries. WGS84, by contrast, or the World Geodetic System of 1984 to give it its snappy formal title, provides a common position reference for the whole world, using a very accurate virtual model of the earth’s surface. WGS84 is used by the US Global Positioning System (GPS), and it’s also used in most commercial GPS units.

Of course, each system models the world in slightly different ways. As a result, they can often assign different positions to particular objects. The Mariner’s Handbook illustrates this by comparing the position of South Foreland Lighthouse, a few miles north-east of Dover, using 3 separate datums, as shown below.

The difference between these positions is not huge – less than 200 metres at most – but in some parts of the world the inter-datum errors can be a lot bigger. Where the differences between datums are of such a size that it is likely to have an impact on the safety of navigation, they are set out in the title block of the chart, and when you see this, you really do need to take account of the errors.

3 CHART DATUMS FOR SOUTH FORELAND

HORIZONTAL DATUM

GEOGRAPHICAL POSITION

Referred to OSGB36 datum (the Ordnance Survey Datum for the UK)

51° 08’.39N 001° 22’.37E

Referred to the European (1950) Datum (which is the continental datum)

51° 08’.47N 001° 22’.35E

Referred to the World Geodetic System 1984 (WGS84) Datum (the worldwide datum used by the GPS)

51° 08’.42N 001° 22’.27E

WGS84

It’s worth spending a bit of time discussing WGS84 because, if you use GPS in your boat, it will in all probability use WGS84 as its horizontal datum.

An increasing number of maritime charts produced by the UK and US Hydrographers, and many other national charting agencies, are now drawn to be compatible with WGS84. In a nutshell, when you see the words ‘WGS84 POSITIONS can be plotted directly on this chart’, similar wording, or just ‘WGS84’, you can safely plot positions from the US GPS system directly onto your chart without correction.

Check for these words (or similar) before plotting GPS positions on your chart

There are still a number of charts, however, which are not drawn to the WGS84 datum, where you will have to correct the GPS position before plotting it on the chart. There is no ‘WGS84’ notation in the margins of these charts. Instead, you will find a small table of inter-datum corrections in the title block.

This chart, for instance, covers a part of the northwest coast of France. It has been drawn with reference to the European Datum (Circle 1), and it tells you (Circle 2) that you need to correct GPS positions before plotting them on the chart and tells you how to correct for this.

Always check the title block before you use a chart, because some of these corrections are quite significant. The largest known discrepancy between the charted position and the WGS84 position is a massive 7 nautical miles in the middle of the Pacific Ocean. Most electronic chart plotters automatically make any necessary datum corrections for you, but it’s worth checking to make sure.

Chart drawn using European Datum

RECORDING & COMMUNICATING POSITIONS

Mariners generally define a position at sea in one of 2 ways:

■ Range and bearing from a fixed object

■ Latitude and longitude

RANGE & BEARING

If you’re sitting at home, speaking to a friend on the phone, and you wanted to tell them where to park their car, you might say something like:

”Drive to the village pub, and the car park is about 200 metres down the High Street.”

In other words, you give them a range and bearing from a conspicuous reference position. This is a simple way of defining a position on land, and its equally simple at sea where you could, for instance, describe your position as:

210° Portland Bill Lighthouse 13.2nm

Meaning that you are 210° from Portland Bill Lighthouse at a distance of 13.2 nautical miles. This is precise and unambiguous, and it’s a widely used way of defining a position at sea. (The common convention at sea is to define a position as a range and bearing FROM a conspicuous point. Using the bearing of Portland Bill Light from you (030°) would cause great confusion!)

LATITUDE & LONGITUDE

The alternative is to use a pre-defined grid system, just like the road system in New York City, where you could say that the Empire State Building is on 5th Avenue and W 34th Street. That’s it. No ifs, and no buts. No need to tell someone to take the third right after Tescos. Anyone can find their way there.

You can define the position of the Empire State Building using the grid of streets (© TierneyMJ / shutterstock.com)

The maritime equivalent is latitude and longitude (always in that order). This too is completely unambiguous. So, using the WGS84 datum, the position of North Foreland Light that we established earlier in this chapter is:

51° 08’.42N 001° 22’.27E

Note that when writing positions, the ° indicates degrees and a single apostrophe (’) designates minutes, a minute being one 60th of a degree. For reasons lost in the mists of time, the apostrophe is always placed after the whole number of minutes, and before the decimal point. Sometimes, although less commonly, you will see a position described using seconds (”) rather than decimals, a second being one 60th of a minute. So, the same position is also:

51° 08’25”N 001° 22’16”E

Importantly, one minute of latitude is one nautical mile in length11, and one tenth of a minute of latitude (a little over 200 yards) is commonly referred to as a cable.

All positions, both latitude and longitude, are measured as angles from the centre of the earth.

LATITUDE

The North and South Poles – that is the True North Pole and the True South Pole – lie on the axis of the earth’s rotation. Their positions are constant, and don’t shift with time. The half-way point between the 2 poles is the equator, a great circle whose position is also fixed. The equator is defined as 0° north or south, and the poles are 90° north and south. The latitude of every intermediate position is simply measured from the centre of the earth as the angle between the equator and that point on the surface of the earth.

LONGITUDE

Longitude is not quite so simple as latitude. It is defined by ‘meridians’, which are great circles passing through both North and South Poles, running down the surface of the earth from True North to True South.

The reference, or ‘prime’ meridian could have been placed anywhere, but for historical reasons, the location of the observatory at Greenwich in east London was chosen as the Prime Meridian, and the longitude of every position on earth is defined by the angle east or west of Greenwich, as measured from the centre of the earth12.

Rather like the segments of an orange, the distance between meridians reduces as you move from the equator towards the poles. So, while 1 degree of longitude spans 60nm at the equator, the distance between the meridians steadily diminishes as you move north or south until you get to the North Pole, where submarine crews can stick a pole in the ice and complete a ‘Round the World Race’ in just a few paces.

MAGNETIC NORTH & SOUTH POLES

The True North and South Poles are defined by the rotation of the earth, and do not change. The Magnetic Poles, however, are defined by the earth’s magnetic field which, is caused by the flux of molten material within the earth’s core. And the magnetic poles do move, rather erratically, with the passage of time.

For the last 150 years the Magnetic North Pole has been drifting aimlessly around the islands of northern Canada. But recently it has sped up and started moving across the Arctic Ocean towards Russia. It is currently moving about 35 nautical miles each year.

In 2020, the North Pole was estimated13 to be at about 86°N 163°E, while the Magnetic South Pole was located rather further from the pole, at about 64°S 136°E.