The last navigator

Do you really prefer hitech modern satellite navigational gadgets to the romance of the stars and traditional methods, asks a friend? Actually, I think the question raises another: is there really a low tech traditional method in our modern sense of it? For most people, tradition means compass, sextant and chart.

A Micronesian sailing canoe, from The Last Navigator

As I mentioned in an earlier post, I have been catching up with a second hand copy of The Last Navigator by Stephen D Thomas, printed in 1987, which explains exactly how Polynesian navigators have been achieving remarkable feats of accuracy for thousands of years before even the compass was invented, let alone the sextant. The book has underlined for me how hi-tech our “traditional” methods of navigation really are.

A diagram of a sailing canoe from The Last Navigator

For the last 250 years navigators have in reality relied on precision instruments and almanacs full of mathematical calculations of great sophistication. These are methods, materials and advanced technologies that would have astonished the ancients.

A maritime chronomoter, key to longitude measurement, was a major technological achievement of the 18th century, while in its day the modern sextant was a leap forward in precision engineering. It simply was not possible before the 18th century to make instruments of such accuracy.

A great deal of learning and practice went into using both chronometer and sextant, in combination with the immense data resources published in nautical almanacs and the even bigger resource of worldwide nautical charts created by meticulous surveying.

Actually, you don’t even need to learn where all the stars and constellations are to use a sextant. The tables tell you where to look, using your compass and sextant and a rough approximation of how far you have gone since your previous position fix. That surprised me when I learnt how to use mine (a late starter because it was only 20 years ago). There is satisfaction in being able to find and recognise the constellations and the main stars from memory, but it is not at all essential.

Contrast all that with the methods of Polynesian navigators. They are well known to have had no instruments and charts, and to have relied entirely on reading the stars and waves and watching for birds and sea creatures. An aura of mysticism and magic surrounds those achievements. But The Last Navigator brings out the detail of how they actually did it, and it reads as an astonishing feat of memory and long practice, which in its underlying method turns out to connect directly with what we do nowadays.

They used the rising and setting positions of many different stars as their compass; they read information about currents and directions in subtle changes in wave patterns and the surface ripples of water; and bird and sea creature behaviour gave them information about distances from land.

In their heads, they carried the equivalent of a chart of Pacific islands. They fixed their position on it by seeing, in their minds, where lines from the rising and setting points of stars on the horizon intersected, using a reference island chosen for the particular passage they were on.

It was the use of position lines that I found most intriguing. Far from being some unknowable ancient and mystic puzzle, Polynesian navigation essentially did what we do now. Underneath everything was a geometry common to all methods of position fixing. Finding the intersection point of lines from stars and other known objects is the basis of every type of navigational position finding.

Intersecting lines are used by the computer in your GNSS receiver to calculate a position using signals from satellites. “Traditional” navigation using a sextant and chart uses lines measured from precise instrument readings of angles in the sky. For thousands of years Polynesian navigators have achieved a similar feat using no aids at all but memory, observation and experience.

If you want truly to learn to navigate with the romance of the stars and a deep understanding and feel for the natural world, you will have a long way to go. According to Wikipedia, by 2014 these skills were still taught only in the outlying Polynesian island of Taumako in the Solomons.

A new edition of The Last Navigator is still available in paperback, though search now under Steve Thomas, not Stephen D. The charm of the book is that it combines the story of a young man’s ocean adventuring with his quest to be taught the old arts before they disappear.

I also found on Amazon what looks like a much more detailed manual of techniques: We the Navigators: Ancient Art of Landfinding in the Pacific by David Lewis. If I could find a copy for less than £30 I would buy it! There are several blogs for enthusiasts of Polynesian and Micronesian navigation and quite a few replicas have been built to sail.

Relearning old navigation lessons

Two letters in the latest issue of the excellent Cruising Association magazine* claim it is now safe to rely entirely on electronic charts because backups to the boat’s main chartplotter are so cheaply available. You can leave your old paper charts abandoned in the bottom of a locker somewhere, the argument goes.

However, while researching updates for the yachtmaster book I mentioned last month, I’ve been re-reading intensively about old fashioned navigation techniques I learnt a long time ago, some of which were beginning to fade from memory. It has reinforced for me how important it is to avoid total reliance on electronics and have a portfolio of navigation skills including the old ones.

Traditional chart table at the end of a sail home from Scotland, and the bottle we brought back to celebrate our return .

So I think discussion of electronic charts needs to be a lot more nuanced than some of their more enthusiastic users suggest. Yes, on Spring Fever we have also relied heavily on electronics for many years, but my worry is how easy it is for phone, tablet and laptop software as well as hardware to fail, which can take a long time for the less than totally expert to sort out on passage.

It is certainly true that most well equipped offshore boats will have one or more extra chartplotters loaded on tablets, phones and laptops. Some are backups in case the main plotter system run from the boats batteries fails – that only takes a bust alternator or an engine that refuses to start. I also hear that an increasing number of people are dispensing with dedicated marine chartplotters altogether and relying on tablets etc.

Two examples of what can go wrong: my tablet’s GPS receiver failed last year. After a great deal of googling back home I found out why – I had not updated it for the GPS clock change that took place last year (little noticed, but the navigation equivalent of the millennium bug in 2000). Updated this winter using wifi at home, the GPS started working again, ready for this season. Hands up all those who knew about this and fixed it in advance. My problem was an older tablet. Recent ones would have updated for it automatically.

This was specifically a US GPS problem, and I don’t think it affected the wider GNSS, which includes the Russian, European and Chinese satellites processed by newer receivers than the one in my tablet.

More seriously, my laptop had earlier ground almost to a halt because of some unknown and untraceable software issue picked up I know not where – probably not a virus, or at least not one my Norton protection could find. I abandoned it for navigational backup and planning in favour of the tablet, well before that too showed it could let us down.

This winter, having bought a new laptop, I did a factory reset on the old one, wiping everything except the Windows 7 operating system. Miraculously, it now works like new. So I have loaded the charts, GPS driver and related software again and it is going back on the boat and staying there. I will keep it bug free by not connecting to the internet.

The laptop will be used for planning, with the cockpit chartplotter still our main instrument. But the laptop is old and has a hard disk drive rather than a solid state one, so can mechanically wear out. We may add a further backup in the shape of a small, cheap 7 inch tablet from visitmyharbour.com that’s tough and cockpit proof. (£170 loaded with 2020 raster charts for the UK, France and Atlantic Spain and Portugal).

Computer hotshots might have done fault-finding at sea or in a marina but it took me a whole day at my desk to figure out the laptop solution and sort it, using a high speed internet connection. The software fault had even disabled the DVD drive, which is now working again.

This is not the end of the list of issues. Tablet and phone navigation relies on apps and they do sometimes misbehave. Often the solution is to uninstall and download again – not an option for us at sea. That problem cropped up last year not with a chartplotter app but with one of the best tide prediction apps, though it could easily have been one of the chart apps. This all tells me – a reasonably capable but far from expert user – that the issue of electronics reliability, even with multiple backups, is complicated.

And that is without more basic questions, the first of which, GNSS reliability, I have covered in two previous posts. (On that subject, I have just read a report that GPS spoofing equipment can now be bought for only $100, and how to use it is widely discussed on line).

There is of course the more widely considered question of what happens to your backup tablets, phones and laptops in a knock down, a partial flooding of the saloon, a fire that you may put out but which causes damage or – a nightmare – a lightning strike.

I’ve had a strike on a previous boat, while it was moored on a river – I was ashore – and it destroyed all the electronics and made for a large insurance claim. High voltages nearby can induce large currents in equipment even if it is not wired into the boat’s own circuits. The advice I hear to stick your laptop, tablet and phone in the oven to shield them if lightning is near I find the complete opposite of reassuring!

So we don’t plan to chuck the paper charts, dividers, almanac and ruler in the bottom of a locker somewhere – they’ll always stay ready in the chart table, and we’ll use them from time to time so as not to forget how.

By way of contrast: now reading this 1987 book, which makes even our traditional navigation seem hi-tech. It’s about the last island outposts of Pacific Ocean navigation skills going back thousands of years, which use only knowledge of stars, waves and birds, and no instruments, achieving remarkable accuracy.

* Cruising, Spring issue.

February – averting satellite disaster

The British government turns out to have been ahead of the game on the  satellite risks I mentioned last month, with a £36 million programme just announced to  prevent navigational satellite failures damaging the economy by as much as £1 billion a day. It is feared that the entire country has become over-dependent on a handful of satellite systems.

Emergency services, the energy grid, mobile phones, Satnav, broadcasting and other communications, the Stock Exchange and an array of other activities all rely heavily on the super-accurate timing provided now by navigational satellites such as GPS and similar systems. There are life-threatening risks from failure, says the government.An image of a third generation Lockheed Martin GPS satellite

The new investment is in a National Timing Centre to create a network of super-accurate atomic clocks around the UK, accessed through ground-based communications, so that the economy will no longer be over-reliant on timing from GNSS signals from the sky.

GNSS is the term that embraces the US  GPS, the first system, Russia’s GLONASS satellites, Europe’s new Galileo and also a rapidly developing Chinese system.

Galileo failed completely for a while last year during its start up phase, because of operator errors, and there are now many examples of interference with GNSS systems and malicious ‘spoofing’, in which navigation instruments are fooled into thinking they are somewhere else. The heart of all navigation by satellite is accurate timing, without which positions cannot be fixed.

The Business Department said the centre will provide additional resilience against the country’s reliance on accurate timing, which underpins many every day technologies.  “ If there were a large-scale failure, economic impact to the UK would be £1 billion a day”, the department said. “Loss of this accurate data would have severe and life-threatening effects, such as on getting ambulances to patients or getting power to homes around the country”.


The plan puts the UK ahead of the US, where the issue has been rumbling on since President Bush announced a new satellite resilience plan, and a law to authorise it was passed. Those early moves have not actually produced any results.

President Trump’s White House has just announced a new programme but it appears to be no more than studies for future investment rather than actual projects, according to the Resilient Navigation and Timing Foundation (RNTF), a Washington group campaigning on the issue. The Foundation said the UK plan was a great initiative.

Almost nostalgically for small boat navigators with pre-satnav experience, the UK still operates a ground-based Loran navigation system, though receivers for yachts are no longer available (as far as I know). The RNTF believes that Loran may be expanded and incorporated in the UK timing resilience network, which it thinks will be wireless-based rather than land lines.

Amazing to think how dependent we are on one technology at the moment. Ten years ago we used to worry that the US might degrade the accuracy of GPS or switch it off in a war. That is now inconceivable, so dependent is the entire US economy on it at the moment.

More realistic dangers  include accidental  system failures, as with Galileo, deliberate or accidental interference on a regional or local level, and spoofing. If you think about what some idiot can do with a drone near an airport, imagine the chaos if disruptive forces started jamming and spoofing GNSS. The expertise to do that has been nakedly displayed in recent years in the Gulf and the Black Sea near Ukraine.

Back to earth: as I mentioned in an earlier post, plans are always tentative at this time of year, and so that is proving. The passage to north-west Spain I was writing about in December and January has run into the problem of coordinating two lots of family diaries, which can be tricky with joint ownership and joint sailing plans.

Various unmissable events on both sides now mean shorter periods on the boat this year, so the plans are evolving. Maybe we will go back
to south Brittany, where we spent three seasons recently.  Spring Fever on the River Vilaine, southern Brittany

We could always go down from there to Spain next year. There are so many lovely places on the French coast that it would be no hardship if that is what we decide.

Boat maintenance has been brought back home for me this month, with various canvas repair jobs to do on the safety equipment, sail cover and dinghy cover, plus rope whipping, and also ordering new lifelines and jackstays.

Got a great new  sewing  awl for canvas that is much quicker than a needle and palm..

January – satellite scares, and getting ready for Biscay

At the Royal Institute of Navigation’s small boat conference in Lymington earlier this month, I learnt a lot about  new risks of error  in satellite navigation : I did not know, for example, that it is possible with quite cheap local equipment to fool the GPS on a plane, ship or even a missile into thinking it is somewhere other than its real position.

There are now tens of thousands of reported incidents of errors, deliberate, accidental or of unknown cause, with a substantial number of them unsurprisingly in sensitive areas such as the Gulf, and the Black Sea near Ukraine, suspected to be hostile activity.

Reports of accidental errors include a couple of local failures when US naval vessels arrived in the port of San Diego, apparently forgetting to switch off unspecified electronic equipment, which interfered with satellite-derived positions for miles around.

These are serious issue in defence circles, and of course for boats and ships of any kind near a source of interference or one of the ‘spoofing’ attacks. It is easy to imagine some disruptive force deciding to get hold of the equipment and blocking position-finding in a sensitive commercial area such as near an airport or in a vital shipping lane.

Satellite positions are integral to the operation of every form of commerce, from planes and ships  through mobile phone masts and everyday driving, Amazon deliveries and Uber – you name it and somewhere in the business location finding is vital. So there is increasing pressure to redevelop  an old ground-based system called Loran as backup. Loran has not been switched off, but would now need a lot of development and investment. If you want to know more, look at the website of the Washington-based Resilient Navigation and Timing Foundation.

GPS is just one of five satellite position-finding systems in operation, all now called GNSS (Global Navigation Satellite System). GPS, like the word hoover, is a US name that is in danger of wrongly becoming the label for  a whole range of products.

Other GNSS now running include the Russian GLONASS and the EU’s GALILEO – so in one sense there’s a lot better backup against total failure. GALILEO was completely out of operation for a while last year after operational mistakes were made by its staff. It was only in the start-up phase but the fault  proved that a single system can go down.

The cover of the current 5th edition of Pass Your Yachtmaster

I was at the conference because I have been commissioned to update an excellent sailing book, David Fairhall’s Pass Your Yachtmaster, illustrated with hilarious cartoons by the late Mike Peyton.

Completely coincidentally, the cover of the current edition has a nice but anonymous photo of a Sigma 362, the same model as Spring Fever. Having done 11 seasons polishing and antifouling, I recognise the detail of that bow!

David has updated previous editions of the book, first published in 1982, but asked me to take the baton for the next.  The book is a primer for students doing the Royal Yachting Association yachtmaster exams and tests.

I have to say that re-reading  David’s book and many others on navigation has reminded me of several things I ought to remember, and certainly knew when I did the exam and practical test in the 1980s, but had forgotten, so it’s a good exercise for more than one reason.

Back at the yard, plans for sailing to Spain are firming up so we’re making a couple of small improvements to the boat and some minor repairs. We’ve also booked for the Cruising Association’s Biscay Day in London in March where we will meet others sailing that way.

One annoying issue on Spring Fever has always been the need to lift the anchor from the bow roller a couple of feet along the side of the boat when retrieving it to stow in its locker. I’m always afraid that this is going  to damage someone’s back, holding a 15 kilo weight plus chain at a very awkward angle on a potentially rolling boat.

The reason is that the drum of the Furlex reefing gear almost touches the deck, a feature from the boat’s racing days to maximise sail area, so it blocks the anchor from being lifted straight from the bow roller. We are looking for a rigger willing to shorten the forestay with the mast up, to lift the drum 8 inches so the anchor can be pulled up through the roller, and if necessary lashed and dealt with later. Our usual rigger is not keen because he works alone and it needs two experienced  people, he says.

The sail luff can be shortened at the head because it is very narrow there. Our sailmaker is taking 9 inches off but we will lose only about a square foot of sail area.

We’ve also decided to install a holding tank, so the lavatory can be used when we’re somewhere, er, sensitive to such things eg a marina or swimming bay. Nobody ever asks in the UK or France but Spanish authorities are thought to be more vigilant in checking that boats have holding tanks to store their waste on board, an EU requirement. Very few places have pump out facilities to empty tanks so the usual procedure is to empty them by opening the discharge valves a few miles out to sea.

Spring Fever is ashore this winter at Kingston, the Cowes Harbour Commission yard, rather than at Cowes Yacht Haven, whose prices have been creeping up by the addition of extras. The Haven also focuses on expensive racing boats whereas the Kingston yard, which is very friendly, is mainly full of older boats like ours, so we immediately felt more at home. It is not as conveniently sited, but it is looking like a good switch.

Is your chart relying on an 1860 survey?

Footnote to cruising the Scillies: piloting there is a reminder of the importance of proper Admiralty charts, because they show the age of the surveys on which they are based, unlike any of the proprietary ‘vector’ charts available on chartplotters.

The Scillies is a mixed area from this point of view. Some of the surveys of the area were last done in 1860 – 1904 by lead line, probably from boats carried on naval survey ships and rowed up and down in straight lines quite a long way apart, so rocks could easily be missed. Other parts of the islands were surveyed at a range of different dates in the 20th century.

The little diagram showing which area of the Scillies was surveyed when is very instructive, and is a warning that in certain places extra care is needed. Interestingly, one small but vital area, the ferry route in and out of Hugh Town, was surveyed in 2012-14, by the Duchy of Cornwall, the landowners. At least we should be able to rely on that route.

This does not mean you have to stick with paper. We would have spent at least £5,000 in recent years if we had bought all paper charts for our cruises. UKHO (in other words Admiralty) charts for the whole of the UK and Ireland, complete with all the supplementary information, can be bought as a single package for very reasonable prices online from the Cowes firm visitmyharbour.com, and combined with the excellent Marine Navigator chartplotting app they make a fine tablet chartplotter as backup to the main system. What you are looking at then is a computer image of an official UKHO chart (technically, a raster rather than a vector electronic chart). In fact, the UKHO charts are now printed from electronic files anyway, so it is exactly the same image.

The popular yacht chart systems such as Navionics, Garmin and C-Map, leave out vital information such as survey age. They are not images of official charts but combine the data in their own proprietary way. They also contain potential traps because they are organised in layers, and as you click from one to another you get more or less detail. If you are on the wrong level you may miss something important. To have the UKHO charts as backup on a tablet is reassuring.

Furthermore, we have seen a number of examples over the years where Navionics has missed out features altogether or got information wrong. There was one example in Tean Sound, where Navionics showed one power cable crossing the sound under water. The UKHO chart, and to its credit C-Map, showed all four.

Philosopher of sailing

The letter below was published in Cruising magazine this month:

If it does not prompt a few cross letters from traditionalists in the next edition, I’ll be surprised, especially after the recent finish of the Golden Globe round-the-world race using sextants and traditional navigation – just as they did on the first race 50 years ago, which was won by Sir Robin Knox Johnston in Suhaili. It was also the race in which the sad figure of Donald Crowhurst cheated in desperation and then disappeared from his yacht.

A victorious Knox-Johnston 50 years ago

Electronics were banned in the anniversary race, though I did read somewhere that they all had to sneak in a satellite phone just in case.

If the old guard don’t complain about my letter, then things have changed more than I expected….

 

PS I know I’m more than old enough to be part of the Old Guard, but I think modern electronic navigation is wonderful, and it leaves more time for actually handling the boat.

Chart errors (contd).

Several earlier posts covered inaccuracies in charts. Last week I came across yet another example in Cala de Portinax, a bay at the north end of Ibiza in the Balearics. This screen shot from my Navionics chart shows depths in a fair amount of detail: yet the numbers were wrong by a factor of three or four. IMG_2586 Because of the modest depths shown, we motored in with great trepidation in Olivia Jane, a Beneteau 43 with a 2 metre draft. Yet we found the depths in the middle of the bay were all in the 11-13 metre range and even close in to the rocky shore we anchored in 8 metres. In contrast, the C-Map chart on the cockpit plotter gave no depths at all inside the bay, which is a safer option than mistaken information. We’ve fed this back to Navionics and await a reply, but the nagging question will always remain in future, even if this proves to be a rare mistake: can we trust the inshore information on these charts? What if the mistake had been the other way, showing 11 metres when only 2 metres was there? Do other brands of charts have similar errors? Previous posts on this subject were mainly about errors arising from old surveys; many of these were to do with 19th century measurements of longitude, which become glaringly obvious with GPS. This Navionics chart seems in contrast to suffer from a basic data problem. I’ll update when Navionics replies.

May 14  Here is part of the Navionics reply – they put it right but there is no indication of the frequency of the problem.

Thank you for bringing this issue to our Cartography Department. Navionics appreciates your input and we are pleased to inform you the issue has been corrected. The updated data is currently available for Navionics Mobile by updating your maps.  We have asked to write us from the App, in order to know which App version you were using and you need to update it. Always make sure you are running the most current version of the mobile app. 

However, when I went on line, the app told me that no updates were available at this time. Are the above lines just their formula for telling people to go away? The chart of the bay is unchanged.

11 June
finally got the problem of the Navionics chart errors sorted, with help from the company by email. I deleted and reinstalled the whole app and succeeded in getting the updated version, which matched the plotter and our own depth measurements.

Lessons:
(1) one could go on for a long time not knowing that the charts are failing to notify updates via the App Store. So best to ask it regularly to update and if it keeps saying no updates available get in touch with Navionics help, or simply delete and reinstall (after logging on and syncing to avoid losing data). There must be a bug lurking somewhere.

(2) the chart was bought late last year and is still current – so the error was there very recently. There will always be a nagging question in my mind about whether other mistakes are lurking in the charts.

PS Am now looking into why my 2015 edition of Memory Map’s UK charts, which I have on my laptop, does not mark the position of the massive new breakwater at the entrance to Cowes Harbour, my home port. It says in general terms ‘works in progress’ with no position or indication of what. In Navionics’ favour, they show the new breakwater accurately. I have asked Memory Map what the actual cut off date is for its 2015 edition but have had no reply. 

For more on this subject, see:

Med Charter Disaster? Orkney Roulette Marine Survey Accuracy


Marine survey accuracy

Data published by the International Hydrographic Organisation shows up a surprising fact: the UK and Ireland are below Turkey in the league table of survey quality by area of national waters. Spain, Portugal and France score much higher than the UK.

  

Source : selected countries  from an International Hydrographic Organisation table.

 % of area at depths from zero to 200 metres which has been adequately surveyed

% which requires re-survey at larger scale or to modern standards

% which has never been systematically surveyed

Mediterranean

 

 

 

Monaco

100

0

0

Spain

97

3

0

France

95

4

1

Gibraltar

95

5

0

Turkey

88

12

0

Slovenia

80

20

0

Italy

70

25

5

Croatia

39

39

22

Greece

35

55

10

Morocco

30

0

70

Albania

25

45

30

Serbia Montenegro

0

100

0

Cyprus

0

100

0

 

Atlantic, NW Europe 

 

 

 

 

 

 

 

Portugal

100

0

0

Faeroes

100

0

0

Belgium

100

0

0

Germany, N Sea

96

4

0

Denmark

95

5

0

Canaries

95

5

0

Spain

94

6

0

France (Channel, North Sea)

89

0

11

France Atlantic

81

0

19

Norway

60

31

9

Azores

60

40

0

Netherlands

55

35

10

Iceland

52

48

0

UK

49

22

29

Ireland

26

74

0

 Greenland

25

25

50

Svalbard

10

1

89

Jan Mayen

0

0

100

 

Baltic

 

 

 

Germany

98

2

0

Finland

44

49

7

Poland

40

60

0

Sweden

25

74

1

Lithuania

16

84

0

Estonia

13

87

0

 

 

 

Checked January 2013

 There is one obvious reason why the UK’s position is relatively low; with a much larger area of shallow continental shelf, the British Isles has proportionately a far bigger survey to do than a country such as Turkey, where depths in many places reach 200 metres relatively close to shore.

Even so, it is widely known that there are cruising areas off the Turkish coast where chart accuracy is poor. Rod Heikell’s pilot warns that they can be out be by up to 2 minutes of longitude, considerably more than the warnings for anywhere on the British and Irish coasts.

 UKHO chart Q 6090 looks more deeply into the situation in the UK and Northern Ireland. It colour codes areas of the seabed by survey quality.  In the January 2013 edition, large parts of the coast of the Scottish mainland, Hebrides and northern islands, Wales, the Channel Islands, Lancashire and Northern Ireland are still marked as surveyed by leadline, or unsurveyed.

Follow this link to see UKHO chart Q6090

In an earlier 2006 version published in the AIB report on the Octopus (see Orkney Roulette page), the entire coast of  Ireland was marked as leadline surveyed. The republic’s coast has been left out of the latest Q6090, but the Irish government has now produced its own version of the chart, which shows a very large surveying effort since 2006.

Link to Ireland survey coverage chart

Interactive chart with local detail of Irish surveys

One result of the new surveys is that there have been new editions of 22 of the 60 or so admiralty charts for Irish waters since 2009, according to the Irish Cruising Club.

For practical decisions during a cruise, we have to rely on what electronic and paper chart publishers tell us about the underlying quality of their products, backed up, of course, by pilot book advice. This information can be pretty patchy. For example:

  • UKHO charts   have source data chartlets, a benchmark for quality.
  • Imray charts have no source data.  Some Imray charts do have general warnings on chart accuracy, eg for the Ionian and Turkey, where recent editions have quantified errors in the area as up to one minute and half a minute of longitude respectively.
  • Pilot books vary on the issue: Heikell has general warnings in his introductions of much larger errors in Greece and Turkey, of 1.5 minutes and 2 minutes longitude maximum respectively, but that covers a vast area.    
  • The Irish Cruising Club pilot’s latest edition for the West and South coasts of Ireland has gone much further, with localised chart accuracy information for different chapters, a good benchmark to aim for.  
  • Some other pilot books I have checked recently  do not seem to mention the accuracy issue at all, even where they cover cruising grounds which the tables above show have known  chart quality issues (for example parts of the Adriatic).
  • Many leisure electronic charts lack information on source data or other indications of underlying survey quality. They also lack last correction dates, and there is no detail of how comprehensive the updating is. Memory Map does reproduce source data diagrams on its raster charts but not edition dates, even though it is not hard to find UKHO charts for sale now whose most recent edition was three decades ago, for example on the West of Ireland.
  • To find comprehensive source information on an electronic  chart, you have to move to those used in Electronic Chart Display and Information Systems on ships, a different world,  where vessels equipped to a high enough standard are now allowed to dispense with paper charts altogether.

In preparation: articles on GPS accuracy and on what makes leisure charts unsafe compared with those used in ECDIS systems.