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


Maskelyne v Harrison – the longitude show in London

Harrison's third clock
Harrison’s third clock

 

We finally managed to take in the excellent Ships, Clocks and Stars exhibition at the National Maritime Museum, Greenwich, which runs only until 4 January. Among other things, it puts the record somewhat straighter on the eminent 18th century Astronomer Royal, Nevil Maskelyne, chief villain of Dava Sobel’s best selling nautical history of the race to measure longitude at sea. Maskelyne was the man who conspired against John Harrison, the genius who built the first true chronometer, or so the narrative goes in Sobel’s book, Longitude.

Not so, it is clear from the detailed story set out in the exhibition, which of course includes the famous clocks, which are extraordinary creations. It seems the rival method using the moon to measure time (and thus longitude) worked perfectly well, though not as accurately. Far from being biased against Harrison in favour of the lunar method, Maskelyne seems to have been rather even handed, encouraging rigorous trials of both.

In the trials, the lunar method turned out to be good to a degree, or 60 miles, which was a lot better than most navigators had been able to achieve before on long passages, but chronometers could reduce the error to as low as 10 miles. By producing an accurate timepiece that could withstand violent movements and large temperature changes over many months, Harrison revolutionised navigation, made it possible for sailors to know reliably where they were, and thus saved countless lives and cargoes from being wrecked.

Both the chronometer method and the lunar method required measurements by sextant followed by calculations using detailed astronomical tables. One of Maskelyne’s great achievements was to set up a system for producing the annual nautical almanac, which is still published today. The calculations were done by computers, a term which in those days referred to people. In fact it was an attempt to automate the calculations in the early 19th century that led Charles Babbage to design the first mechanical computer, a small part of which is in the exhibition. The attempt failed because the costs of building the machine escalated out of control.

Harrison also seems to have been a bit of a curmudgeon, falling out with Maskelyne when pressure was put on him to demonstrate that his watch design could be reproduced by others for more general use at sea. Some of his rewards were held back until he published and publicly explained the details of the clock’s construction and made two copies. In the end, he won the argument by appealing to Parliament, which awarded him more than had been promised originally.

IMG_2195
The fourth clock, dramatically smaller.

There was much more to discover in the exhibition. I was particularly interested in the method of finding the time and thus the longitude by observing the moons of Jupiter, which proved impossible to do on a heaving ship, but was accurate on land. This giant solar system clock was used to redraw the maps of Europe. When the French Atlantic coastline proved to be much further east than earlier maps had shown – shrinking the country’s area significantly – the King of France remarked that the cartographers had stolen more land from him than all his enemies combined.

 The new book, published this year to coincide with the exhibition, is Maskelyne, Astronomer Royal, edited by Rebekah Higgitt (one of the curators).