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.