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. Continue reading “Is your chart relying on an 1860 survey?”
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. Continue reading “Marine survey accuracy”
Not long after a jack-up rig called Octopus ran aground in Stronsay Firth in the Orkneys in 2006, we were feeling our way into a bay at nearby Stronsay. We crept through shoals and reefs relying on chartplotter and echo sounder, in strong winds and bad visibility, confident that our plotter would get us through: after all, we had checked it frequently on our passage up the English and Irish coasts and through the Hebrides, and had been pleasantly surprised never to find the GPS positions more than 50 metres out on the chart.
So confident had we become in our first chartplotter that a few days earlier, in fog rolling off the island of Hoy so thick that the boat’s bow was only just visible, we used it, along with the depth sounder, to find our way at dusk into the harbour of Stromness, which involves a tight turn round a reef at a point where the tide can run at up to 8 knots into Scapa Flow, right across the entrance to the inlet leading to the town.
If only we had known more about the quality of the local charts ….. much later, I came across a Marine Accident Investigations Branch report on the grounding of the jack-up rig Octopus, under tow by the tug Harald, which popped up in a web search for something else. (MAIB Report 18 2007).
Continue reading “Orkney Roulette”