Originally Posted by mikecambrai
Care needs to be taken when using plotters due to the way that they are made up. ...
Don't forget that few people can update their electronic charts, and papercharts are only as good as the survey
information on which they are written and the date of the last Notice to Mariners corrections.
Paper charts are only as good as the survey
information on which they are based and the date of the last Notice to Mariners corrections AND MANUALLY APPLIED TO THE CHART. But remember that Notices to Mariners are only issued after someone notices an error or someone changes the position of a buoy etc. They don't mean that the chart is now completely error free.
I know that I have some old charts in my cupboard for areas where I haven't sailed for a while. When I last needed to update my folio for the Ionian
Sea (Greece), because I was going back there for a trip I had 50 pages of corrections to apply!
The cheat in the UK is to order new Admiralty charts (the full size ones, and not the Small Boat Folios) from a registered Admiralty Chart Agent. They have to employ people who will apply all the published corrections to the charts before they send them to you.
Their corrections are also a lot neater than I can do when I apply any later changes.
My chart plotter (OK it is only a handheld Garmin
, but has the same charts as their larger models) can really get confused when the chart data that they have based the electronic version on was not compatible with the WPS84 spheroid used for GPS
. (You know the old charts that tell you that GPS positions need to be moved x east and y south before being applied to the chart. I have had my plotter showing my track going the wrong side of a lighthouse when I was going into a harbour in Croatia
, difficult to do in practice as it was on the mainland!
On another trip if I had based my approach to a harbour on the west coast
on the electronic chart as shown on the plotter I would have gone straight through the harbour wall instead of through the entrance.
As my first sailing instructor said to me in Greece
, many years ago, "If you have brought a GPS with you it will tell you exactly where you are on the planet, but not where you are on these charts. They are still based on a survey taken by HMS Hydra in 1870, and that was done using manual triangulation and lead lines!"
Only a year or so ago the Admiralty issued a "Notice to Mariners" alerting shipping
to a sea-mount that had just been discovered between the Orkney Islands and the Shetland Islands in the UK. It was discovered after a heavily laden container ship reported hitting an object in that position resulting in damage to its bottom. They dispatched one of the Royal Navy
survey vessels to investigate and there it was, an underwater spike that no-one knew existed.
I would recommend that anyone who can get hold of a copy reads a book called "How to Read a Nautical Chart" by Nigel Calder. Some of the examples of old surveying errors that he shows (and explains the reasons for) will make you cringe, and wonder how much you can really rely on any chart that has not been based on a survey done recently with modern equipment
Personally I keep both electronic and paper charts and use both, OK I probably use the electronic ones most of the time but the paper ones make it easier to see the big picture. But I don't rely on automatic course corrections at my way-points and always keep a good lookout. I also know that when the visibility goes down then I cannot necessarily rely solely on the electronic charts to get me out of trouble. They are an aid but I still plot my position on my paper chart and keep my eyes open and the echo sounder pinging away.