|Skysail Training © Keith Bater|
1:1 ON YOUR DATE
There is very little in coastal navigation that can be considered 100% accurate.
Although we can calculate courses, tide speeds, and positions with some precision, that does not ensure accuracy. We can consider that we are not navigating a boat, but an area whose accuracy is decreasing and size is increasing the further we travel without an accurate fix.
Errors will arise from helming, errors in the tide, the log, the compass, leeway, waves, surface currents induced by prolonged winds, etc, etc.
If the conditions are favourable, with fair weather, calm seas and neap tides, the error might be 5% per hour of travel. With spring tides and rough seas, the error might increase at 15% per hour. A position line such as a depth contour or bearing on a lighthouse can be used to reduce the area of error.
Most Probable Position - a good article
Position Probability Area
Causes of error
Navigator's error - what the Admiralty calls a 'blunder'. Can be found by cross checking.
Equipment malfunction - can be hard to detect
Equipment breakdown - easy to detect.