navigation_errors_probable position

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Navigation Errors

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.

The area of uncertainty will grow over time of travel. Any opportunity to check position via a position line, depth contour or bearing should be taken.

This reduces the uncertainty, keeping in mind that position lines also have errors.

Most Probable Position - a good article

Position Probability Area

Causes of error

Faults

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.

Systematic errors

Helming errors

Compass errors

Log errors

Random errors

Tide speeds