Marine Glossary

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error current

When the DR track from the last fix at time T1 to the time of the next fix (T2) does not agree with the new fix, the difference between the T2 DR position and the T2 fix is often attributed to the current in effect. The set of the current being the direction from the DR position to the new fix position, and the drift (speed) of the current equal to the the distance from DR to Fix divided by T2-T1.

That is often a good first guess, but the result can often be productively generalized. For example, we could have unaccounted for leeway contributing to the discrepancy, or we could have a log or compass error we did not know about. Or we could have simply not steered the average course we thought we did between T1 and T2.

Thus is it often useful to think of the “current” as not just the motion of the water, but a combination of all effects that led to the discrepancy, in which case it is called an "error current."

With an assumed constant error current, we know that after sailing a given period of time, we can improve our DR by applying this current, without specific regard to what causes it.

Note that on different headings, a pure motion of the water current will be the same, assuming the water current is indeed constant over time and place covered, but an error current with other factors involved will likely be different on different headings, even if the water current component did not change.

You can experiment with these ideas using our DR computer.

See also Dead Reckoning