Worked DR to EP: Coordinates, Uncertainty and Verification

Start from a fix at 50°00.0′N 001°00.0′W. Steer 090°T at 6 kn for two hours, with no leeway: the DR displacement is 12 NM east. At latitude 50°N, one minute of longitude spans approximately cos 50° = 0.643 NM, so 12 NM east is 12 ÷ 0.643 = 18.7 minutes of longitude. The DR is therefore 50°00.0′N 000°41.3′W.

Apply a steady tidal stream setting 180°T at 2 kn for two hours. Its drift is 4 NM south, equal to 4 minutes of latitude. The EP is 49°56.0′N 000°41.3′W. Label the start fix, water-track vector, DR, tidal vector, EP, and time so another navigator can reconstruct the method.

The calculation assumes the helmsman held the ordered water track and speed, the tidal estimate was representative, leeway was negligible, and the short east-west distance can be converted at 50°N. At Yachtmaster level, name those assumptions without being prompted. If a log reading, engine setting, wind, sea state, stream boundary, or position check contradicts them, revise the EP rather than defending the original plot.

An EP is not upgraded into a fix by precise-looking coordinates. Carry an uncertainty allowance that reflects the quality and age of the inputs, use a conservative edge of that allowance near danger, and seek position evidence independent of the original estimate. Depth, visual bearings, radar range, GNSS and transits each have different failure modes; choose checks that genuinely reduce shared error.