Module 10 - Chartwork

Running Fix (Transferred Position Line)

What you will learn

Transfer an earlier line of position to a common later time using estimated ground displacement, then state the uncertainty inherited by the running fix.

A running fix combines observations made at different times. The earlier line is moved parallel to itself by the vessel's estimated movement over the ground before it can intersect the later line.

Take and time the first bearing to an unmistakable charted object, then plot its line of position. Between observations, estimate the vessel's ground displacement from course, distance, leeway and tidal stream. At the later time, move the first line parallel to itself by that displacement and plot the second observed line. Their intersection is the running fix in the course convention.

The construction is useful when only one suitable object is visible, but it is less independent than two simultaneous observations. An error in the first bearing, the later bearing or the estimated movement shifts the result. The transfer uses movement over the ground; moving the line by log distance and course steered alone silently omits environmental effects.

Doubling the angle on the bow is a special geometry shortcut, not a universal range instrument. With a steady straight ground track and reliable distance made good, when the relative bearing doubles the slant range at the second observation equals the ground distance travelled between observations. Course alteration, uncertain stream, leeway or inaccurate distance breaks the shortcut.

Worked example

A lighthouse bears 035°T at 1015. By 1030 the best EP movement is 1.2 M on 082°T, and a second bearing of the lighthouse is 320°T.

  1. 1Plot the 1015 line of position through the lighthouse using its reciprocal direction from the object.
  2. 2Draw the 1.2 M ground-displacement vector on 082°T for the 1015–1030 interval.
  3. 3Move the complete 1015 line parallel to itself by that vector and label it transferred to 1030.
  4. 4Plot the 1030 line of position; its intersection with the transferred line is the 1030 running fix.
  5. 5Check the result against depth, DR/EP trend and another observation before using a small danger margin.

Sense check: Every point on the transferred line should have moved in the same direction and by the same 1.2 M displacement as the vessel estimate.

Use one light without pretending it is two simultaneous objects

At night one lighthouse remains visible while the coast and horizon obscure every other useful visual mark.

  1. 1. Time and identify

    Confirm the light and record the first bearing and time.

  2. 2. Protect the transfer

    Maintain the intended movement, record changes and use the best ground-displacement estimate rather than course steered alone.

  3. 3. Cross-check the result

    Treat the running fix as dependent on the transfer and compare it with depth, radar or checked electronic evidence.

Sense check: If a plausible stream change moves the running fix across danger, reduce reliance on the construction and obtain stronger evidence.

Common mistake or limitation

  • The first line is moved parallel; rotating it changes the original observation.
  • The ground displacement is estimated, so the running fix is not equivalent to two simultaneous independent lines.

Recap

  • Transfer the earlier line to the later observation time.
  • Use estimated movement over the ground and keep its uncertainty visible.
  • Bound special bearing shortcuts by their geometry and distance assumptions.

Optional quick check

Section 4 of 11

What movement should transfer a 1015 position line to a 1030 observation?

Choose one answer
Sources and factual review

Reviewed 2 September 2026 - Compass Revision editorial review.

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