Under some circumstances, such as low visibility, only one line of position can be obtained at a time. In this event, a line of position obtained at an earlier time may be advanced to the time of the later LOP. These two LOPs should not be parallel to each other; remember that the optimal angular spread is 90°. The position obtained is termed a running fix because the ship has “run” a certain distance during the time interval between the two LOPs.
It is more commonly used when only one object is available for bearings and there is no means of measuring the range. In this case there is a planned delay between bearings so that the change in bearing will provide an acceptable angle of cut.
The running fix (or transferred position line) is a method to determine your position if you only have one visual reference point (like a single lighthouse). The methodology is as follows;
- As you approach the single point, take a bearing on it with your hand held compass. Make a note of the log, and course being steered.
- Once there is an appreciable change in the angle to the object, take another bearing and note the new log reading.
- Plot the 1st bearing, and anywhere along this position line, make a mark and from this point, plot the course steered. Measure down this track the distance travelled (the difference between the 2 log reading). and make a second mark.
- Transfer your first position line, keeping it parallel, down to your second mark.
- Where this transferred line crosses the second compass bearing is your position.
Consider the accuracy of this method? How well was the course steered? How accurate is the log? Was there any tidal streams of leeway?
These can all be factored in to improve the fix, but generally, if the distance between the 2 bearings is not too large, it will be accurate enough.