How navigators turn movement into position — and why this method punishes assumptions
Contents
Use the links below to jump to any section:
- What a Running Fix Really Is
- Why Running Fixes Exist
- The Concept of Advancing a Line of Position
- When Running Fixes Are Appropriate
- Step-by-Step Logic (Without Maths)
- Accuracy and the Cost of Assumptions
- Common Errors in Running Fixes
- Running Fixes in Coastal and Confined Waters
- When Not to Use a Running Fix
- Professional Use of Running Fixes on the Bridge
1. What a Running Fix Really Is
A running fix is a position determined using one object observed at two different times, combined with the ship’s movement between those observations.
It exists because reality is imperfect.
Sometimes there is:
- only one good landmark
- poor visibility in one direction
- limited charted features
A running fix accepts this limitation and works with it.
It is not a precise fix.
It is a disciplined estimate with structure.
2. Why Running Fixes Exist
Running fixes exist to answer a simple operational question:
“Where am I now, given where I was earlier and how I moved?”
They allow navigation to continue when ideal fixing conditions do not exist.
Historically, they were essential.
Today, they remain valuable because they force awareness of movement, something electronics often hide.
3. The Concept of Advancing a Line of Position
When a bearing is taken, it creates a line of position.
That line is true only at the moment it was observed.
In a running fix, that line is then moved forward along the ship’s estimated course and distance run to the time of the second observation.
This creates an advanced line of position.
Where the second bearing intersects the advanced line is the estimated position.
This method assumes one thing only:
The ship actually went where you think it did.
4. When Running Fixes Are Appropriate
Running fixes are most useful when:
- only one suitable object is available
- visibility is limited in one sector
- landmarks appear sequentially
- coastal features are sparse
They are common in open coastal waters and approaches where fixes are infrequent but progress must still be monitored.
5. Step-by-Step Logic (Without Maths)
The logic of a running fix is simple:
First, take a bearing to a known object and plot it.
Then, continue on a steady course and speed.
Later, take a second bearing to the same object.
Advance the first line by the distance run.
The intersection gives your estimated position.
The method works because time creates separation between observations.
6. Accuracy and the Cost of Assumptions
Running fixes are sensitive to error.
If any of the following are wrong, the fix degrades:
- course made good
- speed through the water
- set and drift
- steering accuracy
Unlike other fixes, errors here are invisible. The fix may look neat and still be wrong.
This is why running fixes reward honesty and punish optimism.
7. Common Errors in Running Fixes
The most common mistakes include:
- assuming course made good equals course steered
- ignoring current
- advancing the line incorrectly
- taking bearings too close together in time
- trusting the result more than the inputs
A running fix is only as good as the navigator’s discipline.
8. Running Fixes in Coastal and Confined Waters
In confined waters, running fixes become risky.
High traffic density, frequent alterations, and variable currents make assumptions unstable.
They should be used cautiously and always backed up by other information such as depth, transits, or ranges.
A running fix should never be the sole source of confidence near danger.
9. When Not to Use a Running Fix
Running fixes should be avoided when:
- the ship is altering course frequently
- speed is changing
- current is strong or uncertain
- visual identification is doubtful
- precision is required immediately
In these situations, waiting for a better fix is safer than forcing an estimate.
10. Professional Use of Running Fixes on the Bridge
Professional navigators treat running fixes as trend indicators, not absolute truth.
They are used to:
- confirm general progress
- detect unexpected drift
- support planning decisions
- bridge gaps between stronger fixes
They are rarely used alone and never without caution.
Closing Perspective
Running fixes teach a crucial lesson:
Navigation is not about perfect information.
It is about understanding uncertainty and managing it honestly.
A navigator who understands running fixes understands the cost of assumption — and that understanding carries into every other method.
Tags
running fixes · advancing line of position · coastal navigation · visual navigation · bridge watchkeeping · chartwork