DR & EP Knowing where you should be — when you don’t know where you are
Contents
Use the links below to jump to any section:
- Why DR and EP Still Matter
- What Dead Reckoning Really Is
- What an Estimated Position Actually Means
- Why DR Is Never “Wrong”
- How EP Improves on DR
- Set, Drift, and Leeway (Conceptual Only)
- How DR and EP Support Visual Navigation
- DR, EP, and Error Growth
- Common Civilian Mistakes
- Professional Use of DR & EP on the Bridge
1. Why DR and EP Still Matter
Dead Reckoning (DR) and Estimated Position (EP) exist because fixes are intermittent.
Even in perfect conditions, you are not constantly fixing the ship’s position. Between fixes, the ship still moves. DR and EP fill that gap.
They answer a quiet but essential question:
“Where should the ship be right now?”
When the answer to that question stops matching reality, danger begins to form.
2. What Dead Reckoning Really Is
Dead Reckoning is the process of projecting the ship’s position forward from a known point using course steered and speed through the water.
It assumes:
- the ship went where it was told
- speed remained constant
- no external forces acted
Those assumptions are rarely true — and that is the point.
DR is not a claim of accuracy.
It is a baseline expectation.
3. What an Estimated Position Actually Means
An Estimated Position improves on DR by acknowledging reality.
EP takes the DR position and adjusts it using known or suspected influences, such as current, wind, or steering error.
This is where navigation becomes judgement rather than calculation.
EP openly admits uncertainty.
That honesty makes it safer than pretending precision exists.
4. Why DR Is Never “Wrong”
Dead Reckoning cannot be wrong because it does not claim truth.
It simply states:
“If nothing interfered, this is where we would be.”
When fixes disagree with DR, the DR is not disproved — it is explained.
The difference between DR and fix tells you what the sea has been doing to your ship.
5. How EP Improves on DR
Estimated Position incorporates awareness.
It accepts that:
- current exists
- wind affects the ship
- steering is imperfect
- the sea is not neutral
By applying reasonable corrections, EP often sits closer to the real position than DR.
However, EP is still an estimate — not a fix.
The danger lies in forgetting that distinction.
6. Set, Drift, and Leeway (Conceptual Only)
Set is the direction water pushes the ship.
Drift is the speed of that push.
Leeway is sideways movement caused by wind.
You do not need to calculate these precisely to use DR and EP effectively.
What matters is recognising that the ship is rarely moving exactly where it points.
Ignoring this reality makes DR meaningless.
7. How DR and EP Support Visual Navigation
DR and EP act as expectation generators.
Before taking a fix, you should already have an idea of where the ship ought to be.
If the fix appears far from that expectation, something must be questioned:
- the fix
- the inputs
- the assumptions
Without DR and EP, fixes arrive without context and errors go unnoticed.
8. DR, EP, and Error Growth
DR error grows steadily over time.
EP error grows unevenly, depending on how accurate your assumptions are.
Both remind the navigator that time without fixing increases risk.
This is why fix frequency matters more near land than offshore.
DR and EP do not replace fixing — they demand it.
9. Common Civilian Mistakes
Civilians new to navigation often:
- treat DR as a fix
- believe EP is “close enough”
- forget to advance positions
- stop updating when busy
- rely on electronics instead
The most dangerous mistake is letting DR or EP fade into the background.
When expectation disappears, surprise follows.
10. Professional Use of DR & EP on the Bridge
Professional watchkeepers maintain DR and EP continuously, even when electronics work perfectly.
They use them to:
- anticipate hazards
- detect unexpected set or drift
- verify fixes
- judge fix reliability
- decide when to slow down
When electronics fail, DR and EP are already alive — not being rebuilt under pressure.
Closing Perspective
Dead Reckoning and Estimated Position teach the most important navigation lesson of all:
Navigation is not knowing where you are.
It is knowing where you should be — and noticing when reality disagrees.
Everything else in coastal navigation depends on that awareness.
Tags
dead reckoning · estimated position · coastal navigation · chartwork · bridge watchkeeping · navigation fundamentals