Dead Reckoning — What It Means in Navigation | Maritime Encyclopedia

Dead Reckoning: What It Means

Dead reckoning (DR) is the method of estimating a ship’s present position by advancing from a last known position using three inputs:

Course steered (compass heading, corrected for variation and deviation)
Speed through water (from log)
Time elapsed since last known position

The result is plotted on the chart as a DR position — marked with a dot inside a circle.

How Dead Reckoning Works — Step by Step

Step 1: Mark your last confirmed position (fix) on the chart
Step 2: Draw a line in the direction of your course steered
Step 3: Calculate distance = Speed × Time
Step 4: Mark the DR position along that line at the calculated distance
Step 5: Note the time at the DR position

Example: Ship at position A, course 090°T, speed 12 knots. After 2 hours: DR position is 24 nautical miles east of A.

Why DR Is Still Essential in the GPS Era

GPS and ECDIS have made navigation far more accurate. Dead reckoning is still critical because:

GPS can fail — satellite issues, jamming, spoofing
GPS can be wrong — errors in datum, chart datums don’t match
Regulations require it — STCW competency assessments test DR ability
Cross-check tool — a navigator who understands DR spots GPS errors immediately
Bridge Team Management — DR is part of passage planning and watchkeeping discipline

DR vs EP (Estimated Position)

	DR Position	EP (Estimated Position)
Inputs	Course + Speed + Time only	Course + Speed + Time + Current + Leeway
Accuracy	Less accurate	More accurate
Symbol on chart	Dot in circle	Dot in triangle
When used	Quick estimate	When current data is available

EP is a more refined DR — it accounts for tidal streams, ocean currents, and wind leeway. Navigators should aim for EP when crossing tidal areas.

Dead Reckoning in Exams

For IMU-CET and MEO Class 2/1 orals, expect questions like:

“How would you maintain the DR plot during restricted visibility?”
“Your GPS fails — what is your procedure?”
“What is the difference between a DR position and an EP?”

The answer always starts with: maintain a current DR, take all available cross-bearings (radar ranges, soundings, visual bearings), and treat the DR as your fallback until a new fix is obtained.

Need help with navigation problems or exam preparation? Chat with SailorGPT — India’s AI mentor for seafarers.

Frequently Asked Questions

What does dead reckoning mean in navigation?

Dead reckoning means estimating your current position by calculating forward from a last known position using course steered, speed through water, and elapsed time. The result is a DR position — less accurate than a fixed position but always available regardless of GPS or visibility.

Why is it called dead reckoning?

The term likely derives from 'ded. reckoning' (deduced reckoning) — deducing position mathematically rather than observing it directly. Some historians connect it to the phrase 'by dead reckoning' meaning navigation by deduction alone.

When is dead reckoning used on ships today?

Even with GPS, DR is maintained as a continuous practice: during GPS outages, in restricted areas where GPS jamming is suspected, as a cross-check against GPS, and in exam settings where every officer must demonstrate the skill.

What are the errors in dead reckoning?

DR accumulates error over time. Main error sources: current and leeway (pushing ship off course), speed log inaccuracy, compass error, and helmsman error. The longer since the last fix, the greater the DR uncertainty — which is why regular position fixes are essential.