Diagram 5 of 5 · Long-Distance Passages

Navigation Fix Diagram

How to establish your position offshore: DR (dead reckoning), GPS cross-check, and running fix from two bearings — the three pillars of bluewater navigation.

Method 1 & 2
DR Position & GPS Cross-Check
Plot DR from last known fix using COG + SOG × time. Verify against GPS. Difference = current set & drift.
N S W E Fix A 08:00 COG 040° DR Est. 12:00 GPS FIX 12:00 actual Set/Drift SOG: 6.2 kn ±2 nm error Known fix (circle) DR position (dash) GPS fix (filled) Current vector
Method 3
Running Fix — Two Bearings
Take a bearing to a landmark, advance the LOP along your track, then cross with a second bearing. Intersection = running fix.
LIGHTHOUSE Pos. 1 09:00 Bearing 1: 042°T Track: COG 090° Pos. 2 11:00 Adv. LOP 1 Bearing 2: 008°T RUNNING FIX Run: 18 nm @ 9 kn × 2 h LOP 1 (1st bearing) LOP 2 (2nd bearing) Running fix
Combined View
Position Fix Quality Triangle & Navigation Labels
When three LOPs are used simultaneously, they form a triangle (cocked hat). Your position lies within it — closer to the center in calm conditions.
COG From SOG ×T COG / SOG POSITION TYPES DR Dead Reckoning COG+SOG×Time EP Est. Position DR + current FIX Confirmed Fix 2+ LOPs / GPS R.FIX Running Fix Advanced LOP Cocked Hat (3 LOPs) Pos. inside △
COG — Course Over Ground The actual direction of travel relative to Earth's surface, accounting for leeway and current.
SOG — Speed Over Ground Actual speed over Earth's surface. Different from boat speed (STW) due to current set and drift.
DR — Dead Reckoning Estimated position calculated from known fix + COG × SOG × elapsed time. Accumulates error over time.
EP — Estimated Position DR position adjusted for known current set and drift. More accurate than DR alone.
Fix Confirmed position from two or more simultaneous LOPs, GPS, or celestial observations crossing at an angle >30°.
Running Fix Single-landmark fix obtained by advancing an earlier LOP along the vessel's track to cross a later bearing.
LOP — Line of Position A bearing line, circle of equal altitude, or depth contour on which the vessel must be located.
Cocked Hat Triangle formed by three non-intersecting LOPs. True position lies within; assume the most dangerous corner if near hazards.
DR + GPS Cross-Check Procedure
1
Mark last fix Record time, position (lat/lon), and log reading at last confirmed fix.
2
Plot DR track Advance along COG using SOG × elapsed time to get estimated position.
3
Apply current (EP) Adjust DR for known tidal stream or ocean current to get estimated position.
4
Read GPS fix Note actual GPS lat/lon at the same time as your DR plot.
5
Measure error vector The line from DR to GPS fix = unexplained current set and drift. Update passage plan.