How two marks become one truth — and why professionals trust them
Contents
Use the links below to jump to any section:
- What a Range or Leading Line Really Is
- Why Ranges Are So Powerful
- Types of Leading Lines
- How to Identify a Range on the Chart
- Using a Range to Steer a Safe Track
- Ranges as Lines of Position
- Combining Ranges with Bearings and Depth
- Accuracy, Limits, and Common Illusions
- When Ranges Lie or Disappear
- Professional Use of Ranges on the Bridge
1. What a Range or Leading Line Really Is
A range, also called a leading line, is created when two fixed objects are aligned so that, when viewed in line, they define a single straight line on the water.
Operationally, it answers one question with absolute clarity:
“Is the ship on this line, or not?”
Unlike bearings, ranges do not produce angles.
Unlike fixes, they do not produce points.
They produce certainty.
2. Why Ranges Are So Powerful
Ranges are powerful because they remove judgement.
If the marks are in line, the ship is on the line.
If they are not, it is not.
There is no interpretation, no correction, no calculation.
This is why ranges are heavily used in:
- harbour approaches
- dredged channels
- narrow passages
- pilotage waters
They are navigation reduced to a binary state — correct or incorrect.
3. Types of Leading Lines
Leading lines may be created by:
- two charted leading marks
- a lighthouse and a daymark
- a tower and a hill
- a beacon and a conspicuous structure
Some are purpose-built. Others are incidental but equally valid if charted.
What matters is not what the objects are, but that they are:
- fixed
- charted
- clearly identifiable
If one mark moves or is misidentified, the range collapses.
4. How to Identify a Range on the Chart
Charts depict ranges as a straight line passing through two symbols, often annotated with bearings or notes.
This line represents a safe track, not a suggestion.
It may indicate:
- the centreline of a dredged channel
- a clearance line between dangers
- a recommended approach path
Ignoring the charted range while navigating visually defeats its purpose.
5. Using a Range to Steer a Safe Track
When steering by a range, the helmsman keeps the rear mark directly behind the front mark.
Any lateral movement causes the marks to separate.
This makes ranges ideal for:
- steering through narrow channels
- compensating visually for set and drift
- maintaining track without constant correction
Ranges convert lateral error into visible displacement, which humans detect extremely well.
6. Ranges as Lines of Position
A range is also a line of position.
If the ship is on the range, its position lies somewhere along that line.
This means a range can be combined with:
- a bearing
- a depth contour
- a radar range
to produce a fix.
This combination is extremely robust and widely used in pilotage.
7. Combining Ranges with Bearings and Depth
Professional navigators rarely rely on a range alone.
They cross-check it against:
- expected depth
- compass heading
- visual bearings to other objects
If the range says the ship is on track but depth or bearing disagrees, something is wrong.
Ranges provide confidence — they do not remove the need for awareness.
8. Accuracy, Limits, and Common Illusions
Ranges are extremely accurate laterally, but they provide no information about position along the line.
They also have limitations:
- background clutter can hide one mark
- sun glare can obscure alignment
- haze can compress perspective
- incorrect mark identification can mislead
A range that looks correct but feels wrong should be questioned immediately.
9. When Ranges Lie or Disappear
Ranges fail when:
- one mark is extinguished or damaged
- construction changes the background
- temporary structures mimic alignment
- the observer is not where they think they are
This is why ranges must always be backed up by another form of position information.
Blind faith in a single aid is not seamanship.
10. Professional Use of Ranges on the Bridge
On a professional bridge, ranges are used to:
- reduce workload in confined waters
- provide instant lateral error detection
- verify electronic track control
- support pilotage decisions
They are favoured because they are honest. They do not smooth data or average error.
They simply show reality.
Closing Perspective
Ranges and leading lines represent the purest form of terrestrial navigation.
They replace calculation with observation and turn navigation into continuous verification rather than periodic fixing.
This is why, even on the most modern bridges, experienced navigators still seek them out.
Tags
ranges · leading lines · coastal navigation · pilotage · chartwork · bridge watchkeeping · visual steering