Module 6 - Position, Course & Speed

Measuring Distance on the Chart

What you will learn

Measure a straight or multi-leg Mercator route with dividers, use the nearby latitude scale and turn the result into a checked leg-time decision.

Preserve the divider span from route to nearby latitude scale. The method is simple enough to perform and rich enough to expose route, scale, unit and timing mistakes.

Place the divider points at the ends of one straight chart leg. Lift without changing the span and transfer it to the side latitude scale near the route. For practical Mercator chartwork, minutes and tenths on that nearby scale read as nautical miles and tenths.

Use the nearby latitude because Mercator projection scale changes with latitude. Split a bent route into its intended straight legs instead of replacing it with one chord. If a span is too wide for the dividers, step off controlled sub-spans and record the working.

Do not use the longitude border as the nautical-mile ruler. A labelled graphic scale may be valid for the units and chart it states, but it should not be assumed to transfer to another chart or zoom level.

Distance supports a decision only after it is connected to a compatible speed, an ETA, a monitoring point and relevant hazards. Preserve individual leg distances where a waypoint changes course, evidence or contingency.

Divider span

Measure the route first. Do not change the dividers.

Latitude scale

50°24′

23′

22′

21′

20′

For this chartwork measure, read 1′ on the nearby latitude scale as 1 M.

Preserve the divider span and use the latitude scale close to the route; longitude is a coordinate border, not the nautical-mile ruler.

Worked example

Two safe route legs measure 4.8 M and 3.6 M on the nearby latitude scale. Conservative SOG is 5.6 kn.

1Total route distance = 4.8 + 3.6 = 8.4 M.
2Time = 8.4 ÷ 5.6 = 1.5 hours.
31.5 hours = 90 minutes.
4Retain the 4.8 M waypoint because the course, evidence and hazard picture change there; monitor whether 5.6 kn SOG is achieved.

Sense check: At 5.6 kn, the vessel travels 5.6 M in one hour and 2.8 M in half an hour, totalling 8.4 M.

Measuring Distance on the Chart
Step	Action	Error check
1	Select one safe straight leg	Reject a shortcut over land or danger
2	Set and preserve divider span	Recheck the hinge before reading
3	Transfer to nearby latitude	Do not use longitude
4	Read M and tenths	Compare with chart-scale estimate
5	Calculate time and monitor	Use a compatible ground or water speed

Find the shortcut hidden in a route total

A crew member measures one straight line from start to destination as 7.2 M, while the planned safe route bends around a shoal.

1. Re-measure the plan
Measure each safe leg rather than the chord across the danger.
2. Preserve waypoints
Record each leg, course change and monitoring cue separately.
3. Recalculate time
Use the longer safe-route total and the applicable ground-speed assumption.

Sense check: A shorter arithmetic answer is not an improvement when it describes a route the vessel cannot safely follow.

Common mistake or limitation

Changing the divider opening while transferring the span.
Reading the longitude border or a distant latitude scale.
Measuring a bent route as one chord and understating both distance and time.

Recap

Measure the route actually planned.
Use the nearby Mercator latitude scale.
Keep waypoints and leg distances visible.
Connect distance to a compatible speed and a review point.

Optional quick check

Section 6 of 8

A divider span reads 3.6 minutes on the nearby latitude scale. What is the route distance?

Sources and factual review

Reviewed 20 August 2026 - Compass Revision editorial review.

Practice this in context

Chart distance is only one part of the navigation workflow. The full module connects it to bearings, speed, time, and chartwork practice.

Practice the full module