Module 6 - Position, Course & Speed
How to Measure Distance on a Nautical Chart
Place one divider point at each end of a straight chart leg. Lift the dividers without changing their span and transfer them to the side latitude scale near the same latitude as the route. In practical Mercator chartwork, each minute on that nearby scale represents one nautical mile.
Use the nearby part of the scale because Mercator projection scale increases with latitude. The chart is a flat representation of a curved surface; transferring a long span to a distant latitude introduces avoidable error. If the span is larger than the dividers or the route bends, divide it into controlled straight legs and add the distances.
Do not use the longitude border as the nautical-mile scale. Meridians converge on the Earth, and the spacing of longitude is not the constant distance relationship used for this method. Some charts also provide a labelled graphic scale; use it only for the units and chart scale it explicitly states.
Distance becomes useful when it informs a decision. Record the measured value, calculate an estimated leg time with a compatible speed, and compare the result with tidal, daylight, depth, traffic and crew limits rather than treating the measurement as an isolated exercise.
Latitude scale
For this chartwork measure, read 1′ on the nearby latitude scale as 1 M.
Keep the divider span unchanged and use the latitude scale near the route; the top and bottom longitude borders are not the nautical-mile ruler.
| Step | Action | Error check |
|---|---|---|
| 1 | Set dividers between the route points | Confirm the intended leg, not a shortcut across land or danger |
| 2 | Lift without changing the span | Re-seat the hinge if the opening moved |
| 3 | Transfer to nearby latitude | Do not use the longitude border |
| 4 | Read minutes and tenths as M | Estimate whether the answer fits the visible chart scale |
| 5 | Record each leg and calculate time | Use a compatible and realistic speed assumption |
Optional quick check
Section 5 of 6
A divider span reads 3.6 minutes on the nearby latitude scale. What should you record?
Keep revising this topic
Sources and factual 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.