Module 6 - Position, Course & Speed
How to Measure Distance on a Nautical Chart
Quick answer
Measure nautical miles with dividers, then transfer the divider span to the nearby latitude scale at the side of the Mercator chart. For practical chartwork, one minute on that scale represents one nautical mile.
- Use the side latitude scale, not the top or bottom longitude scale.
- Read the scale near the same latitude as the route you measured.
- Write the distance in nautical miles so it feeds speed, distance, and time calculations.
Active recall
Section 5 of 6
Explain the main idea from measuring Distance on the Chart in your own words.
Distance on a nautical chart is measured with dividers and the latitude scale at the side of the chart. One minute of latitude equals one nautical mile, so the latitude scale is the only chart scale you should use for distance.
Place one point of the dividers on the start of the leg and the other point on the end. Without changing the divider span, move the dividers to the latitude scale at roughly the same latitude as your route. Count the minutes of latitude to read the distance in nautical miles and tenths.
Worked example: if the divider span covers 4 minutes and 3 tenths on the latitude scale, the leg is 4.3 nautical miles. At 5 knots, that leg takes about 52 minutes because time in minutes equals distance times 60 divided by speed.
Do not use the longitude scale at the top or bottom of the chart. Meridians converge on the Earth, so a minute of longitude is not a nautical mile; on a Mercator chart, read distance from the latitude scale close to the measured route.
Latitude scale
Count minutes of latitude. One minute equals one nautical mile.
- Step
- 1
- Action
- Set dividers between the two points on the chart
- Reason
- This captures the exact chart distance for the leg
- Step
- 2
- Action
- Move the dividers to the side latitude scale
- Reason
- Latitude minutes convert directly to nautical miles
- Step
- 3
- Action
- Use the scale near the same latitude as the route
- Reason
- Mercator chart scale changes slightly north and south
- Step
- 4
- Action
- Read minutes and tenths of latitude
- Reason
- 1 minute equals 1 nautical mile
- Step
- 5
- Action
- Write the distance on the passage plan or chart
- Reason
- It feeds speed, distance, and time calculations
| Step | Action | Reason |
|---|---|---|
| 1 | Set dividers between the two points on the chart | This captures the exact chart distance for the leg |
| 2 | Move the dividers to the side latitude scale | Latitude minutes convert directly to nautical miles |
| 3 | Use the scale near the same latitude as the route | Mercator chart scale changes slightly north and south |
| 4 | Read minutes and tenths of latitude | 1 minute equals 1 nautical mile |
| 5 | Write the distance on the passage plan or chart | It feeds speed, distance, and time calculations |
Apply it
Worked chart-distance check
Your divider span reaches 6 minutes and 4 tenths on the nearby latitude scale.
- 1
Keep the span
Lift the dividers from the route without changing their opening.
- 2
Use latitude
Move them to the side latitude scale at roughly the same latitude as the route.
- 3
Read and label
Six minutes plus four tenths gives 6.4 nautical miles; record 6.4 M.
Sense check: A longitude scale is not a nautical-mile ruler. If you used the top or bottom border, repeat the measurement on latitude.
Common mistakes
- Using the longitude scale because it is closer to the route.
- Changing the divider span before reading the scale.
- Reading whole minutes only and ignoring tenths of a nautical mile.
Quick practice check
Try a few questions before you move into the full module.
1. After setting dividers between two charted points, what should you do next?
2. A divider span reads 3.6 minutes on the latitude scale. What distance is that?
3. Why read the latitude scale near the route you measured?
Key points
- ALWAYS use the LATITUDE scale (side of chart) for distance
- 1 minute of latitude = 1 nautical mile
- Use the latitude scale at the same latitude as your measurement
- Never use the longitude scale for distance
Common questions
How do you measure nautical miles on a chart?
Use dividers to span the distance, then transfer the dividers to the latitude scale at the side of the chart. One minute of latitude equals one nautical mile.
Which chart scale do you use to measure distance?
Use the latitude scale on the side of the chart, ideally close to the latitude of the route you measured. Do not use the longitude scale at the top or bottom.
How do you measure distance if the route is not a straight line?
Break the route into straight legs, measure each leg with dividers, read each span on the latitude scale, then add the nautical miles together.
Why should you not use the longitude scale for distance?
Longitude spacing varies with latitude on Mercator charts, so it does not provide a constant nautical-mile scale.
Keep revising this topic
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.