Module 8 - Drawing Instruments
Dividers
What you will learn
Transfer a route span without changing it, read distance from the appropriate nearby chart scale, combine multiple legs and check the result against time and chart geometry.
Dividers do not contain a distance scale. They preserve the separation between two chart points so that the same span can be read on the nearby latitude scale, or on a suitable graphic scale when the chart explicitly provides one.
Open the dividers so the tips just meet the route endpoints, then lift and transfer the unchanged span to the latitude scale near the route's middle latitude. In practical Mercator chartwork, one minute on that nearby scale is read as one nautical mile. The international nautical mile is separately defined as exactly 1,852 metres; the chartwork convention is not a claim that every geodetic minute has exactly that physical length.
Read the chart's graduations and units before counting divisions. A longitude minute is not a distance scale because its ground length changes with latitude. Some charts provide a labelled graphic scale that can be used when suitable for the chart and task, so 'latitude always' is too absolute; follow the chart's instructions and use the scale intended for that measurement.
For a longer or bent route, measure each straight leg or set a stable known divider span and walk it along the planned track. Count complete spans, close the dividers onto the final remainder and measure that remainder separately. Keep the tips light on the paper: pressing points into a chart can damage it and does not make a moving chart table safe.
Latitude scale
For this chartwork measure, read 1′ on the nearby latitude scale as 1 M.
Transfer the route span unchanged, then read it on the scale intended for distance near the route's latitude.
Worked example
A two-leg training-chart route measures 3.7 M and 2.6 M. Planned speed through the water for the simple timing check is 4.5 kn.
- 1Measure each straight leg and transfer each span to the nearby latitude scale: 3.7 M and 2.6 M.
- 2Add the route legs: 3.7 + 2.6 = 6.3 M.
- 3Calculate time: 6.3 M ÷ 4.5 kn = 1.4 hours.
- 4Convert decimal hours: 1.4 × 60 = 84 minutes, or 1 hour 24 minutes.
Sense check: At 4.5 kn, the vessel covers about 4.5 M in one hour, so 6.3 M taking a little under an hour and a half is plausible. This timing check does not include tide, leeway or manoeuvring.
| Measurement | Method | Boundary |
|---|---|---|
| Short straight span | Transfer the unchanged span to the nearby latitude scale | Check graduations and unit before reading |
| Long straight span | Walk a stable known distance and measure the remainder | Do not exceed a controllable divider opening |
| Bent route | Measure the component legs and add them | Do not replace the route with a departure-to-destination shortcut |
| Labelled graphic scale | Use it when the chart provides it for the task and units required | Do not assume every chart has or intends the same scale |
Measure without forcing the answer
The direct divider span appears to be 6.3 M, but a crew member reads 8 M from a remote part of the latitude scale and presses the tips into the chart to stop them slipping.
1. Protect the chart
Use a stable working surface and light contact; do not make dimples or punctures to compensate for movement.
2. Preserve the span
Set the route endpoints once and lift the dividers without changing their opening.
3. Use the local scale
Transfer the span to the latitude scale nearest the route's middle latitude, or to a suitable labelled graphic scale where the chart provides one.
4. Repeat and sense-check
Measure again independently and compare the distance with chart geometry, planned speed and expected passage time.
Sense check: A large disagreement between two scale readings is evidence to repeat the transfer and check the chosen scale, not to average the answers.
Common mistake or limitation
- Using the longitude scale as though a minute of longitude were one nautical mile at every latitude.
- Reading from a distant part of a Mercator chart instead of the latitude scale near the route's middle latitude.
- Changing the divider opening during transfer, missing the final remainder or cutting across bends in the planned track.
- Pressing the points into the chart; a light dimple is still damage and does not control the hazards of a moving boat.
Recap
- Keep the divider span unchanged between the route and the chosen chart scale.
- Use the nearby latitude scale for practical Mercator distance work unless a suitable chart instruction or graphic scale applies.
- Measure remainders, protect the chart and check distance against route geometry and time.
Optional quick check
Section 4 of 5
A two-leg route measures 3.7 M and 2.6 M. At 4.5 kn, what simple no-tide timing check should you expect?
Common questions
Why is distance measured on the latitude scale?
For practical Mercator chartwork, the nearby latitude graduation supplies the nautical-mile scale at that latitude. Longitude spacing changes with latitude and is not used as a nautical-mile scale.
Can I use a graphic scale instead?
Yes when the chart provides a suitable labelled graphic scale for the task and unit. Read the chart's instructions rather than applying a universal rule from another chart.
Should divider points be pressed into a paper chart?
No. Use light contact on a stable surface. Pressing or dimpling damages the chart and does not make a moving workspace safe.
Keep revising this topic
Sources and factual review
Reviewed 20 August 2026 - Coastal Skipper Revision.
Continue studying Drawing Instruments
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