Module 8 - Drawing Instruments
Dividers
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.
| 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 |
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.
Optional quick check
Section 4 of 4
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?
Keep revising this topic
Sources and factual review
Reviewed 20 August 2026 - Day Skipper Revision.
Continue studying Drawing Instruments
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