Module 8 - Drawing Instruments
Proprietary Plotters
Course plotters combine a straight plotting edge with a protractor scale and parallel reference lines. The USCG describes aligning a plotter reference or bull's-eye with a chart meridian so a course or bearing can be read or drawn. RYA's own Handy Plotter is recommended for shorebased navigation students and marks compass direction on its plotting wheel.
Start by identifying the model's north arrow, reference grid, centre or bull's-eye, direction scales and straight edge. Align the reference exactly with a chart meridian or other direction permitted by the maker, keep the orientation arrow pointing the correct way, place the edge through the required point, then read or set the intended three-figure direction. If the plotter presents reciprocal values, resolve the route's sense before drawing.
A plotter is not inherently more accurate than parallel rulers and is not automatically reliable on a moving chart table. A skewed grid, wrong scale, reversed north arrow, thick line or displaced centre still gives a wrong result. Secure the chart, use controlled pressure and verify the completed line from a second chart reference or compass rose.
| Plotter cue | Question to answer | Failure it catches |
|---|---|---|
| North arrow or compass direction | Does it agree with chart north? | Reversed orientation |
| Reference grid or bull's-eye | Is the correct mark aligned with a meridian? | Skewed direction |
| Two reciprocal scales | Which value points from start to destination? | 180-degree error |
| Straight edge and centre | Does the edge pass through the exact chart point? | Parallel displacement |
Common questions
What is a course plotter?
It is a transparent direction instrument combining a straight edge, angular scale and reference marks that align with the chart grid. It can replace parallel rulers for many plotting tasks.
Are Breton, Portland and other plotters used identically?
No. They share direction-transfer principles, but their arrows, rotating parts, reference grids and scales differ. Use the instructions and markings for the exact model.
Is a course plotter more accurate than parallel rulers?
Not inherently. Either can support accurate work when correctly aligned and checked; either can produce a wrong result through poor set-up, movement, line width or reciprocal error.
Optional quick check
Section 3 of 4
A plotter gives 238°T for a planned leg that should run generally north-east. What is the strongest first check?
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Sources and factual review
Reviewed 20 August 2026 - Day Skipper Revision.
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