Module 8 - Drawing Instruments
Proprietary Plotters
What you will learn
Use the orientation marks and scales on a suitable course plotter to draw or read a direction, then independently detect alignment and reciprocal errors.
A transparent course plotter can read or draw a direction from the chart grid without walking rulers to a compass rose. Different designs use different wheels, arrows and scales, so the exact model's markings and instructions govern the method.
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.
Worked example
The plan calls for 058°T, but the first plotter reading appears as 238° because the reciprocal scale was read.
- 1Confirm the plotter's north arrow and align its reference grid with a chart meridian using the model's instructions.
- 2Place the plotting edge through the departure point and identify the route towards the destination.
- 3Choose 058°T on the scale that points along that route; 238°T is the same line in the opposite direction.
- 4Draw and label the line, then re-read it from another meridian or the true compass-rose scale.
Sense check: 058°T points generally north-east. If the line arrow points south-west, the reciprocal direction has been selected.
| 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 |
Diagnose a plotter disagreement
Two crew members use the same transparent plotter on one line. One reads 112°T and the other 292°T.
1. Keep the line fixed
Recognise that 112 and 292 degrees are reciprocals, so the physical line may be aligned correctly.
2. Check orientation
Verify the plotter's north arrow and reference grid against the chart meridian, following the exact model's markings.
3. Use the route sense
Compare the start and destination, select the value that points along the intended route and label it T.
4. Read independently
Use a second meridian or the labelled true compass rose to confirm the answer before relying on it.
Sense check: A 180-degree disagreement usually signals direction sense, but the grid alignment and chart point still need checking.
Common mistake or limitation
- Applying one memorised sequence to every branded plotter even though reference marks, wheels and scales differ.
- Aligning a convenient line on the plotter with a chart meridian without confirming that it is the model's intended reference grid.
- Reading the reciprocal scale or reversing the north arrow while the edge still appears to fit the route line.
- Calling a plotter easier in rough conditions or more accurate by design; movement and technique affect every paper-chart instrument.
Recap
- Learn the exact plotter's orientation marks and scale before using it.
- Align the intended reference accurately with the chart grid and resolve the reciprocal sense.
- Re-read the finished line from an independent chart reference.
Optional quick check
Section 3 of 7
A plotter gives 238°T for a planned leg that should run generally north-east. What is the strongest first check?
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.
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Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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