Module 8 - Drawing Instruments
Essential Plotting Instruments
What you will learn
Choose the right paper-chart tool for direction, distance or position work, prepare it for accurate use, and leave a light, labelled plot that another navigator can check.
Plotting tools do different jobs. A direction tool transfers or reads an angle, dividers transfer a distance or coordinate interval, and a sharp erasable pencil records the result. Accuracy comes from the whole method, not from owning a particular brand of instrument.
For Coastal Skipper chartwork, use parallel rulers or a suitable course plotter for directions, dividers for distances and coordinate intervals, and a sharp erasable pencil for the plot. The RYA recommends plotting instruments for its shorebased navigation courses, but the exact model is less important than understanding its scale, orientation marks and instructions.
Prepare a clean, stable chart table and inspect the chart before plotting. Confirm the chart edition or training-chart task, the units, the latitude and longitude graduations, the compass-rose labels and any printed instructions. Check that hinged rulers move without obvious play, the plotter is undamaged, the dividers hold their span and the pencil makes a narrow, readable line without heavy pressure.
A neat plot is auditable. Use the symbol, arrow, time and reference suffix required by the exercise or vessel procedure, such as T, M or C for a direction. Keep construction lines light and read the finished result back independently. The instrument only transfers the angle or span you set; it cannot detect a wrong input, wrong scale, 180-degree reciprocal error or mistaken chart point.
| Task | Suitable tool | Independent check |
|---|---|---|
| Transfer or read direction | Parallel rulers or a suitable course plotter | Re-read the line and confirm the 180-degree sense and T/M/C reference |
| Measure route distance | Dividers and the nearby latitude scale or a suitable labelled graphic scale | Repeat the transfer and check the unit and route midpoint |
| Plot a coordinate | Dividers or a drafting compass with the chart's latitude and longitude scales | Read the plotted point back from both scales |
| Record the result | Sharp erasable pencil where the adopted procedure calls for it | Check symbol, time, label, line width and chart point |
Prepare one short paper-chart leg
You need to draw a planned course through a waypoint, measure the next two route legs and leave enough information for the skipper to check the work before departure.
1. Match tool to task
Use a direction instrument for the course, dividers for each route span, and the chart's own labelled scales for the readings.
2. Set up the surface
Flatten and protect the chart, check the graduations and labels, and make sure the tools hold their angle or span.
3. Plot lightly and label
Draw only the line needed, add its intended direction and reference suffix, and record the relevant waypoint or time using the adopted procedure.
4. Read it back
Re-measure the course, distance and endpoints without relying on the first reading. Challenge any result that does not fit the charted geometry.
Sense check: A polished line can still be wrong. The check is whether a second reading gives the intended point, direction, reference and plausible distance.
Common mistake or limitation
- Treating the kit list as the method: a premium plotter does not prevent a wrong scale, wrong point or reciprocal-direction error.
- Pressing hard or using a blunt point, which creates a wide line and makes the intended position or bearing ambiguous.
- Writing an unlabelled number such as 064 without showing whether it is true, magnetic or compass and which way along the line is intended.
- Erasing or altering an operational plot without following the vessel's record-keeping procedure; erasable training work is not a universal instruction to remove navigational records.
Recap
- Match the instrument to direction, distance, coordinate or recording work.
- Inspect the chart and the tool before trusting a reading.
- Make light, labelled marks and independently read the result back.
Optional quick check
Section 1 of 7
A plotted line is labelled only '064'. What is the most important correction before another navigator uses it?
Common questions
Which plotting instruments do I need for Coastal Skipper revision?
You need a reliable way to transfer directions, usually parallel rulers or a suitable course plotter, plus dividers and a sharp erasable pencil. Follow the training provider's equipment list and learn the exact instrument's markings.
Must every mark on every working chart be erased later?
No universal rule covers every chart and operation. Keep training construction lines light and erasable, but follow the vessel's chart-correction, passage-record and retention procedures for operational records.
Does a plotting instrument make a result accurate?
No. It transfers the setting you give it. Accuracy still depends on the chart point, scale, alignment, line width, reference, reciprocal sense and independent check.
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Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Drawing Instruments
This public lesson is part of Revision Module 8. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.