Chart tools
How to Use a Breton Plotter — Measuring and Plotting Courses
Published · Last checked October 2026
A Breton plotter is a clear rectangular ruler with a rotating compass rose and grid. To measure a course, lay its edge along the line with the arrow pointing the way you are going, turn the rose until its grid lines up with the chart's meridians and North points up, then read the true direction at the index mark. To plot, set the bearing first, then slide the plotter until the grid lines up and draw along the edge.
The parts of a Breton plotter
- Base plate with a straight edge, scale rulers and a direction-of-travel arrow.
- Rotating rose marked 0 to 360°, with a square grid printed on it.
- Index mark (the 'lubber line') where you read the bearing.
- Variation scales either side of the index, so you can read or set a magnetic bearing directly.
Measuring a course or bearing
- Lay one edge of the plotter along the line on the chart.
- Make sure the arrow on the base points in the direction of travel, not the reverse.
- Turn the rose until its grid lines are parallel with a meridian or parallel of latitude on the chart, and the N on the rose points to the top of the chart (north).
- Read the true bearing at the index mark.
- To get magnetic, read against the variation scale instead: for westerly variation read the figure on the west side, which gives a larger number.
Plotting a bearing
Do the same in reverse. Set the rose to the bearing you want (true at the index, or magnetic against the variation scale). Then place the edge on your starting point or landmark and pivot the plotter until the grid lines up with the chart's grid. Draw along the edge.
| Mistake | Result | Fix |
|---|---|---|
| Arrow pointing the wrong way | Bearing 180° out | Sanity-check against the chart's compass rose |
| Rose grid lined up with a chart border that is not north-up | Error of several degrees | Use meridians and parallels, not the paper edge |
| Variation applied the wrong way | Error of twice the variation | Remember 'West is best, East is least' |
| Rose 'N' pointing south | Bearing 180° out | N on the rose always to the top of the chart |
Breton plotter or parallel rule?
A parallel rule walks across the chart to the compass rose, which is awkward on a small chart table in a seaway. A Breton plotter works anywhere on the chart, which is why most RYA courses use one. Practise both in the course plotting lesson and the compass and charts lesson.