A Breton plotter (French: Rapporteur Breton), also known as a Portland course plotter or Weems protractor named after later manufacturers producing similar devices, is a navigational instrument used for nautical navigation with charts. It is a ruler with a rotating rose that enables to plot a course on charts, while aligning the North of the ruler with the North of the chart.

Breton plotter

The Breton plotter was invented by Captain Yvonnick Gueret, a Breton seaman who developed the plotter during his experiences teaching navigation with a Cras plotter [fr] (Règle Cras) by Jean Émile Paul Cras. Gueret was a commander in merchant vessels, served on small fishing boats, delivered private yachts and taught navigation. He later extended the use of the plotter to air navigation for small aircraft.