Douglas Parallel Rule/Protractor

The American-style navigation plotter/protractor is suitable for lower latitudes where the meridians of longitude are more parallel to each other. In northern latitudes, where the meridians slope toward each other, these plotters become more difficult to use due to visual illusions (it's harder to line the plotter up with the meridians). In northern latitudes, VFR aviators tend to prefer the parallel-rule plotter (the "Douglas protractor") for measuring tracks.

ATP's Douglas Protractor is able to accurately measure a track on any chart because it can be centered easily between converging meridians. The grid of parallel lines allows any track to be offset from another track, and the graduated scale along the edge of the protractor is divided into 1:500,000 scale nautical miles for use when scaling distances on the chart. Only ATP's precision Douglas Protractor offers these features. Other Douglas-type protractors omit the chart scale. ATP's scale is made from space-age unbreakable Lexan.