Definition
A single observation of a bearing, transit, range, depth contour or celestial altitude constrains the vessel to a locus rather than a point. Compass bearings give straight lines through the charted object; distance measurements by radar or vertical sextant angle give circular arcs centred on the object; celestial sights give arcs of position drawn as short straight lines near the estimated position. It is drawn as a single arrowhead line labelled with time and, where relevant, the observed value in degrees true.
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