The bearing disc is a simple device for measuring horizontal angles. It consists of a disc on which a divided circle (divided into degrees , mil , gon etc.) is applied. In the center of the circle is a rotatable arm ( alidade ) with two perforated or slit diaphragms ( diopter ) at a certain distance.
The bearing disc, usually on a tripod , on a built-in boat compass or on the deck of a ship, is used to determine an angle between two visible objects, taking into account your own location.
The main use of the bearing disc is the side bearing to determine the location of a ship by sighting landmarks and drawing the angle on the nautical chart . You can also z. B. can be used to determine a direction starting from a certain and known place, also another place (in the picture example a forest fire, observed from a watchtower).
Simple bearing disks allow readings with an accuracy of 0.5 to 1 ° (gon, mil). If the alidade is equipped with a vernier , higher resolutions are also possible.