Alidade
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An alidade or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site using
A simple alidade for use with a ceiling projector
An example of an alidade on a circumferentor. Taken from the Table of Surveying, Cyclopaedia, Volume 2, 1728
Traverse plane-table alidade, c. 1898
A U.S. Navy sailor using a telescopic alidade
Osborne Fire Finder
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The Osborne Fire Finder is a type of alidade used by fire lookouts to find a directional bearing to smoke in order to alert fire crews to a wildland fire.
Alidade
…the two sights and adjusts them until they are aligned with the source of the smoke (or an observed lightning strike to be monitored for smoke). See Osborne Fire Finder. Gunsight…
An Osborne Fire Finder device used in fire lookout towers.
Osborne Fire Finder at Sand Mountain Lookout, Oregon
A USFS Fire Lookout using an Osborne Firefinder while on duty at Vetter Mountain, California.