The Alhidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down.
In a sextant the alhidade is the turnable arm carrying a mirror and an index to a graduated circle.
The alhidade or alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down.
In a sextant the alhidade is the turnable arm carrying a mirror and an index to a graduated circle.
Alidade tables have also long been used in fire towers for sighting the bearing to a forest fire.
The history of theodolites goes back to so-called plane table alhidades, devices allowing the graphical mapping of the terrain.
These devices consisted of a plane table and a telescope mounted in a fork-like contraption or alhidade[?], allowing it to be aimed out of the horizontal plane.
The whole assembly rested on a plane table, onto which graphing paper was attached; a ruler connected to the alhidade in such a way as to be always pointing in the same horizontal direction as the telescope, was then used to plot the direction to the target.
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