alidade

[al-i-deyd]
noun Surveying.
1.
(in plane-tabling) a straightedge having a telescopic sight or other means of sighting parallel to the straightedge.
2.
the entire upper part of a theodolite or transit, including the telescope, its supports, the level vials, the circle-reading device, and the spindle.
Also, alidad [al-i-dad] .


Origin:
1400–50; variant of alhidade < Medieval Latin alhidada < Arabic al-ʿiḍādah the turning radius (like a clock hand) of a circle; replacing late Middle English allydatha (< Old Spanish alhidada)

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World English Dictionary
alidade or alidad (ˈælɪˌdeɪd, ˈælɪˌdæd)
 
n
1.  a surveying instrument used in plane-tabling for drawing lines of sight on a distant object and taking angular measurements
2.  the upper rotatable part of a theodolite, including the telescope and its attachments
 
[C15: from French, from Medieval Latin allidada, from Arabic al-'idāda the revolving radius of a circle]
 
alidad or alidad
 
n
 
[C15: from French, from Medieval Latin allidada, from Arabic al-'idāda the revolving radius of a circle]

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Example sentences
Most of the actual mapping, however, was done by planetable and telescopic alidade.
When carried horizontally while on the tripod, the alidade's weight is an excessive load for the hollow centerpiece to bear.
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