alidade
American Heritage Dictionary:
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al·i·dade
(ăl'ĭ-dād') 
also al·i·dad (-dăd')
n.
also al·i·dad (-dăd')n.
- An indicator or a sighting apparatus on a plane table, used in angular measurement.
- A topographic surveying and mapping instrument used for determining directions, consisting of a telescope and attached parts.
[French, from Medieval Latin alidada, sighting rod, from Arabic al-'iḍāda, the revolving radius of a circle : al-, the + 'iḍāda, revolving radius (alternate form of 'aḍud, humerus , from 'aḍada, to lop, cut (trees)).]
The part of a surveying instrument which consists of a sighting device, with index, and reading or recording accessories.
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At its most simple, a flat wooden or metal strip with a scaled straight edge on one side and sighting mounts at each end so that it can be aligned on specific target points when used in conjunction with a plane table. More sophisticated alidades are equipped with telescopic sights and a series of levels and scales to measure vertical angles. See plane table.
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American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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McGraw-Hill Dictionary of Architecture & Construction
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc. All rights reserved.
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