error of perpendicularity

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• Theodolite — A theodolite (IPA en|θiːˈɒdəlаɪt) is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a key tool in surveying and engineering work, particularly on inaccessible grounds, but theodolites… …   Wikipedia

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• Setting circles — on an equatorial fork mounted telescope Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemeris …   Wikipedia