/al"i dayd'/ , n. Survey.
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'/ .
[ 1400-50; var. of alhidade alhidada al-'idadah the turning radius (like a clock hand) of a circle; r. late ME allydatha ( alhidada ) ]