Fiducial edge

(Astron. & Surv.) the straight edge of the alidade or ruler along which a straight line is to be drawn.

See also: Fiducial

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
The Q-R-S fiducial edges of the thoracic MQRS and the invasive FQRS signals were obtained for each record.