(1) A pointer, called an alidade , was pivoted at the centre of the disk.(2) Telescopic alidades came into use in Europe in the early 1800s, and were soon introduced to American practice.(3) A wide variety of alidades and plane tables are available, both in manual and electronic versions.(4) Both the alidades and the straight edges of the quadrant are fitted with viewers.(5) In the circle an index on the alidades gives the angle that they form with the vertical diameter.
