nullified. Surveyor's double prism A double prism is a device to measure right angles, consisting of two five sided prisms stacked on top of each other and a plumb-bob. It is used to stake out right angles, for example on a construction site. See also Graphometer Alidade References "Circumferentor". Oxford English Dictionary. Oxford University Press. 2nd edition. 1989. Surveying instruments Angle measuring instruments