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Prismatic compass versus surveyor's compass

Short answer

Prismatic compass: graduated ring fixed to needle, whole circle bearings (0–360°) read through prism while sighting; fast, hand-held. Surveyor's compass: fixed ring, quadrantal bearings (0–90°) read directly; needs tripod, separate sighting and reading.

What are the differences between a prismatic compass and a surveyor's compass?

Both compasses measure the magnetic bearing of a line with a magnetic needle, but they differ in the reading system, graduation and sighting arrangement.

PointPrismatic compassSurveyor's compass
GraduationGraduated ring (card) fixed to the needle, 0∘0^\circ–360∘360^\circ (whole circle bearing)Ring fixed to the box; graduated in quadrants, 0∘0^\circ–90∘90^\circ (quadrantal bearing)
NeedleNeedle is attached to the ring; the ring swings with the needleNeedle swings freely over the ring; the ring is fixed to the box
ReadingTaken through a prism at the eye slit, so the reading is seen while sightingTaken by looking down directly on the ring; cannot be read while sighting
SightingObject vane with a hair and eye slit with a prism; sighting and reading at the same timeSight vanes (slit and hair); the eye must be moved between sighting and reading
TripodCan be used with or without a tripod (hand-held)Needs a tripod
Graduation figuresEngraved upside down (inverted), so they appear erect when read through the prismEngraved erect and read directly; E and W are interchanged on the ring
Reading accuracyEstimated to about 30′About 15′ to 30′
Speed and useQuick, portable, widely used in rough workSlower and used less now

Main advantage of the prismatic compass: sighting and reading are done together, so there is less chance of disturbing the instrument and the survey is faster.

✓ Answer checked by Claude Opus 5.5 (high effort)

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