Last reviewed 16 Sept 2026 · 10 min read
Compass surveying
Compass surveying is a method of traversing in which directions (bearings) of survey lines are measured with a magnetic compass and lengths with a chain or tape.
Suitability: large areas where details are few, rough or preliminary surveys, wooded or crowded areas where triangulation by chain is impossible, and where speed is more important than high accuracy. Unsuitable near magnetic disturbances (iron structures, power lines, ore bodies).
Traverse — a series of connected survey lines whose lengths and directions are measured:
- Closed traverse — starts and ends at the same point, or at points of known position (provides a check).
- Open traverse — starts and ends at different unknown points (roads, rivers, pipelines); no inherent check.
Meridians and bearings
Bearing of a line is its horizontal angle with respect to a reference direction called the meridian.
| Meridian | Definition |
|---|---|
| True (geographic) meridian | Line through the true north and south poles; fixed; found by astronomical observations |
| Magnetic meridian | Direction indicated by a freely suspended, balanced magnetic needle; changes with time and place |
| Grid meridian | Parallel to the central meridian of a map projection (used for national grid systems) |
| Arbitrary meridian | Any convenient direction (e.g. to a permanent object) — for small local surveys |
Bearings accordingly are true, magnetic, grid or arbitrary bearings.
Systems of bearings
| Whole circle bearing (WCB) | Quadrantal bearing (QB) / reduced bearing (RB) |
|---|---|
| Measured clockwise from north from 0° to 360° | Measured from north or south (whichever is nearer), towards east or west, from 0° to 90° |
| E.g. 45°, 135°, 225°, 315° | E.g. N 45° E, S 45° E, S 45° W, N 45° W |
| Used in the prismatic compass | Used in the surveyor's compass |
| WCB | Quadrant | QB (RB) |
|---|---|---|
| 0°–90° | NE | N θ E |
| 90°–180° | SE | S (180° − θ) E |
| 180°–270° | SW | S (θ − 180°) W |
| 270°–360° | NW | N (360° − θ) W |
Fore bearing and back bearing
- Fore bearing (FB) of a line — bearing measured in the direction of progress of the survey (e.g. AB measured at A towards B).
- Back bearing (BB) — bearing in the opposite direction (BA measured at B towards A).
(+ if FB < 180°, − if FB > 180°.) In the QB system, the back bearing has the same numerical value with opposite letters (N ↔ S, E ↔ W): e.g. FB = N 30° E → BB = S 30° W.
Included angles from bearings
The angle between two lines meeting at a point is the difference of their bearings measured from that point:
(add 360° if negative). For a closed traverse of sides, the sum of interior angles and of exterior angles .
Computing bearings from included angles: bearing of the next line = bearing of the previous line + included angle ± 180° (for clockwise angles), adjusting by 360° as necessary.
Compasses
Prismatic compass
- A graduated aluminium ring attached to a broad magnetic needle rotates with it; the ring is graduated in WCB with 0° at the south end (so that the reading through the prism gives the bearing of the line sighted), figures written inverted.
- Sighting is done through the prism slit and the object vane (with a horse hair); the reading is taken simultaneously through the prism.
- Can be used on a tripod or held in hand.
Surveyor's compass
- The graduated ring is attached to the box (does not rotate); the needle moves freely over it.
- Graduated in quadrantal bearings with E and W interchanged (so that the reading at the north end of the needle gives the correct quadrant).
- Sighting and reading are not simultaneous; reading taken directly by looking at the needle from the top; must be used on a tripod.
| Feature | Prismatic compass | Surveyor's compass |
|---|---|---|
| Needle | Broad (edge bar) type, rotates with the graduated ring | Thin needle, rotates over fixed ring |
| Graduated ring | Attached to the needle; rotates | Attached to the box; fixed |
| Graduations | WCB, 0° at south end, figures inverted | QB, 0° at N and S, E and W interchanged |
| Sighting | Prism slit and object vane with hair | Eye vane with fine slit and object vane |
| Reading | Through the prism — simultaneous with sighting | Directly from the top — after sighting |
| Tripod | Optional (hand-held possible) | Essential |
Magnetic declination and dip
Declination
Magnetic declination is the horizontal angle between the true meridian and the magnetic meridian at a place.
- East declination — magnetic north is east of true north.
- West declination — magnetic north is west of true north.
Isogonic lines — lines joining places of equal declination; agonic line — zero declination.
Variations in declination:
- Secular variation — slow, long-period change (over centuries) — most important in surveying (old surveys must be corrected).
- Annual variation — small yearly periodic change.
- Diurnal (daily) variation — daily swing, larger in summer and near the poles.
- Irregular variation — due to magnetic storms, earthquakes, solar flares.
Dip
The magnetic needle, if freely suspended, dips from the horizontal towards the nearer magnetic pole. Angle of dip is 0° at the magnetic equator and 90° at the magnetic poles. To keep the needle horizontal, a small sliding rider (weight) is placed on the higher end. Isoclinic lines join points of equal dip; the aclinic line is the magnetic equator.