Compass and Theodolite Traversing
Core concepts
Compass traversing measures magnetic bearings; theodolite traversing measures angles with greater precision. Know the bearing systems: whole-circle bearing (WCB, 0–360°), reduced bearing (RB, quadrantal), and the conversions. Learn magnetic declination, true bearing, local attraction, and the difference between an open and a closed traverse.
For theodolite work, understand face left/face right, transiting, the telescope's line of collimation, the horizontal and vertical circles, and temporary adjustments (setting, levelling, focusing, parallax removal). Permanent adjustments must also be named.
Traps in questions
- Back bearing = fore bearing ± 180°. Apply the correct sign; do not simply add 180°.
- Local attraction: the observed bearing difference at a station that is free of attraction is correct. Identify the free station, then correct the rest. Interior angles from corrected bearings must sum to (2n−4)×90° for a closed traverse.
- Latitude = L cos θ, departure = L sin θ (θ = reduced bearing). Confusing sin/cos is the classic error.
- Closing error: e = √(ΣL² + ΣD²), and angular error check: sum of observed angles vs theoretical.
Worked example / rule of thumb
A line has FB 40°; BB = 40° + 180° = 220°. If the FB is 300°, BB = 300° − 180° = 120°. Rule: subtract 180° if FB > 180°, add 180° if FB < 180°.
For traverse adjustment, apply Bowditch's rule: correction to latitude of a line = total latitude error × (line length / perimeter). Use transit rule when angles are more precise than lengths. Always close the traverse on a known point for a check.