Underground Surveying

Surveying below ground brings special challenges: transferring coordinates and orientation down shafts, gyro-theodolites, forced-centring traverses, setting drive centrelines and grades, slope reductions, breakthrough accuracy and cavity monitoring.

Underground there is no GNSS and no sky. Surveyors must carry position and direction from surface through shafts or declines, then extend long traverses in dusty, dark, narrow drives — accurately enough for headings driven from opposite ends to meet within centimetres.

An analogy

Underground surveying is like navigating a submarine: once you lose sight of the surface, every small direction error grows the further you travel.

Transferring control

Through declines: continuous traverse from surface. Through shafts: two plumb wires (or optical plumbing) transfer coordinates and orientation; the short wire base makes orientation the weak link. Gyro-theodolites measure true north directly underground and check traverses.

Setting out development

Surveyors paint centrelines and grade lines on drive faces, set laser alignment for long drives, and mark drill patterns (face marking). Drives slope slightly (≈ 1:50) for drainage.

Breakthroughs and checks

When headings meet, the breakthrough error is measured. Forced-centring traverses, repeated measurements and gyro checks every few hundred metres limit error growth.

As-built surveys

Cavity monitoring systems (CMS) and mobile laser scanners measure stopes and drives for reconciliation of tonnes, overbreak and support compliance. SLAM-based scanners map drives while walking or driving.

Formulas

• Slope reduction: HD = SD cos α, VD = SD sin α
• Grade line: RL_B = RL_A + g × distance
• Orientation error from plumbing: δθ ≈ δ / b (rad); lateral error = L × δθ

Worked example: Slope reduction

Given: SD 85.50 m, vertical angle −12°

Find: HD and VD.

• HD = 85.50 × cos 12° = 83.63 m
• VD = 85.50 × sin(−12°) = −17.78 m

Answer: HD 83.63 m, down 17.78 m

Worked example: Grade line

Given: Drive from RL 500.00 over 800 m at +1 %

Find: RL at end.

• Rise = 0.01 × 800 = 8.00 m

Answer: RL 508.00

Common mistakes

• Wrong: Relying on shaft plumbing alone for orientation. — Right: Check with gyro-theodolite observations.
• Wrong: Forgetting instrument and target heights. — Right: Include HI/HT in RL calculations.

Applications

• Shaft sinking: Verticality and orientation control.
• Decline development: Laser-guided alignment and grades.
• Stope reconciliation: CMS scans vs design shapes.

Key takeaways

• Transfer control via shafts, declines, gyros.
• HD = SD cos α; VD = SD sin α.
• Orientation errors grow with distance.
• Scan as-builts for reconciliation.

Going further

• SLAM scanning: Mobile mapping without GNSS.
• Error propagation: Predicting breakthrough error ellipses.
• Inertial surveying: Borehole and drive surveys with IMUs.