Total Station

A total station combines an electronic theodolite with an EDM and a computer. Learn how EDM works, slope-to-horizontal reduction, coordinate computation, atmospheric and prism corrections, stake-out, resection, robotic and reflectorless operation, and data workflows to CAD.

The total station is the workhorse of modern construction surveying: one instrument measures angles and distances to millimetres, computes coordinates on the spot, and guides setting-out of every column, pile and kerb.

Understanding the geometry and corrections behind its displayed numbers is essential to trust and check them.

An analogy

A total station is a theodolite that can also 'shout and listen' — timing a laser pulse to a prism to know how far away it is.

What it measures

Horizontal angle, vertical (zenith) angle and slope distance — from which the onboard software computes horizontal distance, height difference and coordinates. Typical accuracy: 1–5″ angles and 1–2 mm + 1.5 ppm distances.

Setting up

Occupy a known point and backsight another (known station), or do a free-station resection to several known points. Enter instrument and target heights, atmospheric conditions (or ppm) and prism constant. Check against a third control point before work.

Stake-out and topography

Stake-out: the instrument displays the angle and distance to move until the prism is on the design coordinate. Topographic surveys capture points with codes that build linework and DTMs automatically in CAD.

Robotic, reflectorless and monitoring

Robotic total stations track the prism so one person can survey; reflectorless EDM measures to surfaces directly (facades, tunnels). Automated monitoring stations measure deformation of structures and excavations continuously. Imaging/scanning total stations blend with laser scanning.

Formulas

• Horizontal & vertical components: H = S cos α = S sin z, V = S sin α = S cos z
• Coordinates: E₂ = E₁ + H sin Az, N₂ = N₁ + H cos Az
• Elevation: Z₂ = Z₁ + h_i + V − h_t
• EDM principle: D = c · Δt / 2 (pulse) or phase comparison
• Atmospheric correction: D_corr = D (1 + ppm × 10⁻⁶)

Worked example: Slope to horizontal

Given: S = 250.000 m, vertical angle +5°

Find: H and V.

• H = 250 cos 5° = 249.049 m
• V = 250 sin 5° = 21.789 m

Answer: H = 249.049 m, V = 21.789 m

Worked example: Coordinates of a new point

Given: Station (1000, 2000); Az = 60°, H = 249.05 m

Find: E, N.

• E = 1000 + 249.05 sin 60° = 1215.684
• N = 2000 + 249.05 cos 60° = 2124.525

Answer: (1215.684, 2124.525)

Common mistakes

• Wrong: Wrong prism constant or heights entered. — Right: Verify settings and check to a known point before work.
• Wrong: Ignoring atmospheric correction on long distances. — Right: Enter temperature/pressure — 10 °C error ≈ 10 ppm.

Applications

• Construction setting-out: Grids, columns, anchor bolts, road alignments.
• Topographic & as-built surveys: Coded point capture into CAD/BIM.
• Deformation monitoring: Automated prisms on buildings, tunnels and dams.

Key takeaways

• Measures Hz, zenith and slope distance → coordinates.
• H = S sin z; V = S cos z.
• Set up on control, check a third point.
• Stake-out and as-built are daily tasks.

Going further

• Scale factors: Grid vs ground distances differ on map projections — apply combined scale factors.
• Machine control: Total stations guide excavators and pavers in 3D.
• Laser scanning: Millions of points for point clouds, BIM and tunnel profiles.