Instrumentation

Modern surveying isn't one piece of equipment doing everything — it's a family of specialised tools, each best at a narrow slice of the work. Picking the right instrument for the job (and knowing the next-best fallback when one's broken) is half the skill.

• Total station. Measures horizontal angle, vertical angle, and slope distance to a reflector. The general-purpose survey instrument — boundary, construction, topographic. Everything in this guide builds on it.
• Automatic level (+ rod). Measures elevation differences only. The cheapest, most reliable, most precise way to carry vertical control across a project. See the dedicated leveling guide for the math.
• GNSS receiver. Computes absolute position from satellite signals, RTK-corrected to centimetre-grade. Faster than a total station for open-sky work; useless under heavy canopy.
• Terrestrial laser scanner. Captures millions of points per second over a 360° sphere. Used for as-built documentation, dimensional control, deformation monitoring.
• UAS / drone. Carries a camera (or LiDAR) over a project to capture topographic surface data and orthomosaics. Best for large areas with open access.

Everything else flows from these pieces working in agreement. Learn the names — not the brand-specific marketing names, the survey names that show up on every spec sheet you'll ever see.

• Telescope. The sighting optic. Rotates in the vertical plane around the trunnion (horizontal) axis. Reticle inside has crosshairs aligned to the line of collimation.
• Standards. The two vertical posts that hold the telescope. Define the trunnion axis. Any rotation around the vertical axis swings the entire telescope-and-standards assembly together.
• Plate (alidade). The horizontal surface the standards sit on. Rotates around the vertical axis. The plate bubble lives here — getting it level is the central act of setup.
• Tribrach. The three-screw base that levels the plate over the tripod head. Three foot screws + the plate bubble = your levelling system.
• Optical (or laser) plummet. An optical or laser projection that shines straight down from the vertical axis. The job of setup is to make that down-line land on the survey point, with the plate bubble centred. When both are right you're plumb over the point.

Setting up is iterative — you don't level perfectly on the first try and you don't centre over the point on the first try. The trick is knowing which adjustment to make next so you converge in three iterations instead of fifteen.

• 1. Plant the tripod roughly over the point. Eye-level it. The tripod head should look horizontal-ish from a few steps back. Spread the legs to about a 60° interior angle — too narrow and the rig is wobbly, too wide and you can't fine-adjust by leg.
• 2. Lock the optical plummet on the point with the tribrach screws. Sight through the plummet, adjust the three tribrach foot screws until the crosshair sits on the survey point. Ignore the plate bubble at this stage.
• 3. Centre the plate bubble by adjusting tripod-leg length. Shorten or lengthen one leg at a time. Watch the bubble travel — match the direction of leg adjustment to the direction the bubble needs to move. Don't touch the tribrach screws yet.
• 4. Re-check the plummet, re-centre the bubble. Repeat. After step 3 the plummet usually drifts off the point. Slide the instrument across the tripod head (loosen the centre fixing screw, slide, re-tighten) to bring the plummet back over the point. Re-level. Loop until both are right at the same time.
• 5. Final precise level with the foot screws. Once the bubble is within a quarter-division of centre and the plummet is sitting on the point, finish with the tribrach foot screws. A 30-arc-second tilt at this stage corrupts every observation that follows.

The choreography of a real setup is easier to learn from watching than from reading about it. This is the procedure above, performed on a stake in the field.

Every total-station shot produces three raw observations — nothing else. Everything you compute downstream (azimuths, coordinates, elevations) is reduced from these three numbers plus the instrument and rod heights.

• Horizontal angle. Measured in the horizontal plane from the backsight direction to the foresight direction. Always “angle right” in modern practice (clockwise from backsight). 0°–360°, never wrap to negative.
• Vertical angle (or zenith angle). Most modern instruments display zenith angle — 0° straight up, 90° horizontal, 180° straight down. Older field books used vertical angle (0° horizontal, ±). Know which your instrument uses.
• Slope distance. The straight-line distance from the instrument's reference point to the prism. Reduce to horizontal using the vertical angle: H = S · sin(Z) where Z is zenith.

Manufacturer spec sheets list angular and distance accuracy as a single number (e.g. “1″ angular, 2 mm + 2 ppm distance”). That's the optimistic floor under ideal conditions. The real-world budget includes setup error, atmospheric refraction, target alignment, and the human pressing the button.

For a 100-m boundary leg, 1″ angular error projects to about 0.5 mmacross the sight. A 5 mm centring error at either the instrument or the rod dwarfs that. Setup precision matters more than instrument spec for most work.

• Sun on the instrument body. Direct sunlight warms one side of the standards and the vertical axis tilts a few arc-seconds. Use the umbrella; observe both faces (direct and reverse) and average them to cancel the bias.
• Reflector constant mismatch. Every prism brand has a slightly different offset (0 mm, -30 mm, -34 mm). Setting the wrong constant on the gun puts a fixed bias on every distance for the entire job. Confirm the constant at the start of each day.
• Pole not plumb. A 1° tilt at a 2 m rod height is 35 mm of horizontal error. Centre the pole bubble, or use a pole tripod for any measurement that matters.
• Backsight too short. Orienting off a backsight that's 5 m away amplifies any centring error in the backsight prism into massive azimuth error on the rest of the setup. Backsight to a point at least 30–50% of your typical shot distance.