InstrumentsCalibrated annually to UKAS OSGB36 / ODNCaernarfon
GRATICULE Land & Hydrographic Survey

Instruments

The figures on this page are the ones we hold on site.

Every instrument has a brochure number achieved on a calibration baseline in still air, and a working number achieved on a hillside in February. These are the second kind, measured against our own check network at Doc Fictoria and re-verified every quarter.

Instrument register — current fleet
Instrument Count Working tolerance Typical use Calibration
Robotic total station, 1″ 4 ±1–3 mm at 200 m Control, setting out, monitoring Annual UKAS + quarterly two-peg
Digital level, invar staff 3 ±0.7 mm per km double run Precise levelling, settlement Annual UKAS + monthly collimation
GNSS rover, network RTK 6 ±8–15 mm plan, ±15–25 mm height Topographic detail, control extension Annual, plus daily check into known station
Terrestrial laser scanner 2 ±2–6 mm at 50 m Measured building, retrofit, record Annual UKAS + per-project target check
Survey UAV, RTK enabled 2 ±25–35 mm to ground control Volumes, faces, inaccessible ground Camera calibration each mobilisation
Multibeam echosounder 1 ±25–50 mm at 12 m depth Berths, scour, channel bathymetry Patch test each mobilisation
Single-beam echosounder 2 ±30 mm + 0.1% of depth Sections, reconnaissance, volumes Bar check at start and end of survey
GPR, dual frequency 2 Depth ±15% of measured depth PAS 128 utility detection Annual, plus on-site velocity calibration
Tide gauge, pressure 2 ±10 mm Tidal reduction for hydrographic work Levelled to ODN on every deployment

Comparison

The same ground, seven different ways.

Put on one logarithmic scale, the fleet spans nearly two orders of magnitude. The point of the chart is not that the total station wins — it is that a stockpile volume does not need it, and a bearing plate does.

Choosing one band tighter than the job requires typically doubles the field time. Choosing one band looser than it requires means measuring it again.

Precise levelling 1–2 mm
Total station 1–3 mm
Laser scan @ 50 m 2–6 mm
GNSS RTK, plan 8–15 mm
GNSS RTK, height 15–25 mm
UAV @ 60 m 20–35 mm
Multibeam @ 12 m 25–50 mm
1 mm31030100

Logarithmic scale. Figures are one sigma against site control, over a full working day, in the weather this coast actually has.

Calibration

Why we publish residuals

Every network we observe is adjusted by least squares, and the adjustment report goes out with the drawing whether or not anyone asked for it. It contains the residual on each observation, the standard error of each station, and the chi-squared test on the whole network.

Most clients never open it. The ones who do are usually engineers checking whether a settlement of four millimetres is real movement or is inside our noise, and that is exactly the question the report exists to answer. A survey that cannot state its own uncertainty cannot be used to decide anything marginal, and marginal decisions are the expensive ones.

The check network

We maintain eleven stations around the harbour at Doc Fictoria, observed as a closed network twice a year and levelled to a deep datum. Instruments are checked into it before any monitoring contract and after any repair, transport incident or drop. The comparison is what produces the working tolerances in the table above; they are not copied from a specification sheet.

What gets an instrument stood down

  • Two-peg test outside 3″ on a total station.
  • Collimation drift on a digital level between monthly checks.
  • Any drop, however soft the ground, until it is re-checked.
  • Scanner target residuals above 4 mm on a controlled setup.
  • A patch test that will not resolve within tolerance on the multibeam.

An instrument stood down mid-contract is replaced from the fleet the same day. That is most of the reason there are four total stations for seventeen people.