Local Coordinates and GNSS: How to Align Them for Accurate Site Engineering
On most highways and civil engineering projects, site engineers need to work on a local grid—but GNSS equipment like Trimble and Leica naturally works in global coordinate systems such as ETRS89. To make everything line up, we create a site calibration (or localisation) file that transforms GNSS data into local site coordinates.
This process is vital for machine control, accurate setting out, and ensuring all survey and construction equipment references the same grid.
AKN Engineering Surveyors aligning GNSS and total station measurements on a live UK highways project to create a precise local coordinate calibration for accurate setting out.
Why Local Coordinates Are Used
Design teams often prefer a local coordinate grid for practicality:
Convenient numbers – origins like 10,000E / 5,000N keep figures positive and easy to read.
Scale match – distances stay close to ground scale for simpler tape or EDM checks.
Collaboration – multiple contractors can work from the same base without confusion.
But GNSS receivers naturally output coordinates in ETRS89 or OSGB36. To bridge the gap, a mathematical transformation is required.
How GNSS and Local Grids Are Aligned
GNSS provides highly accurate positions in a global reference frame. To align with a local site grid, surveyors:
Select known control points on site (minimum of four, spread across the area).
Observe them with GNSS RTK corrections, e.g. from OS Net or a private base.
Compare the GNSS-derived positions to the design coordinates.
Solve a best-fit transformation, accounting for translation, rotation, scale, and sometimes tilt.
This creates a calibration/localisation file that lets every rover, total station, and machine on site report positions in the project’s local grid.
Trimble vs Leica: Step-by-Step
The workflow is similar whether you use Trimble or Leica:
Trimble Access / TBC (Trimble Business Center)
Leica Captivate / Infinity
Both systems can combine GNSS and total station observations for hybrid control.
Machine Control: Why It’s Critical
Machine control systems rely on a perfect calibration. If the site calibration is wrong, machines may cut, fill, or dig in the wrong place. On highways or earthworks projects, that can mean expensive rework and programme delays.
A robust calibration at the start of the project ensures that every engineer, surveyor, and machine is working from exactly the same grid.
AKN Engineering’s Advice
Use at least four well-spaced points for calibration.
Check horizontal residuals <10 mm and vertical residuals <15 mm.
Back up the calibration file and share the same version across all devices.
Recalibrate if the site expands or new control is introduced.
Need help with local grids, GNSS control networks, or machine control calibration?
👉 Call 01279 927 033 or visit www.aknengineering.co.uk to get dedicated support from your local friendly team.