Drone survey data is put on the National Grid by observing the ground control and the aircraft trajectory in ETRS89 through OS Net, transforming eastings and northings with Ordnance Survey’s OSTN15 grid model, and converting ellipsoidal heights to Ordnance Datum Newlyn with the OSGM15 geoid model. Where a project uses a site grid, the survey is captured on OSGB36 and ODN and related to it by a documented transformation.
This guide supports our topographic survey and aerial survey services and sits alongside two earlier articles: the field workflow in OSGB36 drone survey workflows and the full mathematics in latitude and longitude to OSGB36. Here the focus is the pieces themselves, the Ordnance Survey accuracy figures for each, and the site-grid question.
What are the coordinate systems involved?
| System | What it is | Role in a drone survey |
|---|---|---|
| ETRS89 | The European terrestrial reference system, fixed to the Eurasian plate; realised in Britain by the OS Net GNSS network | The frame every GNSS observation is expressed in: base station, GCPs, aircraft trajectory |
| WGS84 | The global GPS datum, which moves with the whole Earth relative to ETRS89 | What a receiver reports if nothing better is applied; not a survey datum in Britain |
| OSGB36 National Grid | The horizontal datum and Transverse Mercator projection of Ordnance Survey mapping, derived from the 1936 to 1962 retriangulation | The grid on which UK drawings, designs and statutory plans sit |
| Ordnance Datum Newlyn (ODN) | The vertical datum: mean sea level at the Newlyn tide gauge, 1915 to 1921 | The datum for every level, contour and invert |
| OSTN15 | Ordnance Survey’s National Grid Transformation, a 1 km grid of easting and northing shifts between ETRS89 and OSGB36 | Converts ETRS89 grid coordinates to OSGB36 |
| OSGM15 | Ordnance Survey’s National Geoid Model, a 1 km grid of geoid-ellipsoid separations | Converts ETRS89 ellipsoidal height to ODN orthometric height |
Why can’t a simple datum shift be used?
The OSGB36 triangulation carries distortions of several metres across the country, inherited from the way it was observed and adjusted in the mid-twentieth century. A seven-parameter Helmert transformation cannot model that. Ordnance Survey’s own guide states that a Helmert transformation between ETRS89 and OSGB36 is limited to applications requiring an accuracy of only about 3 metres, which is why OS developed the rubber-sheet OSTN15 model instead. A survey processed through a Helmert shift can look perfect in isolation and still be metres off Ordnance Survey mapping and every other survey on the site.
How accurate are OSTN15 and OSGM15?
The figures below are Ordnance Survey’s published statements, not ours.
| Model | Ordnance Survey stated accuracy | Source |
|---|---|---|
| OSTN15, ETRS89 to OSGB36 | New and archived OSGB36 coordinates agree on average to better than 0.1 m (0.1 m rmse, 68 per cent probability) | OS, A Guide to Coordinate Systems in Great Britain, section 6.3 |
| OSGM15, ETRS89 ellipsoidal height to ODN | Nominal accuracy 8 mm rms in mainland UK; better than 2 cm rms for other areas; 3 cm rms for the Isle of Man | OS, section 6.4 |
| Helmert ETRS89 to OSGB36 | About 3 metres | OS, section 6.3 |
Two consequences follow. First, OSTN15 in combination with OS Net now defines the National Grid: a point refixed by GNSS and OSTN15 is correct by definition, and an archived triangulation coordinate that disagrees is the one that is wrong. Second, the geoid model contributes only millimetres to the height budget on the mainland; the height accuracy of a drone survey is governed by the GNSS observations and the control, not by OSGM15.
How does the drone’s data get onto the grid?
- Base station. A GNSS receiver logs over the site for the duration of capture, positioned by static observation tied to OS Net, or occupying known control.
- Ground control. GCPs and independent check points are observed by dual-frequency GNSS against the same base, giving ETRS89 coordinates and ellipsoidal heights.
- Trajectory. The aircraft’s GNSS and inertial data are post-processed against the base (PPK) so that every image or LiDAR pulse has an ETRS89 position.
- Transformation. ETRS89 positions are projected to National Grid eastings and northings, OSTN15 shifts are interpolated and applied, and OSGM15 separations are subtracted from ellipsoidal heights to give ODN levels. This is done in the processing software with the OS models loaded, not by a manual shift.
- Verification. The finished products are compared with the withheld check points and the residuals reported against the RICS band table in Measured Surveys of Land, Buildings and Utilities (RICS, 2014).
Our ground control guide covers step 2 and our QA/QC guide covers step 5.
What is a site grid and when is one used?
The National Grid is a projection, and a projection cannot preserve both shape and scale everywhere. OSGB36 carries a scale factor that varies across the country, so a distance measured on the ground differs slightly from the same distance computed from grid coordinates. For mapping the difference is irrelevant; for setting out a bridge or a track alignment it is not.
Large rail and highway schemes therefore adopt a project or site grid: a local system with a unitary scale factor, often aligned to the route (“snake” grids on rail), so that plan distances are true to the ground along the alignment. Phased developments and earthworks contracts frequently use a contractor-defined grid for the same reason.
For a drone survey the practical rule is:
- Capture and control on OSGB36 and ODN as the master reference, so the survey can always be related to Ordnance Survey mapping, statutory plans and other surveys.
- Deliver on the site grid where the project requires it, using the project’s published grid definition.
- Document the relationship between the two in the report and the drawing title block, so that any future survey can be reconciled without ambiguity.
Never “float” a drone survey on a site grid from a single point and a bearing; the relationship must come from the project’s control.
What about islands and offshore areas?
OSGM15 covers areas whose heights are not related to ODN, including the Orkney and Shetland Isles, the Outer Hebrides, the Isles of Scilly and the Isle of Man, each on its own local vertical datum. Offshore, datums are extended 2 km from land, beyond which an “ODN Offshore” surface of lower quality applies. A coastal or island survey must state which vertical datum its levels are on. On the Isle of Man, which also regulates its own airspace, our five-epoch coastal erosion monitoring programme was controlled by GNSS to OSGB 1936 with semi-permanent stations re-validated before every epoch.
What must the deliverable state?
- Horizontal datum: OSGB36 National Grid, transformation OSTN15
- Vertical datum: Ordnance Datum Newlyn, geoid model OSGM15, or the local datum on islands
- If a site grid is used: name, definition, scale factor and relationship to OSGB36
- How the control was fixed to OS Net
- Check-point residuals against the RICS band specified
If any of those is missing, ask before the data goes into a design.
How Angell Surveys does this
Angell Surveys is regulated by RICS. Every UAV, LiDAR and topographic survey we issue is captured on OSGB36 and ODN through OS Net, OSTN15 and OSGM15, with the transformation and geoid named in the report, and is delivered on the client’s site grid where the project requires it with the relationship documented. For principal contractors on weekly earthworks programmes we establish permanent ground control tied to OS Net and levelled to ODN, agree the coordinate file with the contractor’s setting-out engineer and lock it as the programme datum; for a rivers trust’s upland catchment survey in Scotland the tender specified OSGB36 and OSTN15 explicitly, and every deliverable was grid-tied accordingly. Read the weekly earthworks monitoring case study and the Calair Burn case study.
Related services
Sources
- Ordnance Survey, A Guide to Coordinate Systems in Great Britain, v3.6, 2020: sections 5.3 (Ordnance Datum Newlyn), 6.2 (Helmert transformations), 6.3 (OSTN15) and 6.4 (OSGM15).
- Ordnance Survey, OS Net developer resources: OSTN15 and OSGM15 replaced OSTN02 and OSGM02 in August 2016.
- RICS, Measured Surveys of Land, Buildings and Utilities, 3rd edition, November 2014.
- Angell Surveys guides: OSGB36 drone survey workflows, latitude and longitude to OSGB36.