01The challenge
The client had secured an option on a 40 ha greenfield site and needed a full topographic survey to support drainage strategy, levels design and an outline planning application. A traditional GNSS and total station survey had been quoted at around three weeks on site for a two-person crew, which would have pushed the planning submission back by a month.
- Mature hedgerows with ditches on both sides crossed the site, and hedge-line ground levels were critical to the drainage design.
- Two watercourses needed bed and bank levels for the flood risk assessment.
- The design team required a 1:500 CAD survey on OS National Grid and ODN, compatible with their civils software.
02Our approach
Day one was spent installing and observing 14 ground control points and 20 checkpoints with GNSS rover and a local base, tied to OS Net via OSTN15 and OSGM15. Flights ran on day two: a Zenmuse L2 LiDAR mission to capture ground under hedgerows and field margins, and a Zenmuse P1 photogrammetry mission for a high-resolution orthomosaic and detail interpretation.
| Mission | Specification |
|---|---|
| LiDAR | Zenmuse L2 at 80 m AGL, ≈200 pts/m², five returns |
| Photogrammetry | Zenmuse P1 (45 MP, 35 mm) at 90 m AGL, 11 mm GSD, 80/70% overlap |
| Control | 14 GCPs, 20 checkpoints, static GNSS base with network RTK rover |
| Ground detail | Total station and GNSS pickup of ditch inverts, culverts, manholes and gate posts |
Day three was reserved for ground detail that no aerial sensor can see reliably: culvert inverts and diameters, ditch bed levels under overhanging vegetation, manhole cover and invert levels on the adjacent highway, and boundary features obscured by tree canopy. These observations were merged with the aerial data in the CAD model and flagged by survey method so the design team knew the source of every level.
03The outcome
The finished survey was issued eight working days after the last day on site. Independent checkpoints returned an RMSE of 14 mm horizontally and 19 mm vertically across hard and short-grass surfaces, reported in a control and accuracy statement that the client included with the planning submission.
- Around two weeks saved on site compared with the traditional survey quote, keeping the planning programme on track.
- The LiDAR ground model captured hedge-line levels and ditch profiles that the drainage engineer used directly in the SuDS strategy.
- The orthomosaic became the base plan for masterplanning, landscape and ecology teams.
- The client kept the point cloud, so later design questions, such as a revised access position, were answered without a return visit.
04Deliverables
- 2D and 3D topographic survey at 1:500 (DWG, DXF) with 0.25 m contours
- Digital terrain model (LandXML, GeoTIFF)
- Orthomosaic at 20 mm GSD (GeoTIFF, ECW)
- Classified LiDAR point cloud (LAZ)
- Watercourse long sections and cross-sections
- Control and accuracy report with checkpoint residuals
