Calair Burn UAV LiDAR & Photogrammetry — Ballimore Restoration, Scotland

Project Brief

Angell Surveys was appointed by the Forth Rivers Trust to deliver a fixed-wing UAV LiDAR and photogrammetric survey of the Calair Burn catchment in the Balquhidder glen, Stirlingshire — part of the Trust’s wider Ballimore Restoration project. The survey envelope followed the catchment boundary supplied by the Trust and covered approximately 9.67 kilometres of upland watercourse and the surrounding land that drains into it.

The Ballimore Restoration is a publicly-funded landscape-scale natural flood management and habitat recovery scheme funded by NatureScot and the Loch Lomond and Trossachs National Park. Planned interventions across the catchment include riparian tree planting, exclusion fencing to allow natural regeneration, peatland restoration, large woody debris insertion and green bank reprofiling using willow weaving. Each of these interventions required a survey-grade Digital Terrain Model (DTM) to underpin the engineering, planning and flood-risk evidence base that the Trust’s project team and consultants would build on for the remainder of the programme.

At the point of commissioning, no LiDAR coverage existed for the catchment from the Scottish Government’s national remote sensing programme. The best public-domain spatial data was 30-metre resolution — wholly insufficient for natural-flood-management design, hydraulic modelling or capital-works engineering at the scale of the Calair Burn. Capturing a high-resolution DTM was therefore the foundational dataset for the whole restoration programme.

Survey Objectives & Planning Context

The survey was procured against a tender brief that specified a Digital Terrain Model at 50 cm spatial resolution or better, delivered in the OSGB36/OSTN15 coordinate system with paired orthomosaic imagery, classified point cloud and engineering deliverables suitable for downstream flood-risk and design work.

The data was sized to support four immediate workstreams within the wider restoration programme:

• Flood Risk Assessment (FRA): the DTM is the primary spatial input to the 1D/2D hydraulic modelling that defines pre- and post-intervention flood extents, used both for SEPA engagement and for sizing the natural-flood-management features in the catchment.
• Peatland restoration: identification of degraded peat hags, gullies and erosion features visible only in high-resolution bare-earth terrain — these features routinely fall well below the resolution of 30m public data.
• Riparian planting and fencing layout: burnside geometry, bank slopes and corridor widths driven directly off the high-resolution DTM and orthomosaic so that planting and fencing alignments could be designed in office rather than walked entirely in the field.
• Large woody debris and willow weaving placement: bank geometry, in-channel features and floodplain connectivity all visible from the high-resolution terrain and imagery — enabling engineering design of bionic features before mobilising to site.

Site Constraints

The Calair Burn catchment is a remote upland working farm. The site presented a layered set of operational constraints that drove platform and methodology selection:

• No mobile reception anywhere on site — communications had to be self-contained, with check-in protocols by Iridium / satellite messenger and pre-agreed check-in windows with the Trust representative.
• 4×4 vehicle access required — mobilisation across rough farm tracks, with no hard standing for ground-based equipment.
• Steep elevating boundaries to the catchment with significant relief — terrain that ruled out a purely line-of-sight multi-rotor approach.
• Upland weather exposure — high winds and heavy rain are routine. The survey window had to be planned against forecast windows and re-flown if conditions degraded mid-mission.
• No site facilities — all crew, fuel, batteries, recovery equipment and contingency had to be carried in and out.
• Working farm — full PPE, livestock awareness and farm-traffic protocols throughout.

The Trust handled landowner permissions; Angell Surveys handled CAA Operational Authorisation, public liability and UAV insurance, risk assessments, method statement and the field execution.

Survey Methodology

A combined catchment-scale envelope with significant relief, ≤50 cm DTM accuracy requirement and a tight pre-March delivery window placed the project firmly outside the practical envelope of multi-rotor UAV survey. The methodology selected was a fixed-wing UAV carrying integrated LiDAR and metric photogrammetric payload, operated under a CAA Operational Authorisation permitting beyond-visual-line-of-sight (BVLOS) flight in segregated airspace.

The fixed-wing platform offered three advantages decisive for this site:

1. Mission endurance sufficient to fly the full catchment in a small number of sorties without intermediate landings — critical on a site with no mobile signal where every break introduces operational overhead.
2. Integrated LiDAR and camera on a single GNSS/INS reference, removing the need to separately georeference photogrammetric and LiDAR products and ensuring consistent OSGB36/OSTN15 alignment across all deliverables.
3. Higher airspeed and altitude tolerance allowing the mission to fit between weather windows rather than waiting for prolonged calm — important in an upland Scottish glen in winter.

The LiDAR payload was specified to deliver point density sufficient to support ≤50 cm bare-earth DTM after classification, with overlapping passes across the steep catchment boundaries to ensure return density on every aspect.

Ground control was established at survey-grade GNSS reference points across the site to validate the integrated GNSS/INS solution, with check points held back for independent accuracy verification on the delivered DTM and orthomosaic.

Deliverables

The survey was delivered to the full tender deliverable schedule in OSGB36/OSTN15:

• Nadir orthomosaic in GeoTIFF format covering the full catchment
• Google tile export in .kml and .html formats for desk-based review
• Nadir Digital Surface Model (DSM) in GeoTIFF
• Nadir Digital Terrain Model (DTM) in GeoTIFF at ≤50 cm spatial resolution
• Classified LiDAR point cloud in .las format — ground / non-ground classification suitable for downstream FRA and hydraulic modelling
• Contour lines in .shp and .dxf formats for direct ingest into the Trust’s GIS and engineering CAD platforms

All deliverables were grid-tied to OSGB36/OSTN15 with an accompanying accuracy statement against the independently held check points.

Outcome

The high-resolution DTM and accompanying classified point cloud replaced the previous best-available 30-metre public data as the spatial foundation for the Ballimore Restoration programme. The Trust’s project team carried that dataset directly into the Flood Risk Assessment, the peatland-restoration design, the riparian planting and fencing layout, and the engineering design of the large woody debris and willow-weaving interventions.

For Angell Surveys the project sits in the line of restoration-led catchment surveys we’ve delivered for environmental NGOs, public bodies and conservation trusts — natural-flood-management and habitat-recovery work where the survey is not an end in itself but the spatial backbone of a multi-year landscape programme.