Case study //

60km Cable Corridor LiDAR Topographic Survey

Category: Survey & Mapping

Client: J Murphy & Sons Ltd (JMS)
Norfolk Offshore Wind Zone (NOWZ) – Onshore Works

Date: November and December 2022

Summary: Monthly detailed aerial mapping for construction monitoring of the 60km cable corridor and mobilisation areas for the Norfolk Offshore Wind Zone (NOWZ) projects: Vanguard West, Vanguard East and Boreas.

Aerial topographic surveys carried out by HexCam for Vattenfall in the spring of 2021 gave a good overview of the general topography of the NOWZ cable corridor route running 60km from the converter / substation at Necton to the landfall site at Happisburgh. These photogrammetry-based surveys, together with video fly-throughs, were shared with Tier 1 contractors tendering for the onshore construction contract.

Photogrammetry alone however is unable to penetrate vegetation canopies such as woodland, scrub, overgrown ditches or crop cover and results in the generation of a Digital Surface Model (DSM). What was needed to inform and progress the design of access and enabling works and to understand drainage features along the route was an accurate pre-construction LiDAR-based Digital Terrain Model (DTM) of the project cable corridor and mobilisation areas.

HexCam was engaged by J Murphy and Sons Ltd in autumn 2022 to provide LiDAR-based topographic survey data and 2D orthomosaics for the entire 60km cable corridor as part of the NOWZ Early Works Project. The survey was timed to take advantage of minimal crop, weed cover and deciduous canopy cover.  HexCam’s familiarity with the corridor route and the stakeholder engagement process allowed us to quickly start planning the best locations for Take-Off and Landing Sites (TOLS). Our CAA ‘Operating Safety Case’ (OSC), incorporating Extended Visual Line Of Sight (EVLOS) operations, allowed us to significantly extend our flight distances and to design the most efficient flight routes for good quality LiDAR data capture. We worked closely with JMS and RSK Consents Solutions to ensure that landowners were updated on our flight plans and ground crew access requirements at all times.

Our DJI M300 RTK survey drone, carrying DJI’s ‘L1’ payload, combined with a constellation of surveyed Ground Control Points (GCPs) was used to achieve the client’s requirement for an absolute vertical (Z) accuracy of +/- 100mm, providing a thinned data density of 50 PPM. Processing, by our partners at GeoDrone Survey, resulted in tiled deliverables, keeping file sizes manageable and allowing a staged release of topographic data to JMS.

LiDAR flights for the entire 60km route were completed in just under 6 days with police, landowners and other air users kept constantly updated on our flight locations and rate of progress.