Context 168 - June 2021

24 C O N T E X T 1 6 8 : J U N E 2 0 2 1 A DJI M300 drone equipped with the Zenmuse H20 camera with 20MP zoom and 12MP wide-angle CMOS sensors (Photo: Historic England) regularly use drone-mounted multispectral and hyperspectral imaging sensors to remotely sense surface landscape type, condition and topography. As with other geospatial survey equipment, such as the total station theodolite, GNSS receiver and laser scanner, drones have matured sufficiently over the past decade to become an essential tool in the surveyor’s toolbox for pro- viding high-level coverage of areas that are often difficult to access by ladder, scaffolding tower or cherry-picker. This has been helped by the increasing worldwide popularity of drones that has driven down their cost. Systems suitable for heritage start at around £500, using a standard mobile phone and downloadable app in their remote controller. Although in theory anyone can fly a drone, their use in the UK is regulated by the Civil Aviation Authority (CAA), which now pro- vides the necessary rules and registration of leisure, recreational and commercial pilots. On 1 January 2021 the rules changed when new EU regulations governing the use of drones came into force in the UK and across Europe. Developed by the European Union Aviation Safety Agency (EASA), these new regulations adopted a risked-based approach, no longer distinguishing between leisure and commercial drone activities. Anyone flying a drone commercially must still obtain a ‘permission’ for commercial operations from the CAA by passing a theory and flight test. As part of the process, an operations manual must also be developed and approved by the CAA, detailing every aspect of the proposed drone operation, including risk assessment, insurance, maintenance and their safe flying within the required visual line of sight limits. For commercial, consumer and leisure appli- cations, two types of drone are used (fixed-wing and rotary), each of which has advantages and disadvantages for heritage application. Until 2018 the capture of drone-acquired imagery for Historic England and English Heritage projects was undertaken through com- mercial contractors who possessed the necessary CAA permissions, hardware and personnel. Although some drone surveys are still procured externally, Historic England now has an in-house team of eight qualified drone pilots, and a range of consumer, professional and inspection-grade drones that are being increasingly used across a range of heritage-related applications. Geospatial survey: providing overlapping, low- level aerial imagery suitable for photogrammetric processing, and combining with terrestrial survey data to generate more complete, three- dimensional models from which plans, sections and ortho-rectified elevations can be extracted. Archaeological investigation: providing a rapid and complementary low-level aerial approach for capturing topographical landscape detail into which more detailed or interpretive survey data can be added from the ground. Civil and structural engineering: allowing the FIXEDWING ROTARY ADVANTAGES Shape gives good stability in the air Uses multiple rotors (usually between 4 and 12), providing redundancy if damaged in flight Lightweight construction, so longer battery life and flight time (up to 90 minutes) Heavier payload capacity, so better, multiple sensors can be carried Faster flight speeds (cruise 40 mph, max 90 mph) Good in-flight manoeuvrability Can glide over longer distances and acquire photography in linear grids Ability to hover and operate in confined spaces Enables rapid mapping of large areas and shorter times on site Enables inspection, monitoring, survey and presentational work on buildings and structures DISADVANTAGES Inability to hover Slower speed (max 40 mph) Lower payload capacity Shorter battery life (up to 30 minutes) More automated image capture and exposure control Longer survey times on site