Low-Cost UAVs for Environmental Monitoring, Mapping, and Modelling of the Coastal Zone
COASTAL AND MARINE APPLICATIONS
Unmanned Aerial Vehicles (UAVs) have been used for many different applications around the world in the last 5 years. As their ease of use improves, the number and range of applications has also increased rapidly. In particular, coastal and marine environments requiring high-resolution local data and information or multi-temporal monitoring data and imagery for change detection are well suited to the use of UAVs carrying a range of different sensors including RGB cameras, thermal, and hyperspectral sensors. Klemas (2015) notes that UAVs (now....) have the capability to effectively fill current observation gaps in environmental remote sensing and provide critical information needed for coastal change research... (p. 1265).
In recent years, with the widespread availability of small, off-the-shelf, Ready-to-Fly (RTF) UAVs and accompanying photographic and video sensors, and the rapidly evolving Global Positioning System (GPS), navigational, and sensor technology, a number of coastal/marine applications have emerged that take advantage and demonstrate the value of this technology for use by coastal researchers and managers. There are now many different suppliers of UAV platforms or frames: some are ready built, and others are available as kits for construction. These are alongside some very specialist custom frames costing considerably more. Depending on the requirements of a coastal study, there are many examples of small off-the shelf sport cameras e.g. GoPro and DSLR cameras, all of which can be mounted in gimbals that are in use, alongside more expensive sensors, such as thermal cameras, hyperspectral, and light detection and ranging (LIDAR) sensors, and most recently the bathymetric Bathycopter LIDAR by Riegl.
In this chapter, three different examples are used to illustrate the potential these of small airborne platforms and sensors to gather data on various different aspects of the coastal environment. Spanning some 30years, these provide a good indication of how the aerial platform and sensor technologies have advanced, over a relatively short period of time to the present day, empowering a wide range of people with the potential to acquire and process high-resolution environmental data.
COASTAL UAV APPLICATIONS IN THE LITERATURE
Most of the journal papers on coastal applications of UAVs examined are from the last 5 years, which coincides with the rapidly growing recognition of this technology, its affordability, and ease of use for many different applications ranging from relatively simple photographic and video acquisition to more sophisticated monitoring and small area surveying exercises. Whilst each paper addresses different coastal applications, the large majority of these papers highlight not only the context behind the growth in applications e.g. technology, battery power, GPS accuracy, and survey capability but also the sophistication of the soft-copy photogrammetry software now available to process the imagery, as well as the importance of operational safety, and flight regulations. Additional considerations are the low costs of data acquisition (Ryan, 2012), large-scale mapping, and spatial resolution requirements.
Special UAV environmental considerations are also cited by a number of authors in relation to coastal work. For example, Guillot and Pouget (2015) mention the challenging nature of coastal environments for flying UAVs and the need to take into account parameters such as wind, water, temperature, and climate. Mancini et al. (2013) recorded a number of important observations in relation to the use of UAVs in dune systems which include planning take-off and landing sites to avoid setting the sand in motion by the UAV rotors and avoiding damage to the functioning of the rotors and camera lens. Klemas (2015) observes the value of UAVs to increase operational flexibility and provide greater versatility. In addition, Klemas cites a number of studies that make use of different types of UAV platforms and sensors. Hugenholtz et al. (2012) also note that improvements in the design of flight control systems have transformed these platforms into research-grade tools capable of acquiring high-quality images and geophysical/biological measurements. Klemas (2015) mentions studies that have mapped tidal wetlands, coastal vegetation and algal blooms, sand bar morphology, the locations of rip channels, and the dimensions of surf/swash zones, coastal hazards, fishing surveillance, coastal erosion studies, and combined aerial views from a UAV (drone) with measurements from autonomous underwater vehicles (AUVs) to get an unprecedented look at coastal waters off the coast of southern Portugal. Others have used UAVs in combination with other platforms and sensors to monitor and track marine mammals (e.g. Aniceto et al., 2018).
The role of fixed-wing versus multi-rotor platforms is also discussed in many papers, serving to provide a useful current overview of the status of these platforms and sensors whilst also highlighting the complementary value of these platforms and their sensors for monitoring, mapping, and surveying different aspects of the coastal environment. In addition, reference is still commonly made to other small airborne data acquisition platforms such as tethered kites and balloons of blimps (Ryan, 2012) which also have potential for some applications and can be both cost-effective and easy to use, even if they fall without the UAV domain. As with all such small platforms and systems, however, they also have their limitations.
Some of the applications of UAVs to coastal environments have included monitoring coastal and intertidal habitats such as mangroves, saltmarsh, and seagrass (Ryan, 2012) to gather ortho-photos, as well as the use of video transects for intertidal bathymetric habitat mapping, to assess inaccessible areas; facilitate repeat surveys, extent, and coverage of vegetation; and for monitoring dredging activity and plume extent. Also mentioned are applications that include the generation of 3D point clouds through stereo-photography or LIDAR and routine marine fauna observation (MFO) including cetaceans and turtle nesting activity using fixed-wing aircraft, minor oil-spill contingency tracking, hyperspectral vegetation classification, detailed engineering inspections of pipelines, offshore structures, thermal imaging, and health and safety intervention. Whilst some of the platforms used for environmental applications have been specialised custom examples, the rapid growth in small, low-cost platforms has led to more of these platforms being standard off-the-shelf UAVs e.g. the DJI Phantom has become very popular as an RTF. Inevitably the opportunity to monitor changes to the coastal environment has been one of the applications where UAVs have considerable potential given the ease with which it is now possible to acquire multi-temporal photographic imagery not only to see changes to the coastline in terms of erosion and deposition but also to be able to measure the change e.g. cutback and to determine the rate of coastal erosion. Appeaning Addo et al. (2016) have used a DJI Phantom 3 to monitor and map coastal change due to erosion in relation to coastal protection structures in Ghana along the Volta Delta shoreline. Using a relatively standard and simple platform and sensor configuration, they were able to generate aerial photographs of the protection structures and surroundings, as well the generation of high-resolution ortho-photos and Digital Elevation Models (DEMs) for the detection and analyses of both planimetric and volumetric changes. The practicality of the method for repeated monitoring and survey is also highlighted. Anderson et al. (2015) have utilised UAVs to gather data about spatial ecology and coastal water quality citing key advantages as being that they can be launched, operated, and the data accessed and analysed within hours and systems that can be cheaply and easily used by aquaculture businesses (e.g. pelagic fish and shellfish farms) and by coastal environmental managers (e.g. UK Environment Agency (EA)) to easily and repeatedly monitor near-shore water quality.
In another study, Guillot and Pouget (2015) have used UAVs specially adapted for coastal environments (e.g. salt, wind, and moisture) for monitoring and precision mapping of the dunes and dikes of Oleron Island, in France before and after tides and storms. The UAV - known also as a Coastal UAV - is a modified DJI F550 and is capable of flying in high wind conditions and is designed to be resistant to moisture and sand particles. Interestingly, and perhaps sensibly, they also made use of an off-the-shelf rugged ‘sports camera’ with a standard fisheye lens. Post-processing of the imagery acquired was carried out to remove the fisheye distortion from the imagery and to geo-correct the imagery using the Ground Control Points (GCPs). AgiSoft soft-copy photogram- metric software was used to generate an ortho-mosaic, a Digital Surface Model (DSM), and a 3D PDF (Portable Document File) for input to a Geographic Information System (GIS). A DDVM (Difference of Digital Volumetric Model) was generated for the different dates of imagery to determine change. DSM and DEM files provided the means to generate 2D topographic profiles in the GIS software. Benefits of the research that were identified included the ease with which it is possible to design a low-cost, coastal-specific platform using off-the-shelf technology for use in a relatively inhospitable environment to gather repetitive high-resolution data and imagery from which it is possible to quantify coastal change at a cm resolution from the UAV-derived products. Further projects for coastal monitoring include the study of sand and pebble movement, birds, waste, and seaweed which can provide valuable information to improve ecosystem knowledge in the future.
Mancini et al. (2013) describe how UAVs have the potential to generate data and information to understand coastal processes using low-altitude UAV aerial photographs and Structure from Motion (SfM [SFM]) to generate DSMs comparable to ground surveys of the morphometry of a dune and beach system in Italy. Comparison of ground and aerial surveys using UAVs reveals the considerable potential of the latter to complete accurate surveys of dunes with less cost, time, and manpower requirements. The research used a non-standard UAV with a Digital Single Lens Reflex (DSLR), Real Time Kinematic (RTK) GPS, and autonomous flight control.
Turner et al. (2016) consider UAV technology to be a tried and tested technology for surveying work, specifically engineering applications, and to be cost-effective solutions for coastal zone applications. They also note that no more step changes in UAV technology or ease of usability are required. With the aid of RTK GPS, Turner et al. (2016) now consider this technology to be a practical, effective, and routine way to assess post-storm survey tool for coastal monitoring in Australia at spatial and temporal resolutions not previously possible. They also provide a brief overview of some of the recent UAV applications to coastal engineering and management.
Pereira et al. (2009) focus on the benefits of recent evolutions in UAV technology that include with autonomous take-off and landing capabilities for aerial gravimetry, aerial photography, surveillance and control of maritime traffic, fishing surveillance, and detection and control of coastal hazards. Such developments include advances in the distance and duration drones can fly for maritime surveillance in harsh coastal environments.
Goncalves and Renato (2015) have used UAVs imagery and photogrammetry to derive topographic information for coastal areas, whilst Long et al. (2016) also used UAV imagery to monitor the topography of a tidal inlet.