A New Approach
Sarah M. KandrotGeography Department, University College Cork
Monitoring changes in the morphology of coastal environments is important for understanding how they function as systems and how they can be most effectively managed to offer maximum protection of the coastal hinterland. The quick, precise, and efficient method of topographic data capture associated with a remote sensing (RS) technology called terrestrial laser scanning (TLS), also known as ground-based Light Detection and Ranging (LiDAR), facilitates improved monitoring of morphological changes to coastal environments over traditional survey methods. Terrestrial laser scanning systems are capable of providing extremely detailed 3-dimensional topographic information in the form of a “point cloud” – a densely packed collection of x,y,z coordinates that collectively represent the external surface (often the ground) of a surveyed area. Such detailed elevation information is useful for coastal research, resource management and planning, hazard and risk assessment, and evaluating the impacts of climate change and sea-level rise on the coast. This paper introduces TLS and its applications in a coastal setting and addresses some of the challenges associated with its use as a monitoring tool in vegetated coastal dune environments. Such challenges include optimising time spent in the field, working with large datasets, classifying simple and complex scenes, and analysing multi-temporal datasets.
Keywords: coastal monitoring, terrestrial laser scanning, coastal sand dunes
Coasts are perhaps the most active of all geomorphic environments, and learning how to adapt to this dynamism will be the first global challenge that we, as a society, will face as a result of climate change. Already coasts are experiencing the adverse consequences of hazards related to a warming climate and rising sea-level, including increased incidences of extreme storm events and increased coastal erosion (Meehl et al., 2007; Parry et al., 2007; Trenberth et al., 2007). In the US alone, coastal erosion is responsible for approximately $500 million per year in coastal property loss (Rabenhold, 2012). Coastal research is now more relevant than ever.
One important way in which coastal researchers study the coast is through morphological monitoring – the repeated collection of information about the topography of the submarine, intertidal, and terrestrial land surface. Traditional methods of monitoring coastal change have typically relied on low-quality, and often sparse, datasets, such as aerial photographs, historic maps, and beach profiles. Studies based on two dimensional cross-shore profiles, however, completely ignore the mechanisms by which beaches function in the third dimension (perpendicular and, especially, oblique to the cross-shore profile). In the case of aerial photographs and historic maps, while they may afford a greater coverage area, the information provided is at a lower spatial resolution (ie. it is less detailed) and may only be available at infrequent and irregular intervals.
Light Detection and Ranging, or LiDAR, technology, however, allows for much improved monitoring of coastal morphological change through the provision of three dimensional topographic datasets at spatial and temporal scales that were, until recently, unattainable. Its development has had major implications for coastal research and resource management and planning, with researchers having quickly identified a variety of applications for LiDAR remote sensing (Brock and Purkis, 2009). Such applications include using LiDAR datasets to quantify beach-dune morphological change (Ali et al., 2011; Feagin et al., 2012), to monitor sea cliff erosion (Lim et al., 2005; Rosser et al., 2005), to study Aeolian (or wind-driven) sediment transport (Lindenbergh et al., 2011; Nield and Wiggs, 2011; Nield et al., 2011), and to evaluate the vulnerability of low-lying coastal regions to flooding caused by relative sea-level rise (Gesch, 2009).
While LiDAR is seen as a “remarkable new asset” (Brock and Purkis, 2009, p. 1) to coastal researchers, its use as a morphological monitoring tool is an application still in the early stages. Significant challenges in this area remain, especially in terms of establishing methodologies and best practice standards for collecting and analysing ground-based LiDAR data. This paper will provide some background information on ground-based LiDAR (more commonly known as terrestrial laser scanning, or TLS) and its applications in a coastal setting. It will also provide a first-hand account of the challenges associated with using TLS as a morphological monitoring tool in a vegetated dune environment based on experiences from on-going research in Dingle Bay, Co. Kerry, Ireland.
Terrestrial Laser Scanning: Principles and Applications
Terrestrial laser scanning technology is used to collect extremely detailed 3D information about a surface. TLS sensors use LiDAR, an active RS technology that uses either a reflected laser pulse or, less commonly, differences in phase from a continuous beam, to measure the distance to an object (often the ground surface). Pulse-based sensors sweep millions of laser pulses across a surface and use the time it takes for those pulses to be reflected back to the instrument to measure the distance to the surface. There are two basic types of LiDAR sensors – airborne (fig. 1) and ground-based (fig. 2). Airborne systems are flown on an aircraft, and thus are capable of capturing data over a relatively wide area. They consist of three main parts: the sensor, the inertial measurement unit (IMU), and the global positioning system (GPS) which work together to produce georeferenced topographic data. Ground-based LIDAR sensors, or Terrestrial Laser Scanners (TLS), capture data from a (or, most commonly, several) fixed position(s) on the ground. Georeferencing is usually established through the use of a known benchmark, although newer models may have a built-in GPS and altimeter. The result of a LIDAR survey, airborne or ground-based, is millions of densely-packed 3-D points, each with a unique xyz coordinate, collectively known as a point cloud (fig. 3 and fig. 4). Information about the intensity, or strength of the reflected laser pulse, is also collected. In the examples shown in figure 3 and figure 4, points were measured with positional accuracy of 6 mm at a resolution of approximately 1 cm.
For the study of micro-scale morphologic change (anything less than <0.5 m), ground-based LiDAR is especially useful. Terrestrial laser scanners provide a far greater point density (>3 orders of magnitude) than airborne LIDAR, thus facilitating more complete capture of the spatial heterogeneity of a surface and making them ideal tools for the study of micro-scale morphological features (eg. Nagihara et al., 2004; Dunning et al., 2009) and processes (eg. Travelletti et al., 2008; Lindenbergh et al., 2011; Nield and Wiggs, 2011). Ground based laser scanners can be deployed for immediate data collection, and they are therefore more practical for performing multiple surveys over relatively short time periods. This is especially important in fast changing environments, such as on beaches, where near instantaneous events can result in large morphological changes (Lindenbergh et al., 2011).