Airborne Bathymetric Lidar systems utilize aircraft as the platform performing laser scan for surveying the underwater topography. The spectral band used for this laser is green, which can penetrate water and backscattered on the sea floor. This technology is best suited for area characterized as shallow and low water turbidity. Comparing with sonar based methods, airborne laser hydrography is featured with high efficiency and particularly suitable for areas with shoals and other risks for navigation. In order to fully utilize advanced technology for bathymetric mapping in Taiwan, and collect the bathymetric information of Dongsha atoll, Ministry of Interior planned a surveying project in 2008 for mapping Dongsha atoll and Penghu. This project is granted to National Chiao Tung University for execution. The aerial survey was performed with an AHAB Hawkeye II lidar system installed on a SA-226 aircraft, in August and September of 2010. 500 square kilometers in Dongsha, and about 400 square kilometers in Penghu were surveyed at spatial density of 3.5 meter by 3.5 meter. An area of 20 square kilometers located in the east portion of the atoll is surveyed with 2x2 meter resolution at lower flying height. Both cross flight and overlaps are compared for quality control. Sonar survey is also conduction for further validation. The survey result meets the IHO 1b specification. This article reviews the aerial surveying, data acquisition and editing, accuracy assessment and validation. The critical issues are discussed for the reference of future development and application of airborne bathymetric lidar in Taiwan.

Seagrasses are as important indicator of water quality and pollution in the shallow-ocean zone and extracts from seagrasses can be used widely, leading them to a high value on both ecology and economy. Dongsha Island, surrounded by abundant seagrass meadow and composed of at least six species accounted for 10% of the world, is a main habitat for seagrasses in Asia. We combined field data of the seagrasses with environmental factors derived from LiDAR around the Dongsha Island to understand distributional characteristics of the seagrasses and to compare effects of water depth on seagrasses’ distributions. Our results indicated that, seagrasses were found in the depth of 0 to 6.8m, with the coexistence of 1 to 5 species. The highest number of seagrass species was coexisted in the depth of 1 to 3m while single or two seagrass species were often observed within depth of 1m. In the depth within 3m, the seagrass bed had the highest occurrence frequency, up to 93%, where the most sites had the leaf cover within 50% to 80%. We also found that the seagrasses Cymodocea rotundata and Thalassia hemprichii occupied the depths less than Cymodocea serrulata and Syringodium isoetifolium. Overall, the seagrasses encircled the Dongsha Island show specific preference, and their distributional characteristics and coexistence are affected most by water depth. The environmental data derived from LiDAR can be further useful on species distributions and ecological monitoring.

Airborne LiDAR Bathymetry has been proven as an ideal technique for safely and efficiently operating in both shallow water and tidal flat areas for 3-D topographic mapping tasks. Yet the crucial challenge for acquiring terrain of water bottom is to precisely capture the trajectory of laser pulse when travelling between different medium, in the air, water and at the water bottom. Therefore, the quality of mapping 3-D bottom terrain would depend on how well the waveform analysis on determining the timing hitting water surface and water bottom can be designated. The goal of this research is to focus on comprehending the reaction of laser pulse on water surface, mainly wind-induced capillary-gravity wave facets. In particular, laser scanning geometry and the wind factor that shape the waveform geometrically and radiometrically are analyzed and thus the better timing detection method can be advised. It is suggested that adopting 50% risetime point, among others, would render better timing detection results when operating under moderate to strong wind speeds. In addition, when provided with gentle wind speed, bathymetry scanned in a lower off-nadir angle would gain richer energy. Through this study, better understanding of laser-to-water interaction is realized. Last but not least, this work may contribute to highlighting the significance of water surface detection which current airborne LiDAR bathymetric systems overlook.

Airborne Laser Bathymetry (ALB), enabled by the combination of laser scanning system, inertia measurement unit (IMU), and global positioning system (GPS), is a recent achievement for coastal water mapping. It is especially suitable for generating the digital terrain model and monitoring the change of the tidal area. In this study, the Hawk Eye II system was employed for two coastal areas of Taiwan. And, the results were evaluated against independent multi-beam surveys. It shows that the bathymetry obtained by Hawk Eye II is consistent with that by multi-beam surveys.

Navigation requirement speeds up people’s activity to ocean exploration, from manual lead line survey to echo sounding, methods of sounding improving with time. Such as sonar transport energy by sound wave and airborne bathymetric LiDAR uses doubled frequency green laser to penetrate water. In the past, bathymetric chart can be surveyed by means of echo sounder; laser scanning applications in shallow water expands the coverage of depth measurement without limitation of operational safety. In this article, shipborne sonar and airborne bathymetric LiDAR is investigated and their theory and operation method are introduced.

Bathymetric Lidar uses near infrared and green band lasers to acquired the water depth and underwater topography. The spatial distance can be estimated from the return signals of the surface and the bottom of water. Dongsha and Penghu are the test areas. The test data are acquired by AHAB HawkEye IIb and processed by Coastal Survey Studio (CSS) software. This study analyses the surface definition, water turbidity and bottom topography. The quantitative analysis includes water depth, signal-to-noise ratio and types of sea bottom. The experiment indicates that depths affected by waveform appearances are not obvious due to the Time-varied Gain (TVG). The turbid water makes stronger volume backscatter and decreases the attenuation effect, even leads to the bottom echo undetectable with deeper water. Waveform appearance is varied with types of reflective material and geometry, as well as influencing the strength and pulse width, respectively. This study analyses the relationship between waveform acquired by HawkEye IIb and results processed by CSS software. The experimental results may provide some evidences for further processing such as bathymetric Lidar echo detection, point cloud classification, and others.