Mapping underwater terrain with bathymetric LiDAR

Author:Reka Vasszi

Measuring water depth for nautical purposes dates back to ancient civilisations. As technology has evolved through centuries, besides ship-based echosounding, remote monitoring and mapping of water bodies became more available via LiDAR bathymetric surveys conducted by airborne systems.

测深量调查使专业人员可以测量水深，绘制水下地形，对淹没的植被和栖息地进行分类，以及研究海洋生态，水质，污染物溢出和流体动力学。

借助4倍的测深产品线，Leica Geosystems引入了一种创新的高分辨率技术，用于浅水和沿海地图，与以前的机载系统版本相比，点密度增加了四倍。这Leica Chiroptera 4X, bathymetric and topographic LiDAR system, captures140,000 points per secondand covers shallow water regions down to a25米深。This efficient coastal survey LiDAR sensor, designed to offer more accurate data for environmental monitoring, research and surveying even in turbid water demonstrated its efficiency mapping Cape John peninsula in Canada and the Tonga islands in the Pacific.

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Improving shallow water detection

Dr. Tim Webster, research scientist at the Applied Geomatics Research Group (AGRG) from Nova Scotia Community College, was among the first to use the new Chiroptera 4X to survey the coastal zone of Cape John peninsula in Canada.

使用传统的航空摄影或基于船基的回声声音的浅水和沿海地区的映射浅水和沿海地区可能会昂贵，由于水的清晰度和不可预见的天气条件而耗时和挑战。为了克服这些挑战并提高生产率和数据准确性，韦伯斯特决定在他的地球研究中使用Chiroptera 4X测深和地形激光雷达。

这purpose of the geomatics research was twofold. First, the team mapped thebenthic habitatand the existing aquaculture in the bay, including the mapping of the shellfish infrastructure and the buoys to estimate how much biomass is being grown.

这second goal of the research was to开发流体动力模型to help the lease of new oyster farms and propose the appropriate locations around the bay without damaging the sensitiveZostera marina或所谓的鳗草。鳗草床对于许多鱼类和贝类的沉积物沉积和托儿所很重要，因此它被加拿大渔业和海洋（DFO）用作生态系统健康指标。

韦伯斯特（Webster）于2014年在加拿大海事领域进行了类似的研究，以了解如何使用Leica Chiroptera II来优化浅水地形 - 伴侣型激光雷达调查的数据收集和后处理。这次，教授及其团队有机会部署新的Chiroptera 4X，并将捕获的数据与2014年的结果进行比较。quantify the improvement of the point cloud density and target identificationof the new Chiroptera 4X, the team compared the captured data of three surveying instruments:

• Chiroptera II
• Chiroptera 4X
• 多梁回声声音。

该地理学实验涉及对不同水深的四个1立方米立方体的目标测量，以确定三个不同传感器提供的细节和数据水平。

Flying the Chiroptera 4X over the bay, the research team collected information on the surveying targets and compared the captured data of the terrain and seabed from 2014 with the data from this experiment carried out in 2018. Using the four cubes and other flat targets captured with the three different survey methods, researchers compared:

• Point cloud density
• Orthophoto mosaics
• 数字高程模型
• 激光振幅。

这analysis of the captured data was completed using the discrete points derived from the waveform data inLeica LiDAR Survey Studiopost-processing software for point cloud generation and cleaning of raw LiDAR data.

“This particular experiment was to quantify the accuracy and level of detail of the Chiroptera 4X and we are very pleased with the results,”reported Webster.“结果表明，与其他设备相比，点密度有显着增加，改善了目标细节，检测极限以及对更直接的底栖点分类的潜力。”

利用Chiroptera 4X的近红外激光进行地形数据收集，并结合了用于测深数据收集的绿色激光器，研究团队精确地捕获了水下功能和生成的虚拟现实抬高模型，以研究底栖生物栖息地。此外，研究人员提出了牡蛎养殖的可持续地点，而不会损害浮雕栖息地。

“This advancement of Chiroptera 4X is asignificant breakthrough in terms of point density它改善了我们沿海地区映射的分辨率，”said Webster.“In any application where target detection is important, I would see the increased benefits of the Chiroptera 4X in industries like hydrography and archaeology.”

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Safe and reliable passage through Pacific waters

这Pacific is an extremely culturally diverse region, as exemplified by the more than 1,000 languages spoken and, yet, what connects them is the water. The ocean and coastal seas have long been integral to the Pacific way of life. The economy, transportation and culture of those sharing the Pacific waters is based on the naval and marine infrastructure and ecosystem this ocean provides. Even though marine navigation plays a key role in the archipelago, theSouth Pacific is inadequately charted。

Shallow water depth range data is critical for safe marine navigation and for construction of harbours, pipelines and any other infrastructure in the coastal area or shallow offshore zones. Without precise information on water depth and the exact location of underwater features and submarine canyons, economic and sustainable development is negatively impacted.

iXBlue together with project partners Geomatics Data Solutions (now Woolpert, Inc.) and EOMAP Australia were requested to map the Tongan archipelago and surrounding areas to provide the 3D water depth information and improve the nautical chart of the region.

该项目是太平洋地区航海计划（PRNI）的一部分，这是一个旨在通过太平洋安全通道的总体计划，同时保护环境并允许太平洋岛屿经济发展。该项目由新西兰外交与贸易部（MFAT）与新西兰土地信息（LINZ）合作。

这Kingdom of Tonga, a Polynesian archipelago comprising of169 islands，大约延伸800 kilometresin the South Pacific Ocean. In Tonga, agriculture, fishing and forestry provide the majority of employment, therefore, detailed data on both land and shallow water features can support the sustainable development of the island group.

Considering the relatively large area in need of high-resolution survey and charting, LINZ required a smart solution to bring the most out of the tight budget. iXBlue, together with EOMAP Australia and Geomatics Data Solutions, applied amulti-sensor approach to collect topographic and bathymetric data, including:

• Satellite-Derived Bathymetry (SDB)
• 机载激光测深（ABL）
• Multibeam Echosounder (MBES)
• Tide gauge install and datum computations.

SDB surveys are very effective tools in mapping large areas to the visible water depth, however to捕获土地和水下的高分辨率数据the Chiroptera 4X ALB was used.

这Chiroptera 4X was installed in a Cessna 441 to acquire data for Tonga and Niue, including Beveridge Reef, covering approximately a633平方公里的区域。地理数据解决方案专家在岸上和海上收集了高分辨率的测深和地形数据，并将roptera 4X的结果与SDB进行了比较。

“Having the extra density of the Chiroptera 4X, compared to the Chiroptera II, allows us to run spatial algorithms while reducing the risk that we are accidentally removing seabed objects from the dataset. In other words, it improves our target recognition probability,”Geomatics数据解决方案的Carol Lockhart说。

Beside the bathymetric data of the seafloor, one of the unexpected benefits Geomatics Data Solutions gained from using Chiroptera 4X was the improved penetration of the denser bathymetric laser data on land, providing full coverage even on dense vegetation where the topo laser did not penetrate.

“当地利益相关者将得到完整的图像和the topo-bathy dataset, so they can also benefit from the high-resolution data provided with the new Chiroptera 4X, and can use the data for other purposes in addition to improving the nautical charts,”explained Lockhart.

这combined approach of different surveying equipment提供了所有人的完整知识导航 - 重要功能和其他地形信息的岛屿。由于4 x技术newly-released Chiroptera,experts surveyed at four times the sounding density than it was previously possible。