High-density airborne LiDAR sensor ideal for challenging coastal survey of Sylt

Bathymetric surveying offers a unique opportunity to capture LiDAR point clouds and images of the transitional zone comprising shallow water and coastal land. This information provides valuable insights into environmental conditions, supports research and planning to protect vulnerable locations and aids marine navigation.

Coastal surveys are greatly influenced by the quality of the water, which can be degraded by swirling sand, algae, and seaweed. Difficult conditions in the North Sea compound these issues and increase the importance of fast data collection and reliable operations. Finnish companyArctia-Meritaito Ltd. successfully completed an airborne topographic and bathymetric survey on Sylt, a popular German island known for its vacation resorts and 40-kilometre (km)-long sand beach, using aLeica Chiroptera 4X作为其测量工具。

Sylt Island in northern Germany at the border to Denmark

Threats to the island

The German island of Sylt位于的外缘Schleswig-Holstein Wadden Sea, between 9–16 km off the mainland. While other islands in the North Sea are surrounded by extensive intertidal sand and mud flats ─ called wadden ─ that protect the coastlines, the west coast of Sylt is completely unprotected and exposed to the forces of the water. Harsh weather conditions such as strong currents from tidal ebb and flow, frequent sea storms, and waves up to 10 metres (m) high continuously reshape the island, despite attempts to reduce erosion along its sandy beaches. Since the 1980’s, preservation efforts have focused on pumping sand back onto the beaches every year, rather than constructing unnatural concrete structures.

TheLandesbetrieb für Küstenschutz, Nationalpark und Meeresschutz Schleswig-Holstein(National Office for Coastal Protection, National Park and Marine Protection Schleswig-Holstein) is responsible for drawing up coastal protection plans and implementing appropriate measures. In recent decades, the organization has experimented with different technologies to identify the most effective methods of maintaining the coastlines.

In the past, surveying vessels carried out the bathymetric mapping of the seabed. For profile lines with a total length of approximately 1,100 km, about four weeks was required to allow for the daily varying tide. Mapping was complicated by an underwater reef about 400 – 500 m off Sylt’s west coast, where the depths rise from about -6 m up to 3 m. Outside the reef, the depths in the open North Sea fall again to a depth of about -10 m. This reef has always been an obstacle to measuring the whole area with a vessel due to potential damage to the ship and equipment.

2020年，选择了Arctia-Meritaito与Leica Chiroptera 4X空降测深扫描仪进行调查。脊翅目4X通常可以渗透到25 m，甚至在理想条件下达到30 m的深度。椭圆形扫描模式捕获了前后视图，以提供两个相同点的数据集，从而降低了由波浪引起的噪声并增加了深度穿透。另外，激光束的倾斜视图捕获了垂直对象的数据。尽管Sylt周围有浊度和波浪，但Chiroptera 4X成功穿透了10 m以达到海床，满足了所有项目要求。

SYLT的照片，取自调查飞机

“来自空中的测深激光测量是捕获Sylt沿海地区的一种更合适的方法，”国家沿海保护局，国家公园和海洋保护局Schleswig-Holstein的测量，地形，形态学办公室的测量负责人Lutz Christiansen说。“通过在涨潮时在有利的天气条件下进行调查，Arctia-Meritaito成功地通过飞机实现了所需的深度和准确性，而不是冒着与船只搁浅的风险。基于这些结果，将来将使用空中的测深度测量每三年进行测量。”

有限的机会窗口

收集机载的测深数据的最显着挑战是时间安排。完美的计时会产生一个快速处理的干净数据集，而糟糕的计时会导致一个非常嘈杂的数据集，该数据集无法满足规范，甚至根本没有数据。在SYLT周围的地区，有多个问题。太多的风会产生白水和波浪，而藻类漂浮在表面上，激光挡住了激光，而细沙子则从海床上搅动了数据收集。

“Being ready at the right time and making the best of the good weather window requires dedication and effort,” explains Mikko Ojala, head of airborne LiDAR bathymetry, Arctia-Meritaito. “The weather and water conditions can change very rapidly. We cannot perform topographic surveys when it is raining, and airborne bathymetric surveys are impacted by inflows of water from the land that increase turbidity. When conditions are finally right, the equipment needs to be functioning and ready to go.”

春季和秋季，当北海没有藻类和没有冰时，通常提供最有利的条件。但是，由细沙组成的海床材料总是在移动并导致浊度。随着海床经常发生变化，相隔数天收集的重叠数据将无法匹配，并且需要更长的时间来处理。至关重要的是要尽快覆盖整个项目区域以最大化一致性。为了捕获Sylt的西海岸以及南部和北端的地形和测深数据，Arctia-Meritaito的目标是2020年6月。两天内，总共五个飞行小时，团队收集了70平方公里。

Ojala说：“我们只有两次航班飞行了整个区域，两者之间有一天，而且海床的地形在航班之间发生了变化。”“这引起了处理挑战，因为在某些地方有两个海床，这些地方的数据重叠了。”

除了用于点云处理和QPS水文软件套件外，Arctia-Meritaito还使用了Leica Lidar调查工作室（LSS）to process all waveform and position data and incorporate four-band camera data from the Chiroptera 4X. Arctia has a LiDAR-specific processing setup based on recommendations from Leica Geosystems.

Survey results support analysis

To ensure high quality results that guide preservation decisions for the future, the surveying contract stipulated that measurements should only be carried out under favourable conditions (easterly winds). Deliverables included point clouds with classifications (1 m and 10 m grid) and orthophotos. The height accuracy had to be better than 20 cm and the positional accuracy better than 50 cm. Processing was completely done by Arctia-Meritaito, with results delivered about eight weeks after data collection.

“We achieved a minimum point density of 5 points per square metre (p/sq m) in the water, thus demonstrating the Chiroptera 4X fulfils general industry requirements in shallow water,” says Ojala. “In addition, simultaneous collection of the bathymetric and topographic point cloud along with aerial imagery is highly efficient and produces a seamless data set from the land to sea bottom.”

数据集确认了海岸线的状态，并允许进一步分析。与以前的测量相比，2020年的调查表明，自1980年代以来，添加到海滩上的沙子已促进了海岸的保护。卢茨·克里斯蒂安森（Lutz Christiansen）打算通过定期测量测量来继续监测SYLT。

“我对结果感到非常满意。Chiroptera 4X达到了必要的深度。”克里斯蒂安森说。“数据密度，准确性和处理类型满足了我们所有的要求。这是一个出色的项目，可提供西海岸和海岸地区的最新状态。现在，我们有一个数字地形型号（DTM），从+20 m的陡峭海岸降至-10 m的水深。”

Results of the airborne bathymetric survey with the Leica Chiroptera 4X

测深图的开发

Arctia-Meritaito专门研究水文图，球道维护和海上航行，并由一系列破冰船和测量船进行。该公司的根源可以追溯到芬兰海事管理局及其前任，因此拥有高技能的专业人士，熟悉芬兰群岛及周边地区不可预测的水域。

传统上，Arctia-Meritaito用配备了单光束回声器的容器对较浅的深度进行了调查，该容器以定义的间隔距离而不是完整的覆盖范围产生了深度曲线。这种方法在单个梁数据集中的调查线之间留下了显着的数据差距。

最近，Multibeam Echosounder解决了数据差距问题。但是，在浅水中，多束回声器并不总是有效的。当海底向传感器上升时，调查的侧面变窄。其他缺点包括，如果容器与未知的浅滩相撞，则增加了设备损坏或损失的风险，而回声器无法从水到土地上产生无缝数据。

2015年，芬兰水文办公室进行了一项试点项目，比较了几个机载的测深传感器的性能。该项目证明，机载激光雷达调查可以符合浅水中所需的映射标准。随后在2016年的一个项目有利地将机载激光雷达的结果与单光束和多束回声器进行了比较。为了适应不断变化的行业需求并在技术上保持最新状态，Arctia-Meritaito用机载的测深痛取代了其单光束回声器服务。

“There were other sensors in the 2015 pilot project, but we saw that the Leica Chiroptera 4X provided the most promising results,” says Ojala. “By replacing single beam vessels with airborne LiDAR, we get full coverage in shallow waters with depth penetration down to 25 m depending on the conditions, while minimizing the risk of losing sensors. Now we collect land and water in one effort, instead of executing a separate survey for the land portion.”

The Schleswig-Holstein national agency for coastal protection has been evaluating airborne bathymetric technology since 1992. By 2015, results from deep-sea airborne LiDAR sensors compared to other types of sea measurements showed that airborne LiDAR was suitable and able to penetrate the water up to 3x Secci depth. Secci depth refers to a measurement of the turbidity of water, i.e., the depth at which light can no longer penetrate the water.

Thanks to improvements in laser technology and more sophisticated algorithms, today it is possible to reach depths of -25 m or more with a shallow-water sensor, such as the Chiroptera 4X, previously attainable only with a deep-sea sensor or echosounder.