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.

沿海调查受到水质的极大影响，水的质量可以通过旋转的沙子，藻类和海藻来降解。北海的困难条件加剧了这些问题，并增加了快速数据收集和可靠操作的重要性。芬兰公司Arctia-Meritaito Ltd。成功完成了一项空中地形和测深的调查，该调查是一个流行的德国岛屿，以其度假胜地而闻名40公里（km）的沙滩，使用Leica Chiroptera 4Xas its surveying instrument.

Sylt Island in northern Germany at the border to Denmark

Threats to the island

德国赛尔特岛位于Schleswig-Holstein Wadden Sea的外边缘，距离大陆9-16公里。尽管北海的其他岛屿被广泛的潮间砂和泥平围绕着保护海岸线的瓦登（Wadden），但西尔特（Sylt）的西海岸完全没有保护，并暴露于水力。尽管试图减少沿着沙滩沿线的侵蚀，但潮汐潮流和流动的强烈潮流，频繁的海风暴和高达10米（m）的狂潮，持续不断地重塑了岛上的恶劣天气条件。自1980年代以来，保存工作一直集中在每年将沙子泵回海滩上，而不是建造不自然的混凝土结构。

这LandesbetriebfürKüstenschutz，国家公园和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.

In 2020, Arctia-Meritaito was selected to survey Sylt with a Leica Chiroptera 4X airborne bathymetric scanner. The Chiroptera 4X can typically penetrate down to 25 m and has even reached 30 m depth in ideal conditions. The elliptical scan pattern captures a forward and backward view to provide two data sets of the same point, which reduces noise caused by waves and increases depth penetration. In addition, the oblique view of the laser beam captures data of vertical objects. Despite the turbidity and waves around Sylt, the Chiroptera 4X successfully penetrated 10 m to reach the seabed, meeting all project requirements.

Photo of Sylt, taken from the survey aircraft

“Bathymetric laser surveying from the air is a more suitable method to capture the coastal area of Sylt,” says Lutz Christiansen, head of surveying, topography, morphology at the National Office for Coastal Protection, National Park and Marine Protection Schleswig-Holstein. “By conducting the survey under favorable weather conditions at low tide, Arctia-Meritaito successfully met the required depth and accuracy with an airplane rather than risking running aground with a ship. Based on these results, the measurements will be performed every three years in the future using airborne bathymetry.”

Limited window of opportunity

这most notable challenge of collecting airborne bathymetric data is timing. Perfect timing produces a clean data set that is fast to process, while bad timing results in a very noisy data set that does not fulfil the specification, or even no data at all. In the area around Sylt, there are multiple issues. Too much wind makes white water and waves, while algae floating on the surface blocks the laser and fine sand stirred up from the seabed interferes with data collection.

“在正确的时间,做最好的准备好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.”

春天和秋天,当没有藻类,也没有我ce in the North Sea, typically offer the most favourable conditions. However, seabed material consisting of fine sand is always moving and causing turbidity. As the seabed is frequently changing, overlapping data collected many days apart will not match and will take longer to process. It is crucial to cover the whole project area as quickly as possible to maximize consistency. To capture topographic and bathymetric data of Sylt’s west coast and south and north ends, Arctia-Meritaito targeted June 2020. In two days and a total of five flight hours, the team collected 70 sq km.

“我们整个飞行area with only two flights, with one day between, and still the seabed topography had changed between the flights,” says Ojala. “This created a processing challenge because there were two seabeds in some places where the data from the previous flight overlapped.”

In addition to Terrasolid for point cloud processing and the QPS hydrographic software suite, Arctia-Meritaito usedLeica LiDAR Survey Studio (LSS)处理所有波形和位置数据，并合并来自Chiroptera 4X的四波段摄像机数据。基于Leica Geosystems的建议，Arctia具有LIDAR特定的处理设置。

调查结果支持分析

为了确保高质量的结果指导未来的保存决策，调查合同规定，仅应在有利的条件下（东风）进行测量。可交付成果包括分类（1 m和10 m网格）和正射击的点云。高度准确性必须高于20厘米，并且位置精度优于50 cm。加工是由Arctia-Meritaito完全完成的，结果在数据收集后约八周提供。

Ojala说：“我们在水中达到了每平方米5点（p/sq m）的最低点密度，因此证明了Chiroptera 4X满足浅水的一般行业要求。”“此外，同时收集了测深和地形点云以及空中图像的效率高度高效，并产生了从陆地到海底的无缝数据集。”

这data set confirms the status of the coastline and allows for further analysis. When compared with previous measurements, the 2020 survey indicates that the sand deposits added to the beach have contributed to the protection of the coast since the 1980s. Lutz Christiansen intends to continue monitoring of Sylt with periodic bathymetry measurements.

“I am very satisfied with the results. The Chiroptera 4X reached the necessary depth,” says Christiansen. “The data density, accuracy and type of processing met all of our requirements. It was an excellent project that provides an up-to-date status of the west coast and shore area. Now we have a digital terrain model (DTM) from the steep coast of +20 m down to -10 m water depth.”

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

Development of bathymetric mapping

Arctia-Meritaito specializes in hydrographic mapping, fairway maintenance and maritime navigation, carried out with a fleet of ice-breaking ships and surveying vessels. With its roots reaching back to the Finnish Maritime Administration and its predecessors, the company has highly skilled professionals familiar with the unpredictable waters of the Finnish archipelago and surrounding region.

Traditionally, Arctia-Meritaito surveyed shallower depths with vessels equipped with a single beam echosounder that resulted in depth profiles at defined interval distances rather than complete coverage. This approach left significant data gaps between the survey lines in a single beam dataset.

More recently, the multibeam echosounder addressed the data gap issue; however, a multibeam echosounder is not always efficient in shallow waters. The survey swath gets narrower as the seabed rises toward the sensor. Other disadvantages include an increased risk of equipment damage or loss in shallow water if the vessel collides with an uncharted shoal, and the echosounder cannot produce seamless data from water to land.

In 2015, a pilot project was conducted by the Finnish Hydrographic Office that compared the performance of several airborne bathymetric sensors. This project proved that airborne LiDAR surveys could meet required mapping standards in shallow water. A subsequent project in 2016 favourably compared airborne LiDAR results to single beam and multibeam echosounders. To adapt to changing industry demand and remain technologically up to date, Arctia-Meritaito replaced its single beam echosounder service with airborne bathymetric LiDAR.

“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.”

这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.

得益于激光技术的改进和更复杂的算法，如今，可以使用浅水传感器（例如Chiroptera 4X）达到-25 m或更多的深度，以前只能使用深海传感器或回声器才能实现。