高密度机载激光雷达传感器非常适合挑战SYLT沿海调查

测深测量提供了一个独特的机会，可以捕捉包括浅水和沿海土地的过渡区域的激光点云和图像。该信息为环境条件提供了宝贵的见解，支持研究和计划，以保护脆弱的位置并帮助海洋导航。

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

对岛上的威胁

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（国家沿海保护办公室，国家公园和海洋保护办公室）负责制定沿海保护计划并采取适当的措施。最近几十年，该组织已经尝试了不同的技术，以识别维持海岸线的最有效方法。

过去，测量船进行了海床的测深图。对于总长度约为1,100 km的轮廓线，需要大约四个星期才能允许每日变化的潮汐。在Sylt的西海岸约400 - 500 m的水下珊瑚礁使地图变得复杂，那里的深度从约6 m升至3 m。在礁石外，开放的北海的深度再次落到约-10 m的深度。由于对船和设备的潜在损坏，该礁一直是用船只测量整个区域的障碍。

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

The 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.

“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.”

Spring and autumn, when there is no algae and no ice 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.

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 Survey Studio (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。

“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.”

空载液化测量的结果与Leica Chiroptera 4X

测深图的开发

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.

传统上,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.

Ojala说：“ 2015年的试点项目中还有其他传感器，但我们看到Leica Chiroptera 4X提供了最有希望的结果。”“通过用机载激龙代替单光束容器，我们在浅水中得到全面覆盖，深度渗透到25 m，具体取决于条件，同时最大程度地减少了失去传感器的风险。现在，我们一项努力收集土地和水，而不是为土地部分进行单独的调查。”

自1992年以来，Schleswig-Holstein国家沿海保护局一直在评估空中的测深技术。到2015年，与其他类型的海上测量相比，深海机载激光雷达传感器的结果表明，机载激光雷达适合3倍SECCI深度。SECCI深度是指对水的浊度的测量，即光无法再穿透水的深度。

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.