Due to the specifics of the Baltic Sea, performing accurate measurements in the coastal zone is not an easy task. For the past decade, topographic laser scanners have been used for the periodic monitoring of the sea’s southern coast in Poland, in addition to profiles using GNSS RTK receivers. This article outlines the work to verify the feasibility and accuracy of using airborne Lidar bathymetry (ALB), both on the seabed and on land.
The southern coast of the Baltic Sea is characterized by great variability. Due to storms, sandy beaches are becoming wider and wider in some places, while in other places they are simply disappearing. The desire to prevent these effects has driven the need to permanently monitor changes and respond accordingly. Annual monitoring of the state of the sea shore is a statutory obligation, and is the responsibility of the directors of Poland’s Maritime Offices.
For almost a decade, the Maritime Office in Szczecin and the Maritime Office in Gdynia have commissioned flights using topographic laser scanners. Topographic scanners are hard to beat when it comes to tracking land changes; they enable the entire coastline to be mapped with high accuracy. Unfortunately, however, the laser pulse in the near-infrared range is not capable of penetrating water. This results in a lack of complete information about the coastal zone. Such information is not provided by height profiles either. Theoretically, a solution to this problem cou
The challenges of the Baltic Sea
Due to the specifics of the Baltic Sea, performing measurements in the Baltic coastal zone is not a simple task. Each measurement method has its own advantages and limitations, but they become particularly significant in the case of this body of water. For example, because the Baltic Sea is a shelf sea, it is particularly difficult to map its bottom in the coastal zone using multibeam echosounders. Hydrographic boats generally have a greater draught, so it is impossible to use them. On the other hand, the use of a shallow-draught survey boat requires measurements to be made in near-ideal weather conditions. Otherwise, the undulations of the Baltic Sea could cause the boat to capsize.
Figure 1: Comparison of mapping performance of a multibeam echosounder and ALB. (Image source: see below)
Therefore, the most common method of measuring the coastal zone of the southern Baltic Sea has been to make profiles using a GNSS receiver. The land surveyor’s task is then to make cross-sectional profiles, within which measurements are taken of the dunes, beach and in the water – as deep as possible. This technology is undoubtedly sufficient for the implementation of measurements over small distances, because it is extremely mobile and independent of weather conditions. However, the potentially high waves of the Baltic Sea pose a problem.
Low water clarity
Aerial bathymetric scanners are known to be ideal for mapping mountain lakes or clear oceans and seas. In fact, many suppliers’ promotional materials boast about their scanner’s ability to reach depths of several dozens of metres in clear water. But the Baltic Sea certainly does not fall into this category; it is a body of water with relatively low water clarity.
An additional factor affecting bathymetric measurements from an aircraft is wave action. The Baltic Sea can be turbulent, and the frequency of waves can be much higher than in oceanic waters. All of this makes taking measurements difficult, but not impossible. The key is to have a good understanding of the environment and constantly keep track of changing conditions in the air and on the water.
Figure 2: Average annual Secchi disc depths based on data from 1990-2005. (Image courtesy: Lindgren and Håkanson, 2007)
Testing the feasibility of ALB
Another factor affecting the popularity of a particular measurement method is its reliability. Technologies that have been known and well established for decades are approached differently than niche methods that are relatively new. Having said that, it should be noted that some first attempts to map the southern Baltic coast using ALB were made as early as 2007. Unfortunately, their results were not commensurate with the cost of acquiring the data at that time.
That trend began to reverse in recent years, when public contracts for monitoring selected sections of the Polish coast began to appear. GISPRO SA carried out one of the pilot projects in this regard under an order from the Maritime Office in Szczecin. The purpose of the undertaken work was to verify the feasibility of using ALB on the Baltic coast and, more importantly, to verify the accuracy of measurements using ALB both on the seabed and on land.
Technical conditions and equipment
Profiles were to be taken at least every 200m from the dune/cliff crest, across the beach and under the shore, up to a distance of 500m into the sea (from the shoreline). Measurements in profile were to be taken every 5-10m and at characteristic locations. The measurement process was to be a continuum, using a GNSS RTK receiver on land and shallow water, and a singlebeam echosounder while in the water. Aerial scanning was to be performed for the entire 10.4km stretch of the coastline, both for the land surface and the bottom, with a density of no worse than 6pts/m2 for the last return.
A Riegl VQ-880-G II was chosen as the airborne laser scanner, which emits laser pulses in the green (532nm) and near-infrared (1,064nm) ranges. It has integrated PhaseOne XM-100 cameras, and is also equipped with an advanced Trimble Applanix AV-610 GNSS positioning system with a high-end IMU-57 inertial unit. The SonarMite BTX singlebeam echosounder (SBES), integrated with a Trimble R8 GNSS RTK receiver, was installed on a lightweight survey boat. A Trimble R12 GNSS RTK receiver was used to measure profiles on land and in the shallow water.
Measurement campaign and products
The measurements were taken on 30 October 2022 under good weather conditions. The sky was overcast with clouds, but there was no precipitation. The wind blew at a speed of only 1m/s. Bathymetric laser scanning was performed from an altitude of 530m AGL at a speed of 100kn.
The result was a point cloud with a minimum density of 6.42pts/m2. Measurements using the SBES were made after calibration using a calibration bar. After sound velocity profiling (SVP), the average velocity in water was determined to be 1,465m/sec. After data processing, photo sketches were prepared showing profiles taken with the GNSS RTK receiver and SBES, RGB CIR orthophotos and a georeferenced point cloud for the coastal area and the underwater part.
Figure 3: ALB point cloud with GNSS RTK-SBES elevation profiles. (Image courtesy: GISPRO SA)
Evaluating ALB accuracy
As part of the work carried out, the accuracy of measurements taken by various techniques was verified. To this end, after classifying the point cloud from the bathymetric scanner, a digital terrain model and a digital bottom model were made. Points measured with a GNSS receiver on land and GNSS working with a singlebeam echosounder were then dropped onto the surface. In this way, the height coordinates of the same points were obtained using different measurement technologies. Thus, it became possible to compare the values obtained (see Table 1).
Table 1: Comparison of the values obtained using different measurement technologies.
Conclusions
It is particularly challenging to take bathymetric measurements in shallow water because the use of traditional surveying techniques poses many problems. The shallow depth essentially eliminates the point of using a multibeam echosounder. The scanning strip of the echosounder would be small, and the echosounder itself could be damaged by entering a shallow area, or by an object lying on the bottom.
Therefore, the only alternative remains to make profiles using a singlebeam echosounder and a GNSS RTK receiver. Unfortunately, this method is fraught with very poor performance, and its results are limited only to depth information within the profile. In the test area, it was recorded that the time required for land-water profiles was 40 human-hours. In contrast, the flight time using an airborne bathymetric scanner was about 1 human-hour.
One major drawback of ALB technology is the limitations associated with its use. Both suitable atmospheric and environmental conditions are necessary. Cloud cover, wave action, wind direction and strength, and water clarity are important. It is crucial to conduct recording at the moment of least contamination with organic and inorganic matter above the bottom.
Regardless of the above, bathymetric laser scanning was demonstrated to obtain the most complete image of the shallow-water area in a relatively short time. This made it possible to create a reliable model of the bottom containing complete information. As a result, it is easier to not only identify objects lying in the water, but also track changes resulting from the activity of the sea.
The positive results obtained from the pilot project undoubtedly contributed to the tenders for monitoring the entire Baltic coast with airborne Lidar bathymetry. As a result, GISPRO SA has already had the opportunity to map the entire coastline for both the Maritime Office in Szczecin (in 2023) and the Maritime Office in Gdynia (in 2024). During the work carried out, in addition to satisfactory mapping of coastal geometry and the seabed, it was even possible to scan some wrecks. One example is the wreck of the concrete ship Karl Finsterwalder off the coast of Wolin Island.
Figure 4: The wreck of the concrete ship Karl Finsterwalder, captured in the ALB data. (Image courtesy: GISPRO SA)
Acknowledgements
These works were carried out using equipment and knowledge obtained during the ‘Research and development works on the creation of a complete, multimodal mapping system for the needs of inland and sea waterways and exploitation areas’ project, number POIR.01.01.01-00-1372/19, funded by The National Centre for Research and Development, Poland.
Figure 1 is sourced from the article titled Przegląd współczesnych metod satelitarnych i lotniczych wykorzystywanych w mapowaniu dna morskiego (Review of contemporary satellite and aerial methods used in seafloor mapping), originally written in Polish.
Further reading
Average annual Secchi depths
