Plitvice Lakes National Park (cro. Nacionalni Park Plitvička jezera) is the oldest and largest national park in the Republic of Croatia. The park is located in the mountainous region of Croatia, as the map in Figure 1
shows. With its exceptional natural beauty, this area has always attracted nature lovers, and as early as 8 April 1949 it was declared the first national park in Croatia. Dominik Vukasović, the parson of the Plitvice Lakes area in 1777, was the first to mention the name “Plitvička jezera”. The name is most likely derived from the Croatian word for shallow water (pličina, or plitvak) [1
The process of tufa formation, which results in the building of the tufa, or travertine barriers and thus to the creation of the lakes, is the outstanding universal value for which Plitvice Lakes were internationally recognised on 26 October 1979 when they wew inscribed on the UNESCO World Heritage List UNESCO. In 1997, the boundaries of the National Park were extended, and today it covers an area of almost 300 km2
The lake system consists of 17 named and several smaller unnamed lakes, which cascade into each other. Due to the geological substrate and the characteristic hydrogeological conditions, the lake system was divided into the Upper and Lower lakes. The twelve lakes that form Upper Lakes are Prošćansko jezero (marked 1 in Figure 1
), Ciginovac (marked 2 in Figure 1
), Okrugljak (marked 3 in Figure 1
), Batinovac (marked 4 in Figure 1
). Veliko jezero (marked 5 in Figure 1
), Malo jezero (in Figure 1
marked with 6), Vir (in Figure 1
marked with 7). Galovac (marked 8 in Figure 1
, represented in Figure 2
), Milino jezero (identified by 9 in Figure 1
), Gradinsko jezero (identified by 10 in Figure 1
is 10), Burgeti (in Figure 1
is 11) and Kozjak (in Figure 1
is 12). These lakes were formed on impermeable dolomite rock and are larger, with more articulated and gentler shores than the lower lakes. The Lower lakes, consisting of the lakes Milanovac (marked 13 in Figure 1
), Gavanovac (marked 14 in Figure 1
), Kaluđerovac (marked 15 in Figure 1
), Kaluđerovac (marked 16 in Figure 1
) and Novakovića Brod (shown as 17 in Figure 1
), were formed in permeable limestone substrate cut into a deep canyon with steep cliffs. The lakes end in the impressive Sastavci waterfalls, with the Korana River springing from the base of the falls.
In Plitvice Lakes National Park there are partitions between the lakes, over which waterfalls and cascades fall, that occur in a special natural way. Under certain physicochemical and biological conditions, they form tufa and settle on the lake bottom and on submerged objects. Tufa forms underwater thresholds and barriers that rise above the water and grow steadily in height and breadth. This constant tufa formation with warm climate, lush vegetation and undisturbed natural balance was the first factor that made Plitvice Lakes part of the World Natural Heritage [5
Tufa is deposited on the bottom of Plitvice Lakes in the form of microscopic crystals that cover the lake bottom with a thick layer, thus preventing water loss through the perforated karst carrier. Thanks to this, the large Upper Lakes (Lake Prošćansko and Lake Kozjak), which lie on dolomite substrate, do not lose water, but after the beginning of the flow of Korana River, water is lost underground because the travertine processes stop [5
Today, in the area of Plitvice Lakes National Park it is possible to find places where travertine is absent or reduced in intensity. Scientific studies have shown that the increased amount of dissolved organic matter (pollution) stops the travertine creation processes at Plitvice Lakes [1
]. Thus, the inevitable process of eutrophication or the aging process of a lake takes place. This is a natural process that usually lasts thousands of years. However, it can be significantly accelerated by human activities (agriculture, livestock, tourism, sewage, settlements and hotels) [5
Eutrophication is the process of enriching water with organic nutrients that promote the growth of aquatic plants such as planktonic algae, bottom algae and other aquatic plants. Plitvice Lakes are covered with swamp vegetation and lake bottoms are covered with underwater meadows. All this vegetation lays on travertine barriers. Sometimes there are even trees growing on the edges of waterfalls whose weight endangers waterfalls’ stability and threatens to cause waterfalls to collapse. The eutrophication itself is a normal natural aging process of the lakes over thousands of years, while anthropogenic eutrophication is caused by human activities and can destroy the aquatic ecosystem in a very short time. Obviously, Plitvice lakes have been affected by the process of anthropogenic eutrophication in recent decades [5
Depth measurements and the characterisation/classification of the lake bottom are of utmost importance for research of tufa formation and eutrophication processes in the lakes. Major Franz (Franjo) Bach was the first to carry out depth measurements of Plitvice Lakes in 1850 [1
]. His work was improved by contributions from Franić in 1910 [2
], Gavazzi in 1919 [6
], and Petrik in 1958 [7
]. All these studies involved measuring depths directly by a rock tied to a wire, which was then thrown from a small boat at as many points as possible. These measurements were very valuable for their time, but they were not georeferenced and the depth measurements were much less accurate compared to modern indirect depth measurement methods.
It is only in the last 20 to 30 years that we have been able to explore and map the underwater world of the Earth, mainly through technical advances such as acoustic remote sensing. Models based on acoustic data can be used to estimate how underwater locations have changed both recently and far in the past but also to predict how they might change in the future [8
]. These models can then be used as powerful tools in public commitment to environmental protection and conservation. Nothing has advanced in underwater technologies and research areas as much as localisation and environmental imaging devices. Recently, photogrammetry, photo modelling, simultaneous localisation and navigation (SLAM), organised light processing, multibeam and numerous other acoustic sensing techniques have become ubiquitous [10
]. In 2007 and 2010 Pribičević and his colleagues bring modern acoustic methods of bathymetric measurements of the two largest lakes in Plitvice, namely Kozjak and Prošćansko lake [5
Robotics is developing into a successful tool in the exploration of shallow waters and offers a wide range of surveying possibilities (2.5D positioning or landscape mapping without excavation) [15
]. Human operators/data collectors combine high versatility, intelligent coordination and a wide variety of manual skills to reduce operating costs. However, these human advantages tend to disappear as the area to be explored grows or the time required for field operations becomes shorter. In addition, robots can endure longer survey missions without the fatigue that humans are prone to, they can move faster, deeper and higher, they do not risk human injury or fatalities and their deployment is more cost-effective in the long run. They can also be used very well in the cold as well as low visibility/turbid environments and they usually have an array of localisation/survey sensors whose sensory range is much greater than that of a human and/or DSLR camera.
A few decades ago, precise robot localisation was a major challenge and a great stepping stone to the use of autonomous robots for detailed and precise surveying missions in unstructured environments. Navigation and localisation are among the most demanding problems in the production of autonomous vehicles. However, the use of surface vessels based on a combination of Global Navigation Satellite System (GNSS), Inertial Measurement Unit (IMU) and/or Dopller Velocity Logger (DVL) navigation in shallow underwater archaeology can solve these problems. In contrast to the slow acoustic communication channel required underwater, a surface vessel often provides a fast wireless communication link to the base.
Robot localisation equipment and algorithms have improved significantly, allowing the experts using the robots to acquire acoustic/visual data with precise position markers and feed it directly into commercially available bathymetry and/or photogrammetry software. All of the technological improvements mentioned above have led to a significant increase in the use of robotic systems for surveying underwater sites, either with or instead of divers. These robotic systems include remotely operated vehicles (ROVs), autonomous surface vehicles (ASVs), autonomous underwater vehicles (AUVs) and unmanned aerial vehicles (UAVs) for inspections in shallow water. The applications of these vehicles range from underwater archeology, biology, ecology, geology, hydrography, safety and many others.
There are many understandable logistical limitations to working in Plitvice Lakes National Park. Due to the minimisation of the carbon footprint that people leave behind in the National Park, any type of gasoline-powered vehicles are strictly prohibited, so only electrically powered vehicles come into play. Even access to the lakes with all the research equipment can sometimes be extremely strenuous. The morphology of the terrain is very rough and steep. Around some lakes there are forest paths only a few meters wide, while some of the lakes are crossed by quite narrow wooden footbridges. The installation of a multibeam sonar on a small boat with electric motor drive seems to be a simple but effective solution. However, the main problem with this human-operated surveying approach is that the planning and execution of surveying missions is prone to human error and could therefore result in some parts of the lake bottom not to be covered by the sonar.
All this leads to the conclusion that it seemed ideal for the research purposes of such a national park to design and use a small, lightweight, electrically driven ASV with engines for such purposes. It would be able to be used by only two people in the lakes, could be easily moved around the lakes on a small trolley, could be transported in a passenger car and could be manufactured with a low-cost, low-power electrically driven thrusters since the water of the lakes would not interfere with the movement of the vehicle. Multibeam sonar mounted on such a vehicle could provide high-resolution acoustic data that could be used for hydrographic, geological and biological research purposes. The coverage of the lake bottom by the sonar would be ensured by the mission plan, which the vehicle would carry out completely autonomously.
This article encompasses the methodology and results of the authors’ recent activities concerning the use of autonomous vehicles for acoustic recording (bathymetric mapping) of most of the Plitvice lakes. The main contribution of this article is the fact that for the first time in history these lakes were recorded by an autonomous vehicle using a multibeam sonar. The authors used an ASV specifically designed for bathymetric surveys of lakes and shallow marine waters and a UAV for visual data acquisition. The results of the postprocessed collected data are presented as a bathymetric map/2.5D model and/or visual recordings by the ASV and the UAV. During the monitoring missions in 2019, a total of 11 of 16 lakes were surveyed with a resolution of 20 cm, together with the lakes Kozjak and Prošćansko, which were previously surveyed by one of the authors in [5
]. The methodology of covering regular and irregularly shaped lakes by sonar swaths is presented together with digital elevation maps (DEMs) for the 11 surveyed lakes. In addition, numerous depressions in tufa at the lake bottom were detected in bathymetric but also in backscatter data. It is assumed that these are natural karst terrain formations. Their detailed exploration in the future with various other sensors such as sub-bottom profilers, cameras mounted on an AUV or an ROV as well as acoustic Doppler current profilers (ADCPs) is necessary for a deeper understanding of this intricate and geologically very dynamic lake system.
The rest of this article is structured as follows: a general overview of the authors’ activities in recent years, including research and technical projects, and the publications contained therein can be found in Section 2
. The autonomous robotic vehicles used for underwater photography and their sensors are presented in Section 3
. The methodology of remote sensing work applied to geological applications is presented in Section 4
. Section 5
presents the results of the Plitvice Lakes’ bathymetric survey missions. The results and future work are discussed in Section 6
. Concluding remarks are given in Section 7
In March and April 2019 extensive bathymetric measurements were carried out on the lakes in Plitvice Lakes National Park in Croatia. The aim of the depth measurements of the lakes is to enable a detailed environmental monitoring of tufa formation and changes over time. The bathymetric measurements were performed by ASV PlaDyBath. In total, three of four of the Lower Lakes and eight of twelve of the Upper Lakes were surveyed.
These lakes have never been surveyed with sonar technology, with the exception of the largest lakes Kozjak and Prošćansko jezero, whose bathymetric surveys were carried out by National Park and its partners some years ago [5
]. The remaining lakes were either too shallow for safe operation of the ASV PlaDyBath or the deployment of the vehicle was too complicated due to the terrain configuration. Challenging work conditions (two-person deployment of the ASV from a narrow forest trail and a makeshift workstation that had to be moved around) at Plitvice Lakes National Park are depicted in Figure 6
. Processed sonar data was delivered in a form of digital elevation maps (DEMs) to National Park Plitvice for further analysis and research of the water column.
A high-resolution georeferenced orthophoto image based on the camera images of the unmanned aerial vehicle (UAV) was used as a basis for the planning of survey missions, as shown in Figure 7
. The methodology to generate such a large-scale orthophoto is partly described in [42
]. This model was calculated before the bathymetry survey missions to serve as a crucial input for the safety of the ASV PlaDyBath. The outlines of the lakes were extracted from these precisely geolocalized orthophoto models with an offset of about 5–10 m from the shore to avoid shallow parts or other potentially dangerous obstacles along the shore, as shown in Figure 8
a and Figure 9
a. Particularly frequent obstacles were the tree trunks that fell into the lakes and were not moved by the National Park staff, as any human intervention in the National Park is strictly prohibited. It can also be noted that the outline of the exact lake boundary does not match the map background from the map loaded in Neptus, because the resolution of the Neptus map layer is not sufficient to recognise the boundaries of the lakes.
The missions were planned as follows: Plan the first mission along the perimeter of the lake, with the sonar beams tilted 30–60° towards the shore and the swath angle at 90–120°. The rest of the lake was covered with a sonar tilt of 0° (looking directly under the vehicle) and a swath angle of 90°. The ping rate of the sonar was set to adaptive in order to obtain as much quality data as possible. This meant that the sonar pinging frequency increased in shallow areas and was automatically reduced in deeper parts of the lake. The ASV’s surge speed was set at 0.7 m/s, which has proven to be a good compromise between energy consumption and surveying time during a number of survey missions. Depending on how shallow/deep a lake is, the rest of the survey for this lake was carried out as follows.
When a lake had a more or less convex shape, as is the case with Malo jezero shown in Figure 8
a, the interior of the lake was partially covered by the so-called lawn mowing manoeuvres with variable lawn mower cross-section widths. In the shallowest parts of the lake denser transects were used, while in deeper parts of the lake wider transects were used, as shown in Figure 8
If the lake was deep or had a rather nonconvex shape, as is the case with Lake Ciginovac (see Figure 9
a), the interior of the lake was covered with concentric, bank-shaped missons, as shown in Figure 9
b. For this type of nonautomated mission planning, it is essential that the vehicle is in WiFi range throughout the time of each circumferential pass, in order to check the depth at each waypoint it passes. The inward offset of the next concentric mission from the waypoints of the current mission is calculated as
, where d
is the inward offset, h
is the depth read by the WBMS software at the specific waypoint reached by the vehicle in the current mission,
is the sonar swath angle and
is the overlap percentage of adjacent sonar swaths. We have used
, which gives
. We also smoothed sharp curves as much as possible to reduce the fan-out effects in the sonar footprint, which result in much sparser sonar pings on the outside and unnecessary multiple overlap of the pings on the inside of the turn towards the centre of the lake.
While PlaDyBath performs its tasks autonomously, the operator processes sonar data in low resolution using the Qimera software to monitor the quality of coverage. As soon as the batteries are depleted (approx. every 1.5–2 h), the vehicle was lifted to the lakeshore, the batteries were changed and high-quality sonar data were transferred to the computer of the ASV operator. Initially, the break in surveying operations was used not only for data transfer but also for passive cooling of the Applanix SurfMaster navigation system. During the 1.5–2 h surveys on lakes, especially in shallow lakes where the adaptive pinging rate would increase significantly, it heated up very much. As mentioned before, the adaptive ping rate was used to get as much data as possible w.r.t. of the given water column depth; therefore. it was not an option to lower it. After the first visit to Plitvice Lakes National Park, a cooling system consisting of a small fan and a heat outlet was developed, which significantly improved the performance of the ASV during the second visit to Plitvice, as it solved the problem of overheating.
Bathymetric data of all survey lakes were processed in the QPS Qimera software to create 2.5D manifolds representing interpolated depth profiles. The reconstruction parameters used for the reconstruction, based on sonar and IMU datasheets, were heading unceartainty:
, roll and pitch uncertainty:
, and the unit cell size for surface interpolation was set to 0.2 m. Bathymetric DEMs of two Plitvice Lakes are given in Figure 10
and Figure 11
. They were created in Global Mapper sotfware based on processed point cloud exported from Qimera and displayed with depth increments of 1–5 m depending on the maximum depth of each lake. Note that for each lake there is a black outline extracted from the digital orthophoto of Plitvice Lakes from the photos taken with the UAV BEE-G3. Within the outline is the generated bathymetric DEM. The white areas where the bathymetric data are missing are the shallowest parts of the lakes where the depth is less than 0.5 m and which the ASV PlaDyBath could not traverse due to safety reasons in order not to damage multibeam sonar. However, with the methodology of tilted sonar swath a good part of these shallow parts were nevertheless recorded. Moreover, it is important to note tufa formations in the middle of the Malo lake, as Figure 11
In addition, the digital elevation maps (DEMs) extracted from the 2.5D bathymetry manifolds were then overlaid with the digital orthophoto of the area based on UAV camera images. Upper Lakes DEM is shown in Figure 12
and Lower Lakes’ DEM is given in Figure 13
. Note that the darker parts on the right side of the lakes are only the result of the standard shader in the Global Mapper software used to merge the bathymetric DEMs with photogrammetric orthophotos in Figure 12
, Figure 13
, Figure 14
and Figure 15
. It is interesting to see how the tufa-shaped lake bottom does not slope evenly from the shore to the middle of the lake. On the contrary, it contains many depressions, which are the result of the interaction between water and tufa bottom. Several detailed views of some tufa depressions in the lake bottom are shown in Figure 14
and Figure 15
. Experts in underwater acoustics and geology were consulted to explain these depth jumps. Underwater acoustics experts could not explain such measurement results with significant measured depth shifts in a small area. For this reason and because of the specific soil composition of Plitvice Lakes, geological experts were consulted. They explained that depressions at the bottoms of the lakes are typical for karst terrains. These depressions represent sinkholes caused by the collapse of an underlying structure.
However, in order to be able to draw final conclusions, it is necessary to repeat the hydrographic measurements with the same and other instruments to make conclusions about the cause of the occurrence of anomalous depth values. Some of the depressions can be detected in bathymetric measurements. The analysis of the backscatter data, as shown in Figure 16
may reveal some anomalies that cannot be detected from bathymetric data because there is no expressed depression on the lake bottom. Furthermore, the difference in the intensity of the returned signal’s intensity and the discernible circularity indicate a different sediment on the lake bottom. The use of a sub-bottom profiler or other underwater mapping solutions for the detailed mapping of structures under the lake bottom would also be of great benefit. This technology could further uncover the intricate connections between lakes with suspected underwater caverns and canals.
The raw data and the processed bahtymetry maps of the Plitvice National Park were then analysed and marked by the National Park staff. In the future, these bathymetric surveys will become a regular and periodic method for the geologists of the National Park to track changes in this geologically very dynamic environment.
summarises the data obtained through acoustic surveys of Plitvice Lakes, including the precisely measured altitude of the lakes’ waterline, the maximum and average measured depth and the calculated area and volume. It is important to note that Petrik in [7
] gathered in total 5400 limnological measurements of various types about all 16 lakes during a four-year period in the 1950s. Based on Table 1
, the total number of data points processed (or so-called cells in the interpolated surfaces) in the QPS Qimera software is over 7.5 million for 13 lakes. This number is an order of magnitude lower than the number of raw sonar measurements. Collecting the measurements took 7 working days, and processing the sonar data took another 8 days, for a total of 15 days. This means that the state-of-the-art autonomous robotic systems, equipped with the latest visual and acoustic remote sensing technologies, enabled the researchers to collect over 1400 times more bathymetric measurements in a time frame more than 60 times shorter than that of [7
Plitvice Lakes National Park is protected under UNESCO’s World Heritage List since 1979, so the requirements for any kind of data collection are remarkably high. The use of gasoline engines is strictly prohibited, so the only remaining option is electric propulsion. In addition, any kind of human intervention within National Park is forbidden, e.g., for the transport of remote sensing equipment and vessels, the removal of tree trunks fallen on the trails, vegetation in the vicinity and in the lakes. All these logistic constraints have led to the development of a small, two-person portable, electrically driven autonomous surface vehicle that can fulfil all the challenging tasks. The main advantages of this vehicle are its high portability, the ability to move omnidirectionally to follow highly rugged lake shores and the robustness to avoid potential hazards, such as branches and logs falling into the shallows of lakes, waterfalls, etc. Furthermore, the vessel is autonomous, so the bathymetric recording missions were carried out by the autopilot. The high-resolution orthophoto, which was generated based on the UAV’s inspection of the entire area of Plitvice Lakes National Park, served as input for mission planning and to avoid potentially hazardous obstacles.
Images captured by the UAV can be processed using photogrammetry software to produce bathymetric maps of shallow lakes (less than 2 m) as well as near the edge of each lake where the ASV could not approach for safety reasons and multibeam sonar data was unavailable even after slanting sonar swath. In addition, the areas at the edge of the lakes are covered with dense thicket (marsh butterbur, sedges, willows). Upper Lakes are very inaccessible and the lake shores are surrounded by dense forests of fir and beech that overhang the lake shores. It is rather difficult to apply remote sensing methods in these areas, so these areas should be covered by a suitable direct measurement method. It would be necessary to cover these areas by remote sensing methods with a UAV at a lower altitude and with an oblique camera capture mode. The photogrammetry-based digital surface model could also be used for bathymetric mapping of the lake shallows. Based on the results from [43
], an optimal method for correcting depth errors caused by water refraction could be chosen. It could then be merged with the acoustically recorded bathymetric maps of the deeper parts of the lakes to produce a complete DEM. This is an objective of authors’ future work. Another possibility is to use a waterborne Ground Penetrating Radar (GPR) as in [45
] or an airborne UAV-mounted GPR [46
A great potential for the use of autonomous vehicles in remote sensing studies is presented in this article. Here an autonomous surface vehicle and an unmanned aerial vehicle were used for hydrology related autonomous remote sensing survey missions of Plitvice Lakes National Park in Croatia. The efficiency of using autonomous vehicles for such applications, the quality and quantity of data in time and the precision of georeferencing the data, especially in larger areas as in the authors’ case study, has been shown to far exceed human performance. A total of 11 of 16 lakes in the National Park were surveyed using acoustic surveying methods with a multibeam sonar.
Interesting depressions at the bottom of the lakes were discovered in the bathymetric data, as well as in some previous research studies on Plitice lakes. It is assumed that these depressions in the lake bed are geomorphological features of the karst terrain that makes up the area. Reviewing and rerecording these anomalies, possibly even with a ROV or AUV for visual inspection, could have a major impact on understanding the intricacies of the Plitvice Lakes system. Their research will provide new insights into the system of lakes and its evolution. Previous studies have also reported such depressions but only a few of them. Now it has come to our attention that there are many more than expected still waiting to be explored.
The paper presents the scientific basis and methodology used in the modern autonomous robot geodetic measurements in the area of Plitvice Lakes National Park, its processing and the development of a digital three-dimensional geodetic models of lakes. These digital bathymetric maps will be the basis for further research purposes in GIS environment by experts in scientific disciplines such as biology, geology, hydrology, ecology, etc. Decisions on the way and form of protection of the underlying phenomenon of tufa formation in Plitvice lakes would be made on the basis of these state-of-the-art maps. Based on the raw sonar ping data, snippets and backscatter or the entire unprocessed point cloud, a future objective of the authors is to develop methods for characterising the lake bed type. This, together with in-situ sampling, would lead to a definitive classification of the lake bed types.
Looking to the future, the authors’ research objective is also to develop higher levels of vehicle autonomy, which would include autonomous exploration/inspection path (re)planning based on online processed sensor data and artificial intelligence to further optimise the performance of autonomous vehicles. If these investigations are carried out regularly, the geologists, biologists and ecologists at Plitvice Lakes National Park will be able to track changes of this very dynamic environment over time, based on high-resolution opto-acoustic models georeferenced with an accuracy of a few centimetres.