remote sensing

Article Geospatial Modeling of the Tombolo Phenomenon in Sopot using Integrated Geodetic and Hydrographic Measurement Methods

Mariusz Specht 1,* , Cezary Specht 2 , Janusz Mindykowski 3 , Paweł D ˛abrowski 2, Romuald Ma´snicki 3 and Artur Makar 4 1 Department of Transport and Logistics, Gdynia Maritime University, Morska 81-87, 81-225 Gdynia, Poland 2 Department of Geodesy and Oceanography, Gdynia Maritime University, Morska 81-87, 81-225 Gdynia, Poland; [email protected] (C.S.); [email protected] (P.D.) 3 Department of Marine Electrical Power Engineering, Gdynia Maritime University, Morska 81-87, 81-225 Gdynia, Poland; [email protected] (J.M.); [email protected] (R.M.) 4 Department of Navigation and Hydrography, Polish Naval Academy, Smidowicza´ 69, 81-127 Gdynia, Poland; [email protected] * Correspondence: [email protected]



Received: 31 January 2020; Accepted: 21 February 2020; Published: 23 February 2020


Abstract: A tombolo is a narrow belt connecting a mainland with an island lying near to the shore, formed as a result of sand and gravel being deposited by sea currents, most often created as a result of natural phenomena. However, it can also be caused by human activity, as is the case with the Sopot pier—a town located on the southern coast of the Baltic Sea in northern Poland (ϕ = 54◦26’N, λ = 018◦33’E). As a result, the seafloor rises constantly and the shoreline moves towards the sea. Moreover, there is the additional disturbing phenomenon consisting of the rising seafloor sand covering over the waterbody’s vegetation and threatening the city's spa character. Removal of the sand to another place has already been undertaken several times. There is a lack of precise geospatial data about the tombolo’s seafloor course, its size and spatial shape caused by only lowering the seafloor in random places, and the ongoing environmental degradation process. This article presents the results of extensive and integrated geodetic and hydrographic measurements, the purpose of which was to make a 3D model of the phenomena developing in Sopot. The measurements will help determine the size and speed of the geospatial changes. Most of the modern geodetic and hydrographic methods were used in the study of these phenomena. For the construction of the land part of geospatial model, the following were used: photos from the photogrammetric flight pass (unmanned aerial vehicle—UAV), laser scanning of the beach and piers, and satellite orthophotomaps for analysis of the coastline changes. In the sea part, bathymetric measurements were carried out with an unmanned surface vehicle (USV).

Keywords: geodetic and hydrographic measurements; geospatial modeling; tombolo phenomenon; terrestrial laser scanning (TSL); unmanned aerial vehicle (UAV); unmanned surface vehicle (USV)

1. Introduction In the last century, as a result of climate change observed on a global and local scale [1], there has been an increase in sea level. This leads to an increased impact of waves on the sea shore, causing the shore to move inland [2,3]. Therefore, coastal states are increasingly forced to take a number of actions to maintain the current coastline course [4] and protect coastal infrastructure [5]. In parallel, it should be ensured that the steps taken minimize interference with and the impact on the natural environment [6].

Remote Sens. 2020, 12, 737; doi:10.3390/rs12040737 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 737 2 of 18

Morphological changes in the coastal zone and the coastline course are most often due to natural factors. However, they can also be the result of human activity related to direct interference in the environment, as is the case here, where the marina construction caused a slowdown in the sediment transport along the coast, initiating the process of creating a tombolo. Sopot is one of the major Polish holiday and spa resorts situated on the Baltic Sea coast. The city has the longest wooden pier in Europe, which is regularly damaged by storms. In October 2009, a violent storm completely destroyed the wooden structure of the pier groyne. The only economically viable method for protecting the pier was to build two breakwaters from the southern and eastern sides. The waterbody bordered between these breakwaters and the pier groyne and head has become a natural marina [7]. In 2010, expert discussions and their opinions resulted in a decision to build a yacht marina in Sopot (3 basins, a maximum of 103 vessels: 40 large ones up to 14 m in length, and 63 smaller ones up to 10 m in length) for PLN 72 million. This seemingly undoubted decision is currently becoming a serious problem for the city, as the construction of the marina led to the local stoppage of the sand transport along the coast, which resulted in its accumulation between the marina and the shore and the shift of the coastline towards the sea (by approx. 50 m), and initiated the process of the inevitable formation of a peninsula in Sopot. Such an oceanographic phenomenon, known as a tombolo [8], is most frequently influenced by the course of beaches and coasts under natural conditions but can also result from human activities, as is the case in Sopot [9]. In the Bay of Gda´nsk,the strongest surface wind waving is generated from direction N towards E. The waves that reach the beach in Sopot from the E direction hit diagonally against the shore and cause the movement of bottom sediments along the coast. After the marina was built, its breakwater significantly decreased the wave energy, moreover, waves are deflected at its ends (Figure1b), which results in the formation of two vortexes (directed opposite to each other). Consequently, the seafloor between the marina (obstacle) and the shore is elevated upwards, which resultsRemote Sens. in the 2020 development, 12, x FOR PEER of REVIEW a morphological formation known as a tombolo. It should be stressed3 of 19 that this phenomenon in Sopot is unique in Poland.

(a) (b)

FigureFigure 1.1.The The naturenature ofof thethe phenomenonphenomenon that causes the formation formation of of the the tombolo tombolo effect effect (a (a),), and and the the wavywavy surface surface of of the the sea sea in varyingin varying directions directions resulting resulting from from the ditheffraction diffraction of waves of waves around around the marina the breakwatermarina breakwater in Sopot in (b Sopot). (b).

ADepending detached on breakwater the value (beach,of LB*, two coastal, basico accumulationffshore) as a marinaforms can in be Sopot created provides on the shelterbeach: from the waves,• wherebySalient: When the littoral LB* is transport less than behindapprox. the0.6 to breakwater 0.7, a bell-shaped is decreased salient and in the the shoreline transport pattern adjacentwill to form the breakwaterin the lee of isthe modified. breakwater. These However, characteristics parameters of a other breakwater than the are breakwater utilised in different wayslength for various and distance types of also breakwaters influence bythe varyingaccumulation relevant pattern. parameters. The main parameter determining• theTombolo: future morphologicalWhen LB* is greater form, than which approx. will be 0.9 shaped to 1.0, the asa sand result accumulation of the obstacle behind in Sopot, the breakwater will connect the beach to the breakwater in a tombolo formation. But again, parameters other than the breakwater length and distance influence the accumulation pattern. In the case of the marina in Sopot, LB* is 0.78, hence it is likely that a tombolo will eventually develop, which means that in the near future, a strip of land will be created connecting the pier with the shore and protruding above the water. The purpose of this publication is to present the results of extensive and integrated geodetic and hydrographic measurements that enabled creation of a 3D model of the phenomenon developing in Sopot. This will enable the size and speed of geospatial changes to be determined.

2. Materials and Methods

2.1. Study Area In the literature on the subject, the process of modelling three-dimensional sediment transport has been described extensively and in detail in many publications [11–12]. It includes both the impact on the sea shore, as well as the influence on infrastructure and marine structures [13–14]. The diverse research approach to the authors' problem results from different methods of modelling being taken into account: evolution [15], fixed profile [16], coupling area-line [17], area cross-shore–alongshore transport coupling [11,18], diffusion [14], wet-dry [19], hybrid [20], and cut-cell models [19]. It should be noted with reference to [21] that general wave-average area models may also have merits in simulating shoreline evolution for arbitrary geometry. However, the difficulty in the treatment of the shoreward boundary has hindered the use of this type of approach for simulation of shoreline evolution. The major defect is that it is impossible to simulate morphologic change above the wave- average water level, although it is more or less hidden for a macro-tidal environment. A preliminary analysis of the tombolo phenomenon in Sopot was made in study [9]. On this basis, the city of Sopot developed an approximate model for the phenomenon development (Figure 2) [22].

Remote Sens. 2020, 12, 737 3 of 18 is the breakwater length relative to breakwater distance to shoreline (LB*) calculated according to the formula [10]: LB LB = (1) ∗ d where: LB–breakwater length, d–breakwater distance to shoreline. Depending on the value of LB*, two basic accumulation forms can be created on the beach: Salient: When LB* is less than approx. 0.6 to 0.7, a bell-shaped salient in the shoreline will form in • the lee of the breakwater. However, parameters other than the breakwater length and distance also influence the accumulation pattern. Tombolo: When LB* is greater than approx. 0.9 to 1.0, the sand accumulation behind the breakwater • will connect the beach to the breakwater in a tombolo formation. But again, parameters other than the breakwater length and distance influence the accumulation pattern. In the case of the marina in Sopot, LB* is 0.78, hence it is likely that a tombolo will eventually develop, which means that in the near future, a strip of land will be created connecting the pier with the shore and protruding above the water. The purpose of this publication is to present the results of extensive and integrated geodetic and hydrographic measurements that enabled creation of a 3D model of the phenomenon developing in Sopot. This will enable the size and speed of geospatial changes to be determined.

2. Materials and Methods

2.1. Study Area In the literature on the subject, the process of modelling three-dimensional sediment transport has been described extensively and in detail in many publications [11,12]. It includes both the impact on the sea shore, as well as the influence on infrastructure and marine structures [13,14]. The diverse research approach to the authors’ problem results from different methods of modelling being taken into account: evolution [15], fixed profile [16], coupling area-line [17], area cross-shore–alongshore transport coupling [11,18], diffusion [14], wet-dry [19], hybrid [20], and cut-cell models [19]. It should be noted with reference to [21] that general wave-average area models may also have merits in simulating shoreline evolution for arbitrary geometry. However, the difficulty in the treatment of the shoreward boundary has hindered the use of this type of approach for simulation of shoreline evolution. The major defect is that it is impossible to simulate morphologic change above the wave-average water level, although it is more or less hidden for a macro-tidal environment. A preliminary analysis of the tombolo phenomenon in Sopot was made in study [9]. On this basis, the city of Sopot developed an approximate model for the phenomenon development (Figure2)[22]. The developed model, or rather an explanatory figure, is of such a general nature that it prevents referring to coordinates and depth, which makes it impossible to precisely plan dredging works that the city of Sopot has been implementing for 3 years. Hence, direct measurements of the geospatial nature of the phenomenon are necessary from the point of view of assessing its size and planning dredging works for the seafloor (Figure3)[ 22,23]. It should be noted that the city’s activity in the field of digging sand from the seafloor should be conducted in such a way as to seek restoration sediment transport along the coast and restart natural processes. Therefore, the geospatial distribution of the beach and the seafloor near the pier plays a key role here. Over the last several years (after the yacht marina was built), the adverse effect of the Sopot tombolo phenomenon on the aquatic environment and humans has been noted (Figure3)[ 22,23]. One of the local internet portals https://sopot.gmina.pl/ explains the cause of the phenomenon: “The emerging tombolo promotes the increasing occurrence of water stagnation and more rapid water heating, which creates more favourable conditions for the blooming of cyanobacteria and other microorganisms” [22]. Remote Sens. 2020, 12, x FOR PEER REVIEW 3 of 19

(a) (b)

Figure 1. The nature of the phenomenon that causes the formation of the tombolo effect (a), and the wavy surface of the sea in varying directions resulting from the diffraction of waves around the marina breakwater in Sopot (b).

Depending on the value of LB*, two basic accumulation forms can be created on the beach: • Salient: When LB* is less than approx. 0.6 to 0.7, a bell-shaped salient in the shoreline will form in the lee of the breakwater. However, parameters other than the breakwater length and distance also influence the accumulation pattern. • Tombolo: When LB* is greater than approx. 0.9 to 1.0, the sand accumulation behind the breakwater will connect the beach to the breakwater in a tombolo formation. But again, parameters other than the breakwater length and distance influence the accumulation pattern. In the case of the marina in Sopot, LB* is 0.78, hence it is likely that a tombolo will eventually develop, which means that in the near future, a strip of land will be created connecting the pier with the shore and protruding above the water. The purpose of this publication is to present the results of extensive and integrated geodetic and hydrographic measurements that enabled creation of a 3D model of the phenomenon developing in Sopot. This will enable the size and speed of geospatial changes to be determined.

2. Materials and Methods

2.1. Study Area In the literature on the subject, the process of modelling three-dimensional sediment transport has been described extensively and in detail in many publications [11–12]. It includes both the impact on the sea shore, as well as the influence on infrastructure and marine structures [13–14]. The diverse research approach to the authors' problem results from different methods of modelling being taken into account: evolution [15], fixed profile [16], coupling area-line [17], area cross-shore–alongshore transport coupling [11,18], diffusion [14], wet-dry [19], hybrid [20], and cut-cell models [19]. It should be noted with reference to [21] that general wave-average area models may also have merits in simulatingRemote Sens. shoreline2020, 12, 737 evolution for arbitrary geometry. However, the difficulty in the treatment4 ofof 18 the shoreward boundary has hindered the use of this type of approach for simulation of shoreline evolution. The major defect is that it is impossible to simulate morphologic change above the wave- averageMs Magdalena water level, Jachim, although the spokeswoman it is more or for less the hidden Sopot Cityfor a Hall, macro-tidal speaks in environment. a similar tone: A “This preliminary is not analysisa thorough of the deepening, tombolo phenomenon but a straightening in Sopot of the was coastline made thatin study will prevent[9]. On thethis accumulation basis, the city of of waste Sopot and algae brought by the sea to the beach in this place” [24]. developedRemote Sens. an 2020 approximate, 12, x FOR PEER model REVIEW for the phenomenon development (Figure 2) [22]. 4 of 19

Figure 2. Forecast development model of the tombolo phenomenon in Sopot.

The developed model, or rather an explanatory figure, is of such a general nature that it prevents referring to coordinates and depth, which makes it impossible to precisely plan dredging works that the city of Sopot has been implementing for 3 years. Hence, direct measurements of the geospatial nature of the phenomenon are necessary from the point of view of assessing its size and planning dredging works for the seafloor (Figure 3) [22–23]. It should be noted that the city's activity in the field of digging sand from the seafloor should be conducted in such a way as to seek restoration sediment transport along the coast and restart natural processes. Therefore, the geospatial distribution of the beach and the seafloor near the pier plays a key role here. Over the last several years (after the yacht marina was built), the adverse effect of the Sopot tombolo phenomenon on the aquatic environment and humans has been noted (Figure 3) [22–23]. One of the local internet portals https://sopot.gmina.pl/ explains the cause of the phenomenon: “The emerging tombolo promotes the increasing occurrence of water stagnation and more rapid water heating, which creates more favourable conditions for the blooming of cyanobacteria and other microorganisms” [22]. Ms Magdalena Jachim, the spokeswoman for the Sopot City Hall, speaks in a similar tone: “This is not a thorough deepening, but a straightening of the coastline that will prevent the accumulationFigure of waste 2. Forecast and algae development brought model by the of sea the to tombolo the beach phenomenon in this place” in Sopot. [24].

(a) (b) (c)

FigureFigure 3.3.Removing Removing sandsand from under the pier in Sopot ( (aa),), pine pine pollen pollen and and rotten rotten sand sand lying lying around around thethe pier pier ( b(b),), transportedtransported toto thethe southernsouthern partpart ofof the beach in Sopot ( (c).).

TheThe quotedquoted statementsstatements and and source source materials materials clearly clearly sh showow that that Sopot Sopot is likely is likely to lose to lose its status its status as asan an international international health health resort. resort. This This is isprimarily primarily due due to to the the blooming blooming of of cyanobacteria cyanobacteria and and other other bacteria,bacteria, which which repeatedlyrepeatedly preventsprevents bathingbathing and swimmingswimming in this this region region throughout throughout the the year. year. In In the the summersummer time,time, aa swimmingswimming prohibitionprohibition has been repeatedly issued issued by by the the Pomeranian Pomeranian Voivodeship Voivodeship SanitarySanitary andand EpidemiologicalEpidemiological Station.Station. In order to to determine determine the the geospatial geospatial scale scale of of the the phenomenon, phenomenon, aa photogrammetric photogrammetric flightflight passpass waswas performed using an an unmanned unmanned aerial aerial vehicle vehicle (UAV) (UAV) in in the the vicinity vicinity ofof the the Sopot Sopot pier.pier. TheThe measurementsmeasurements werewere carriedcarried out at the end of October and and the the beginning beginning of of November November 2018 2018 (a(a period period unfavourable unfavourable toto waterwater blooming).blooming). Based Based on on the the study study results, results, it it can can be be concluded concluded that that the the rangerange of of cyanobacteria, cyanobacteria, algae,algae, andand otherother microorganisms occurring in in this this area area is is significant. significant. On On both both orthophotosorthophotos theythey are marked marked with with a adark dark green/black green/black colour. colour. Opposite Opposite the theGrand Grand Hotel Hotel in Sopot, in Sopot, the theabove-mentioned, above-mentioned, 30 30m wide m wide blooming blooming area area is islocated located along along a 200 a 200 m m stretch stretch of of the the beach. beach. On On the the otherother hand, hand, FigureFigure4 4bb presents presents cyanobacteria cyanobacteria andand algaealgae bloomingblooming along the entire length of of the the yacht yacht marinamarina inin Sopot,Sopot, whichwhich stretchesstretches forfor severalseveral dozendozen meters towards the mainland [25]. [25]. Another factor having an adverse effect on the environment is the accumulation of sand in the vicinity of the marina. Despite the positive fact that the Sopot beach is expanding (under reconstruction), in other places the sand reaching the pier is taken away. The University of Gda´nskProfessor Leszek Ł˛eczy´nski,marine geology specialist, stated in an interview for the Gazeta Wyborcza paper that:

Remote Sens. 2020, 12, 737 5 of 18

“For years, we have been observing abrasion of the Orłowo Cliff, in my opinion, the material originating from there is being accumulated exactly near the Sopot pier” [26]. For example, in February 2018 as much as 1200 m2 of ground originating from this cliff slid down onto a beach in Gdynia Orłowo. The landslide itself was approx. 40 m long [27]. It should also be noted that the Orłowo Cliff is part of the K˛epaRedłowska nature reserve. Therefore, this area represents a landscape typical of Gdynia, with natural characteristics primarily associated with the following values: natural (the cliff has a unique multi-layered structure comprising glaciofluvial sands, gravels and silts, glacial till, sand-gravel sediments, etc. [28]), historical (this is the first nature reserve in Pomeranian Voivodeship and one of the oldest in the country), and landscape (from the cliff one can see a panorama of the Hel Peninsula, the harbour in Gdynia, the Sopot pier, etc. [29]). Remote Sens. 2020, 12, x FOR PEER REVIEW 5 of 19

(a) (b)

FigureFigure 4. 4. WaterWater blooming blooming in inthe the vicinity vicinity of the of theGrand Grand Hotel Hotel in Sopot in Sopot (a) and (a )the and yacht the marina yacht marina in Sopot in (Sopotb). (b).

AnotherThe growing factor coastalhaving beltan adverse also poses effect a on navigational the environment hazard is to the shipping accumulation traffic of in sand this region.in the vicinityThe vessels of the moored marina. to the Despite jetty located the positive in the eastern fact that part ofthe the Sopot pier andbeach in theis southernexpanding part (under of the reconstruction),yacht marina are in at other risk of places damaging the sand their reaching hull structure. the pier This is istaken due toaway. the fact The that University for the vast of Gda majorityńsk Professorof the berthing Leszek area, Łęczy theń waterski, marine depth isgeology actually specialist, lower than stated 1.5 m, in and an theinterview draft of bothfor the sailing Gazeta and Wyborczamotorised paper vessels that: often “For exceeds years, 1–1.5 we m, have thus been preventing observing them abrasion from sailing of the on theseOrłowo waters. Cliff, It in should my opinion,be added the that material sailors originating may not be from aware there of this is being fact, because accumulated the offi exactlycial electronic near the navigational Sopot pier” chart[26]. For(ENC) example, from 2011 in February (Figure5) 2018 shows as thatmuch the as isobaths 1200 m increase2 of ground linearly originating (and at equal from distances) this cliff slid in relation down ontoto the a beachbeach coastline,in Gdynia and Or thełowo. water The depth landslide for the itself berthing was areaapprox. ranges 40 m from long 2 to[27]. 5 m. It Figureshould5 showsalso be a notednavigational that the chart Orł (fromowo Cliff OpenCPN is part 5.0 of software), the Kępa which Redłowska uses offi naturecial ENC reserve. cells used Therefore, by marine this vessels area represents(ships, yachts, a landscape etc.). Therefore, typical itof shouldGdynia, be with stressed natu thatral characteristics currently no o ffiprimarilycial charts associated used in navigation with the followinginclude depth values: changes natural due (the to thecliff tombolo has a unique phenomenon multi-layered found near structure the Sopot comprising pier. Hence, glaciofluvial as part of sands,the preliminary gravels and study, silts, it glacial was decided till, sand-gravel to carry out sedime measurementsnts, etc. [28]), to determine historical the(this current is the first topography nature reserveof the seafloor in Pomeranian in this area. Voivodeship and one of the oldest in the country), and landscape (from the cliff one can see a panorama of the Hel Peninsula, the harbour in Gdynia, the Sopot pier, etc. [29]). 2.2.The Measurement growing Conceptcoastal belt also poses a navigational hazard to shipping traffic in this region. The vesselsIn moored terms of to measurement, the jetty located the in area the eastern where thepart tombolo of the pier phenomenon and in the southern occurs is part specific, of the placing yacht marinahigh demands are at risk on of surveyors damaging and their hydrographers hull structure. regarding This is due the to measurementthe fact that for precision, the vast majority as well as of a theneed berthing to use variousarea, the advanced water depth measurement is actually techniques, lower than such 1.5 as:m, and the draft of both sailing and motorised vessels often exceeds 1–1.5 m, thus preventing them from sailing on these waters. It should Land measurements (geodesy) enable the acquisition of geospatial data of a beach section adjacent be• added that sailors may not be aware of this fact, because the official electronic navigational chart (ENC)to from the sea.2011 Measurements (Figure 5) shows of thisthat type the isobaths can be carried increase out linearly in two (and ways. at Inequal order distances) to obtain in a relationprecise to the 3D beach model coastline, of the beach and the adjacent water depth to the shore,for the theberthing best solution area ranges is the from use 2 of to the 5 m. terrestrial Figure 5 showslaser a scanningnavigational (TLS) chart [30 –(from32]. However, OpenCPN this 5.0 is software), a time-consuming which uses method, official it ENC requires cells expensive used by marinemeasurement vessels (ships, equipment yachts, etc.). and Therefore, advanced it methods should be of stressed point cloud that processing.currently no Theofficial predicted charts used positioningin navigation accuracy include (3D) depth is 2–3 changes cm, for due distances to the up tombolo to 100 mphenomenon (Figure6a). An found alternative near the to TLSSopot is pier. Hence,the use as of part photogrammetric of the preliminary methods, study, where it was the decided beach to model carry is out developed measurements based onto determine a sequence the current topography of the seafloor in this area.

Remote Sens. 2020, 12, 737 6 of 18

of photos from the UAV [33–35]. The predicted model accuracy (3D) depends on the positioning Remoteaccuracy Sens. 2020 of, the12, x globalFOR PEER navigation REVIEW satellite systems (GNSSs) used and the type of photogrammetric6 of 19 camera used, ranging from 5 cm to 1 m (Figure6b). Sea surveys (hydrography) for depths below 1 m require the use of an atypical hydrographic • vessel that has a very small draft and is equipped with a specialized echo sounder dedicated to ultra-shallow waters, the range of which should be at least 30 cm. The predicted accuracy of this solution allows a depth accuracy of 2–5 cm root mean square (RMS) (Figure7a). An alternative solution to the use of a manned hydrographic vessel is the use of an unmanned surface vehicle (USV), which is increasingly used in bathymetric measurements [36–38]. The predicted accuracy of bathymetric measurements is identical to that of manned hydrographic measurements (Figure7b). Sea surveys (hydrography) for depths: 0–0.6 m are specific because this part of the waterbody • could not, for obvious reasons, be measured by a manned vessel. Therefore, no actual bathymetric data that could determine the spread of the tombolo phenomenon were available in this area. Currently, bathymetric measurements in ultra-shallow waters and on the coastline are carried out by either tachymetric [39] or geodetic method with a person stepping into the sea to a predefined depth using a GNSS receiver operating in real time [40–42]. Remote Sens. 2020, 12, x FOR PEER REVIEW 6 of 19 Figure 5. Bathymetric chart of the marina area in Sopot made on the basis of official electronic navigational chart (ENC) data.

2.2. Measurement Concept In terms of measurement, the area where the tombolo phenomenon occurs is specific, placing high demands on surveyors and hydrographers regarding the measurement precision, as well as a need to use various advanced measurement techniques, such as: • Land measurements (geodesy) enable the acquisition of geospatial data of a beach section adjacent to the sea. Measurements of this type can be carried out in two ways. In order to obtain a precise 3D model of the beach adjacent to the shore, the best solution is the use of the terrestrial laser scanning (TLS) [30–32]. However, this is a time- consuming method, it requires expensive measurement equipment and advanced methods of point cloud processing. The predicted positioning accuracy (3D) is 2–3 cm, for distances up to 100 m (Figure 6a). An alternative to TLS is the use of photogrammetric methods, where the beach model is developed based on a sequence of photos from the UAV [33–35]. The predicted model accuracy (3D) depends on the Figure 5. Bathymetric chart of the marina area in Sopot made on the basis of official electronic Figure 5. positioningBathymetric chartaccuracy of the of marinathe global area navigati in Sopoton made satellite on the systems basis of (GNSSs) official electronic used and the navigational chart (ENC) data. navigational charttype (ENC) of photogrammetric data. camera used, ranging from 5 cm to 1 m (Figure 6b).

2.2. Measurement Concept In terms of measurement, the area where the tombolo phenomenon occurs is specific, placing high demands on surveyors and hydrographers regarding the measurement precision, as well as a need to use various advanced measurement techniques, such as: • Land measurements (geodesy) enable the acquisition of geospatial data of a beach section adjacent to the sea. Measurements of this type can be carried out in two ways. In order to obtain a precise 3D model of the beach adjacent to the shore, the best solution is the use of the terrestrial laser scanning (TLS) [30–32]. However, this is a time- consuming(a )method, it requires expensive measurement(b) equipment and advanced methods of point cloud processing. The predicted positioning accuracy (3D) is 2–3 cm, FigureFigure 6. 6.Laser Laser scanner scanner (a ()a and) and unmanned unmanned aerial aerial vehicle vehicle (UAV) (UAV) DJI DJI Mavic Mavic Pro Pro (b ()b used) used for for geospatial geospatial modelingmodelingfor ofdistances of the the beach beach inup in Sopot. Sopot.to 100 m (Figure 6a). An alternative to TLS is the use of photogrammetric methods, where the beach model is developed based on a sequence of • photos Sea from surveys the (hydrography)UAV [33–35]. for The depths predicte belowd model 1 m requireaccuracy the (3D)use ofdepends an atypical on the positioninghydrographic accuracy vessel of that the has global a very navigati small drafton satellite and is equi systemspped with (GNSSs) a specialized used andecho the sounder dedicated to ultra-shallow waters, the range of which should be at least 30 cm. type of photogrammetric camera used, ranging from 5 cm to 1 m (Figure 6b). The predicted accuracy of this solution allows a depth accuracy of 2–5 cm root mean square (RMS) (Figure 7a). An alternative solution to the use of a manned hydrographic vessel is the use of an unmanned surface vehicle (USV), which is increasingly used in

(a) (b)

Figure 6. Laser scanner (a) and unmanned aerial vehicle (UAV) DJI Mavic Pro (b) used for geospatial modeling of the beach in Sopot.

• Sea surveys (hydrography) for depths below 1 m require the use of an atypical hydrographic vessel that has a very small draft and is equipped with a specialized echo sounder dedicated to ultra-shallow waters, the range of which should be at least 30 cm. The predicted accuracy of this solution allows a depth accuracy of 2–5 cm root mean square (RMS) (Figure 7a). An alternative solution to the use of a manned hydrographic vessel is the use of an unmanned surface vehicle (USV), which is increasingly used in

Remote Sens. 2020, 12, x FOR PEER REVIEW 7 of 19

Remote Sens. 2020, 12, 737 7 of 18 bathymetric measurements [36–38]. The predicted accuracy of bathymetric measurements is identical to that of manned hydrographic measurements (Figure 7b).

(a) (b)

FigureFigure 7. 7. HydrographicHydrographic vessel vessel with with a a draft draft of of 30 30 cm cm (a (a) )and and unmanned unmanned surface surface vehicle vehicle (USV) (USV) designed designed forfor coastal coastal measurements measurements ( (bb).).

2.2.1. Geodetic• Sea Measurements surveys (hydrography) for depths: 0–0.6 m are specific because this part of the Creatingwaterbody a 3D beach could model not, can for be obvious done on reasons, the basis be of measured two methods: by ausing manned UAV vessel. photos Therefore, or taking measurementsno basedactual onbathymetric geodetic laser data scanningthat could [31 determine]. The technique the spread of measuringof the tombolo and phenomenon creating a 3D beach modelwere based available on TLS comparedin this area. to theCurrently, method bathymetric using a UAV measurements is much more in time ultra-shallow consuming, but allows theoreticallywaters and higheron the coastline measurement are carried accuracy. out by either tachymetric [39] or geodetic method In viewwith of the a elongatedperson stepping surface ofinto the the beach sea under to a predefined measurement, depth it was using necessary a GNSS to planreceiver and arrange an appropriateoperating in number real time of [40–42]. sites. The measurements were taken with a Trimble TX8 laser scanner without the photo-taking option. Hence, the obtained point clouds had only colours resulting 2.2.1.from Geodetic the calculated Measurements laser beam reflection intensity. To cover the assumed study area, it was necessary to establishCreating 27 a sites3D beach located model at a distance can be done of approx. on the 60 basis m from of two each methods: other (Figure using8 ).UAV In view photos of the or takingsmall numbermeasurements of characteristic based on geodetic objects to laser be usedscanni forng the[31]. recording The technique of a point of measuring cloud in and the fieldcreating at a alater 3D beach time, sphericalmodel based tags on (located TLS compared in the sand to the in a method manner using ensuring a UAV the is stability much more of their time position) consuming, were butused. allows The tagstheoretically had to be higher located measurement a relatively shortaccuracy. distance from the neighboring measurement sites. Therefore,In view they of the were elongated located halfwaysurface of between the beach the un measurementder measurement, sites or it inwas their necessary immediate to plan vicinity. and arrangeSuch an an approach appropriate ensured number that aof relatively sites. The large measurements set of points were on eachtaken spherical with a Trimble tag’s surface TX8 laser were scannerobtained without during the photo-taking measurements. option. This enabledHence, the the obtained precise fitting point of clouds spheres had into only the colours set of points resulting and fromthe determination the calculated of laser their beam midpoints reflection which, intensity. in the recording To cover the process, assumed were study the points area, ofit was adjustment necessary for toparticular establish local 27 sites point located cloud systems.at a distance of approx. 60 m from each other (Figure 8). In view of the Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 19 small number of characteristic objects to be used for the recording of a point cloud in the field at a later time, spherical tags (located in the sand in a manner ensuring the stability of their position) were used. The tags had to be located a relatively short distance from the neighboring measurement sites. Therefore, they were located halfway between the measurement sites or in their immediate vicinity. Such an approach ensured that a relatively large set of points on each spherical tag’s surface were obtained during the measurements. This enabled the precise fitting of spheres into the set of points and the determination of their midpoints which, in the recording process, were the points of adjustment for particular local point cloud systems.

FigureFigure 8. 8.The The location location of of terrestrial terrestrial laser laser scanning scanning (TLS) (TLS) measurement measurement sites. sites.

CombinedCombined pointpoint cloudsclouds comingcoming fromfrom allall stationsstations formedformed aa numericalnumerical representationrepresentation ofof thethe environment.environment. TheThe connectedconnected pointpoint cloudsclouds containedcontained measurementmeasurement noisenoise inin thethe formform ofof wronglywrongly registeredregistered measurement measurement points. points. DevelopingDeveloping aa pointpoint cloudcloud oftenoften requiredrequiredmanual manualremoval removal of of these these measurementmeasurement errors errors (Figure (Figure9). 9).

(a) (b)

Figure 9. 3D point clouds obtained from the terrestrial laser scanning (TLS) containing measurement noise (a) and after their removal (b).

The photogrammetric flight pass was performed using a DJI Mavic Pro drone independently for each of the adopted parts. The data were recorded using a Pix4D Capture mobile application. During the mission, 621 photographs were taken at an altitude of 60 m above the drone take-off level (i.e., the beach). The obtained average ground sample distance (GSD) coefficient value amounted to 2.25 cm/pixel (px) at a camera resolution of 4000 x 3000 effective px. The overlap parameter defining the degree of photograph overlapping in transverse and longitudinal directions was determined to be 80%. The main parameters of the camera are: • Sensor: 1/2.3” (CMOS), effective px: 12.35 M (total px: 12.71 M). • Lens: FOV 78.8° 28 mm (35 mm format equivalent) f/2.2. • Distortion: <1.5% focus from 0.5 m to ∞. • ISO range: 100–3200 (video), 100–1600 (photo). • Shutter speed: 8 s–1/8000 s. • Image max size: 4000×3000.

2.2.2. Hydrographic Measurements The marine vessel that was used in the research was the hydrographic survey motorboat Navigator One AMG (Figure 7a), a manned vessel intended for carrying out hydrographic surveys, with a length of 5.5 m, width of 2.55, draft of 0.3 m, a crew of 6, coastal buoyancy class, and a weight of 780 kg. It is equipped with an outboard motor with a power of 115 hp (Suzuki) and the Lowrance HDS Carbon navigation system with the StructureScan 3D module.

Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 19

Figure 8. The location of terrestrial laser scanning (TLS) measurement sites.

Combined point clouds coming from all stations formed a numerical representation of the environment. The connected point clouds contained measurement noise in the form of wrongly registered measurement points. Developing a point cloud often required manual removal of these Remote Sens. 2020, 12, 737 8 of 18 measurement errors (Figure 9).

(a) (b)

FigureFigure 9. 9. 3D3D point point clouds clouds obtained obtained from from the the terrestrial terrestrial laser laser scanning scanning (TLS) (TLS) containing containing measurement measurement noisenoise ( (aa)) and and after after their their removal removal ( (bb).).

TheThe photogrammetric photogrammetric flight flight pass pass was was performed performed using using a DJI a DJI Mavic Mavic Pro Pro drone drone independently independently for eachfor each of the of adopted the adopted parts. parts. The data The were data recorded were recorded using a using Pix4D a Capture Pix4D Capture mobile application. mobile application. During theDuring mission, the mission, 621 photographs 621 photographs were taken were at taken an altitude at an altitude of 60 m of above 60 m abovethe drone the dronetake-off take-o levelff level(i.e., the(i.e., beach). the beach). The obtained The obtained average average ground ground sample sample distance distance (GSD) (GSD) coefficient coeffi cientvalue value amounted amounted to 2.25 to cm/pixel2.25 cm/pixel (px) at (px) a camera at a camera resolution resolution of 4000 of 4000x 3000 x 3000effective effective px. The px. Theoverlap overlap parameter parameter defining defining the degreethe degree of photograph of photograph overlapping overlapping in transverse in transverse and and longitudinal longitudinal directions directions was was determined determined to to be be 80%.80%. The The main main parameters parameters of of the the camera camera are: are: • Sensor: 1/2.3” (CMOS), effective px: 12.35 M (total px: 12.71 M). Sensor: 1/2.3” (CMOS), effective px: 12.35 M (total px: 12.71 M). • • Lens: FOV 78.8° 28 mm (35 mm format equivalent) f/2.2. Lens: FOV 78.8 28 mm (35 mm format equivalent) f/2.2. • • Distortion:◦ <1.5% focus from 0.5 m to ∞. Distortion: <1.5% focus from 0.5 m to . • • ISO range: 100–3200 (video), 100–1600∞ (photo). ISO• range: 100–3200 (video), 100–1600 (photo). • Shutter speed: 8 s–1/8000 s. Shutter• speed: 8 s–1/8000 s. • Image max size: 4000×3000. Image max size: 4000 3000. • × 2.2.2. Hydrographic Measurements 2.2.2. Hydrographic Measurements The marine vessel that was used in the research was the hydrographic survey motorboat NavigatorThe marine One AMG vessel (Figure that was 7a), used a manned in the research vessel wasintended the hydrographic for carrying surveyout hydrographic motorboat Navigator surveys, withOne AMGa length (Figure of 5.57 a),m, awidth manned of 2.55, vessel draft intended of 0.3 form, a carrying crew of out6, coastal hydrographic buoyancy surveys, class, withand a a weight length ofof 780 5.5 m,kg. widthIt is equipped of 2.55, draft with of an 0.3 outboard m, a crew motor of 6, with coastal a power buoyancy of 115 class, hp (Suzuki) and a weight and the of 780 Lowrance kg. It is HDSequipped Carbon with navigation an outboard system motor with with the aStructureScan power of 115 3D hp module. (Suzuki) and the Lowrance HDS Carbon navigation system with the StructureScan 3D module. The single beam echo sounder (SBES) was calibrated using the tare method, which involves adjusting the average speed of sound in water at known depths to echo sounder indications. The basic method for measuring the speed of sound is the use of a CTD (conductivity, temperature, depth) sensor or a sound velocity profiler (SVP). An alternative is the measurement of water temperature and the determination of the speed of sound based on an empirical equation when the salinity is known. Certain echo sounders determine the speed of sound in water based on the temperature by means of a sensor located in the transducer. Oceanographic measurements which are part of hydrographic surveys include water level observations and measurements of the speed of sound in water [43]. The water level can be observed on a gauging station which is situated in a marina or port and is referenced to the geodetic height system used in a particular country. In Poland, according to the current regulations [44], water levels are referenced to the height system of Kronstadt (PL-KRON86-NH) (until 2019) and Amsterdam (PL-EVRF2007-NH). For the area of Sopot marina surveys, the closest gauging stations are Gda´nsk Port Północny and Gdynia. Interpolation between observations at distant points carries the risk that in the location of bathymetric measurements, the actual water level deviates from the interpolated level Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 19

The single beam echo sounder (SBES) was calibrated using the tare method, which involves adjusting the average speed of sound in water at known depths to echo sounder indications. The basic method for measuring the speed of sound is the use of a CTD (conductivity, temperature, depth) sensor or a sound velocity profiler (SVP). An alternative is the measurement of water temperature and the determination of the speed of sound based on an empirical equation when the salinity is known. Certain echo sounders determine the speed of sound in water based on the temperature by means of a sensor located in the transducer. Oceanographic measurements which are part of hydrographic surveys include water level observations and measurements of the speed of sound in water [43]. The water level can be observed on a gauging station which is situated in a marina or port and is referenced to the geodetic height system used in a particular country. In Poland, according to the current regulations [44], water levels Remoteare referenced Sens. 2020, 12to, the 737 height system of Kronstadt (PL-KRON86-NH) (until 2019) and Amsterdam9 (PL- of 18 EVRF2007-NH). For the area of Sopot marina surveys, the closest gauging stations are Gdańsk Port Północny and Gdynia. Interpolation between observations at distant points carries the risk that in the duelocation to e.g., of bathymetric wind causing measurements, water lifting. the Another actual solutionwater level is todeviates periodically from the measure interpolated the water level level due aboveto e.g. thewind sea causing level using water a GNSSlifting. receiver Another and solution a geoid is model.to periodically measure the water level above the seaDuring level theusing surveys, a GNSS the receiver water levels and a recorded geoid model. at the Gda´nskPort Północny (at a distance of 9.7 km fromDuring Sopot) andthe Gdyniasurveys, (at the a distancewater levels of 7.8 recorded km from at Sopot) the Gda gaugingńsk Port stations Północny were (at identical a distance and of were 9.7 adoptedkm from forSopot) the reduction and Gdynia of depth (at a distance measurement of 7.8 tokm the from chart Sopot) datum. gauging The methodology stations were for identical considering and waterwere adopted level measurements for the reduction during of hydrographic depth measur surveysement isto described the chart in datum. detail inThe [45 methodology,46]. for consideringSounding water profiles level havemeasurements been planned during for hydrographic the marinaand surveys pier is areas, described taking in intodetail account in [45–46]. the followingSounding assumptions: profiles have been planned for the marina and pier areas, taking into account the followingArea includesassumptions: a rectangle 800 m long, calculated along the coast and symmetrical to the pier and • • 500 m wideArea corresponding includes a rectangle to the pier 800 length,m long, excluding calculated the along marina the coast (Figure and 10 symmetrical). to the Profile length:pier and 285–492 500 m m.wide corresponding to the pier length, excluding the marina (Figure 10). • • Profile length: 285–492 m. Total length of profiles: 42 km. • • Total length of profiles: 42 km. Distance between profiles: 8 m. • • Distance between profiles: 8 m. Minimum planned depth: 40 cm. • • Minimum planned depth: 40 cm. Sea level will be provided by the gauging stations of the Institute of Meteorology and Water • • Sea level will be provided by the gauging stations of the Institute of Meteorology and Management (IMGW) in Gda´nskPort Północny and Gdynia. Water Management (IMGW) in Gdańsk Port Północny and Gdynia.

Figure 10. Planned sounding profiles profiles for the hydrographic vessel. 3. Results 3. Results 3.1. Geodetic Measurement Results A photogrammetric model was developed using the Pix4d Mapper Pro software which enables photographic data processing and generation based on three-dimensional models and orthophotomaps. A median of 21,128 nodal points on a single photograph was obtained, which indicates a relatively high number of characteristic points and areas, considering the frequently little diversified coverage of the area surface. Figure 11 shows a 3D model built based on the UAV photos [25]. A comparative picture of point clouds is illustrated in Figure 12, obtained from measurements using TLS and UAV methods. To assess the accuracy of the 3D beach model made, it is necessary to compare it to the state geodetic coordinate system used in Poland (Figure 13)[47]. Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 19

3.1. Geodetic Measurement Results Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 19 A photogrammetric model was developed using the Pix4d Mapper Pro software which enables photographic data processing and generation based on three-dimensional models and 3.1. Geodetic Measurement Results orthophotomaps. A median of 21,128 nodal points on a single photograph was obtained, which indicatesA photogrammetric a relatively high model number was of developed characteristic using points the Pix4d and areas, Mapper considering Pro software the frequently which enables little photographicdiversified coverage data ofprocessing the area surface. and Figuregenerati 11on shows based a 3D on model three-dimensional built based on the models UAV photos and orthophotomaps.[25]. A median of 21,128 nodal points on a single photograph was obtained, which indicates a relatively high number of characteristic points and areas, considering the frequently little

Remotediversified Sens. 2020 coverage, 12, 737 of the area surface. Figure 11 shows a 3D model built based on the UAV photos10 of 18 [25].

Figure 11. 3D model of the area adjacent to the Sopot pier, obtained using an unmanned aerial vehicle (UAV).

FigureA comparative 11. 3D model picture of the ofarea area point adjacent adjacent clouds to the is Sopot Sopotillustr pi pier,ateder, obtained in Figure using 12, an obta unmanned ined from aerial measurements vehicle using(UAV).(UAV). TLS and UAV methods.

A comparative picture of point clouds is illustrated in Figure 12, obtained from measurements using TLS and UAV methods.

Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 19

To assess the accuracy of the 3D beach model made, it is necessary to compare it to the state FigureFigure 12.12.Two Two point point clouds clouds originating originating from from measurements measuremen usingts using terrestrial terrestrial laser laser scanning scanning (TLS) (TLS) and geodetic coordinate system used in Poland (Figure 13) [47]. unmannedand unmanned aerial aerial vehicle vehicle (UAV) (UAV) methods. methods.

Figure 12. Two point clouds originating from measurements using terrestrial laser scanning (TLS) and unmanned aerial vehicle (UAV) methods.

Figure 13. Algorithm of the terrestrialterrestrial laserlaser scanningscanning (TLS)(TLS) methodmethod accuracyaccuracyestimation, estimation, where: where:∆ ΔTx8Tx8

and ∆ΔPL-2000PL-2000 areare absolute absolute errors errors corresp correspondingonding the the Trimble Trimble Tx8 laser scanner and PL-2000 nationalnational

system, respectively. uuTLSTLS—uncertainty—uncertainty of of the the considered considered method. method.

In the discussed TLS case study, the detailed inaccuracy components of the applied laser scanner measurement are equal to [47]: • Inaccuracy of the Trimble Tx8 laser scanner—1 cm horizontally and 1.5 cm vertically. • Inaccuracy of the Trimble R10 GNSS receiver used to determine the coordinates of reference points—1 cm horizontally and 1.5 cm vertically. • Inaccuracy of the PL-2000 system—2 cm horizontally and vertically. A cloud of TLS points after transformation to the PL-2000 system, including the data collected at the determined area (terrain), can be a reference set for the cloud of points obtained by the UAV method in the form of terrain digital photos. A series of photos, processed by the appropriate program, enables creation of a space projection of the investigated area (terrain), in which besides the X-Y coordinates, the Z coordinate is determined in relation to each point of the cloud. On the basis of the reference points coordinates, the obtained local coordinates are transformed in the PL-2000 system. In practice, the sequence of operations realized on the basis of photo data does not allow achievement of a satisfactory level of accuracy of the data on the investigated area [47]. In the UAV method of terrain imaging, the picture processing operation resulted in the generation of a cloud containing 20,666,253 points, which translates into a density of approx. 117 points per m2. Numerical data processing was carried out using a high-parameter workstation (16 GB RAM, i7-6600U, GTX 1070) which lasted for 3 hours 42 minutes. The following values of errors for particular coordinates were obtained: 2.83 m (X), 4.08 m (Y) and 9.81 m (Z) [47]. In the discussed UAV case study, the detailed inaccuracy components of the applied unmanned aerial vehicle are equal to [47]: • Camera px resolution—2.25 cm. • Inaccuracy of UAV position: X, Y—a few meters, Z—several meters. To estimate the quality of the UAV method measurement, it is proposed to use the TLS data cloud as reference data. These data are obtained in the same area as the data in the UAV data cloud. Then, it is possible to estimate the systematic X-Y-Z coordinates errors components in the UAV cloud, as well as their dispersion (noise) in relation to reference values (Figure 14) [47].

Remote Sens. 2020, 12, 737 11 of 18

In the discussed TLS case study, the detailed inaccuracy components of the applied laser scanner measurement are equal to [47]:

Inaccuracy of the Trimble Tx8 laser scanner—1 cm horizontally and 1.5 cm vertically. • Inaccuracy of the Trimble R10 GNSS receiver used to determine the coordinates of reference • points—1 cm horizontally and 1.5 cm vertically. Inaccuracy of the PL-2000 system—2 cm horizontally and vertically. • A cloud of TLS points after transformation to the PL-2000 system, including the data collected at the determined area (terrain), can be a reference set for the cloud of points obtained by the UAV method in the form of terrain digital photos. A series of photos, processed by the appropriate program, enables creation of a space projection of the investigated area (terrain), in which besides the X-Y coordinates, the Z coordinate is determined in relation to each point of the cloud. On the basis of the reference points coordinates, the obtained local coordinates are transformed in the PL-2000 system. In practice, the sequence of operations realized on the basis of photo data does not allow achievement of a satisfactory level of accuracy of the data on the investigated area [47]. In the UAV method of terrain imaging, the picture processing operation resulted in the generation of a cloud containing 20,666,253 points, which translates into a density of approx. 117 points per m2. Numerical data processing was carried out using a high-parameter workstation (16 GB RAM, i7-6600U, GTX 1070) which lasted for 3 hours 42 minutes. The following values of errors for particular coordinates were obtained: 2.83 m (X), 4.08 m (Y) and 9.81 m (Z) [47]. In the discussed UAV case study, the detailed inaccuracy components of the applied unmanned aerial vehicle are equal to [47]:

Camera px resolution—2.25 cm. • Inaccuracy of UAV position: X, Y—a few meters, Z—several meters. • To estimate the quality of the UAV method measurement, it is proposed to use the TLS data cloud as reference data. These data are obtained in the same area as the data in the UAV data cloud. Then, it is possible to estimate the systematic X-Y-Z coordinates errors components in the UAV cloud, as well Remoteas their Sens. dispersion 2020, 12, x FOR (noise) PEER in REVIEW relation to reference values (Figure 14)[47]. 12 of 19

Figure 14. AlgorithmAlgorithm of of the the unmanned unmanned aerial aerial vehicle vehicle (UAV) (UAV) method accuracy estimation, where CloudTLS andand CloudCloudUAVUAV areare clouds clouds of of points points related related to to the the terrestrial terrestrial laser laser scanning scanning (TLS) (TLS) and and UAV UAV methods, respectively, ∆Δ and u areare absoluteabsolute valuesvalues ofof occurredoccurred errorserrors andand relatedrelated uncertainties,uncertainties, uuTLSTLS and uUAV areare measurement measurement uncertainties uncertainties characterizi characterizingng the the TLS TLS and and UAV UAV methods, methods, respectively. respectively.

Dedicated softwaresoftware is is currently currently under under construction construc totion allow to forallow the determinationfor the determination of the uncertainty of the uncertaintycharacteristics characteristics of X-Y-Z coordinates of X-Y-Z collectedcoordinates in largecollected data in files large obtained data files using obtained the UAV using method the UAV [47]. methodResults [47]. of the executed measurements of the terrain geometry are treated as random variables, thatResults is, they of are the subject executed to measurements statistical rules of and the terra the probabilityin geometry calculus are treated is usedas random to assess variables, them. thatThe is, term they “metrological are subject to aspects” statistical in rules this and paper the concerns probability the calculus estimation is used of uncertainty to assess them. in relation The term to “metrologicalthe measurements aspects” of the in sea this shore paper and concerns the seafloor the underestimation investigation of uncertainty [48]. Measurement in relation resultsto the measurementsobtained with useof the of the sea aforementioned shore and the methodsseafloor definedunder investigation as TLS and UAV, [48].respectively, Measurement create results the obtained with use of the aforementioned methods defined as TLS and UAV, respectively, create the series (clouds) of measurement points. Each of them has different references in the X-Y-Z coordinates system. Consequently, it is possible to estimate an uncertainty of determination of space coordinates of each measurement point in cloud by applying the methodology of type B uncertainty determining [49]. Uncertainty of type B concerns mainly the estimation of inaccuracy of measurement instrumentation. Its calculation requires determination of the absolute error threshold value ΔXt and assumption of the given function of probability distribution for this error. Then, for a given component, the type B standardized uncertainty us(x) is expressed as for normal distribution [47]: ΔX ux()= t (2) s 3 or for uniform distribution: ΔX ()= t uxs (3) 3 where: ΔXt—maximum permissible error threshold value. Taking into account that estimation of the type B measurement procedure may concern a few inaccuracy components characterized by the standard uncertainties us1(x), us2(x), ..., and usn(x), we can calculate the complex uncertainty uc(x) as [47]:

()=+++22 () () 2 () uxcss u12 x u x... u sn x (4) where: us1(x), us2(x), ..., and usn(x) correspond to all sources of errors in the considered measurement procedure. Then, for a given component, the type B extended uncertainty UB(x) is expressed as [47]: ()=⋅ () UxBC kux (5) where: k—coefficient of extension (coverage factor).

Remote Sens. 2020, 12, 737 12 of 18 series (clouds) of measurement points. Each of them has different references in the X-Y-Z coordinates system. Consequently, it is possible to estimate an uncertainty of determination of space coordinates of each measurement point in cloud by applying the methodology of type B uncertainty determining [49]. Uncertainty of type B concerns mainly the estimation of inaccuracy of measurement instrumentation. Its calculation requires determination of the absolute error threshold value ∆Xt and assumption of the given function of probability distribution for this error. Then, for a given component, the type B standardized uncertainty us(x) is expressed as for normal distribution [47]:

∆X u (x) = t (2) s 3 or for uniform distribution: ∆Xt us(x) = (3) √3 where: ∆Xt—maximum permissible error threshold value. Taking into account that estimation of the type B measurement procedure may concern a few inaccuracy components characterized by the standard uncertainties us1(x), us2(x), ..., and usn(x), we can calculate the complex uncertainty uc(x) as [47]: q ( ) = 2 ( ) + 2 ( ) + + 2 ( ) uc x us1 x us2 x ... usn x (4)

where: us1(x), us2(x), ..., and usn(x) correspond to all sources of errors in the considered measurement procedure. Then, for a given component, the type B extended uncertainty UB(x) is expressed as [47]:

UB(x) = k u (x) (5) · C where: k—coefficient of extension (coverage factor). First of all, the detailed uncertainty components can be analysed and calculated for the method TLS and related to its measurement instrumentation. Taking into consideration known instrumental and method data, the following error components are considered [47]:

Reference system TLS accuracy. • Satellite receiver R10 maximum errors. • PL-2000 system errors threshold values. • The uncertainty should be estimated for all three coordinates (X, Y, and Z). The uncertainty budget is shown in Table1 for X or Y coordinates and in Table2 for Z coordinate [47].

Table 1. Uncertainty budget (X or Y coordinate) of terrestrial laser scanning (TLS) system.

Quantity Max Error Probability Distribution Standard Uncertainty

(X, Y)Tx8 1 cm uniform 0.58 cm (X, Y)R10 1 cm uniform 0.58 cm (X, Y)PL-2000 2 cm uniform 1.15 cm uTLS(X, Y) 1.41 cm

Assuming the normal distribution of complex uncertainties for all coordinates, the extended uncertainty (5) uTLS is 2.82 cm (X or Y coordinate) and 3.36 cm (Z coordinate) (p = 0.95) [47]. Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 19

First of all, the detailed uncertainty components can be analysed and calculated for the method TLS and related to its measurement instrumentation. Taking into consideration known instrumental and method data, the following error components are considered [47]: • Reference system TLS accuracy. • Satellite receiver R10 maximum errors. • PL-2000 system errors threshold values. The uncertainty should be estimated for all three coordinates (X, Y, and Z). The uncertainty budget is shown in Table 1 for X or Y coordinates and in Table 2 for Z coordinate [47].

Table 1. Uncertainty budget (X or Y coordinate) of terrestrial laser scanning (TLS) system.

Quantity Max error Probability distribution Standard uncertainty (X, Y)Tx8 1 cm uniform 0.58 cm (X, Y)R10 1 cm uniform 0.58 cm (X, Y)PL-2000 2 cm uniform 1.15 cm uTLS(X, Y) 1.41 cm Remote Sens. 2020, 12, 737 13 of 18 Table 2. Uncertainty budget (Z coordinate) of terrestrial laser scanning (TLS) system.

Table 2.QuantityUncertainty Max budget error (ZProbability coordinate) dist of terrestrialribution laserStandard scanning uncertainty (TLS) system. (Z)Tx8 1.5 cm uniform 0.87 cm Quantity(Z)R10 Max1.5 Errorcm Probability uniform Distribution Standard 0.87 cm Uncertainty (Z)PL-2000 2 cm uniform 1.15 cm (Z)Tx8 1.5 cm uniform 0.87 cm (Z)R10uTLS(Z) 1.5cm uniform 1.68 0.87 cm cm (Z) 2 cm uniform 1.15 cm Assuming PL-2000the normal distribution of complex uncertainties for all coordinates, the extended uTLS(Z) 1.68 cm uncertainty (5) uTLS is 2.82 cm (X or Y coordinate) and 3.36 cm (Z coordinate) (p = 0.95) [47]. 3.2. Hydrographic Measurement Results 3.2. Hydrographic Measurement Results Hydrographic measurements enabled creation of the bathymetric chart of the tombolo Hydrographic measurements enabled creation of the bathymetric chart of the tombolo phenomenon in Sopot. This map was developed based on measurements realised in October 2018. phenomenon in Sopot. This map was developed based on measurements realised in October 2018. In In addition, bathymetric data (isobaths) from the 2011 ENC were processed and supplemented with a addition, bathymetric data (isobaths) from the 2011 ENC were processed and supplemented with a point cloud from the beach and pier (Figure 15). point cloud from the beach and pier (Figure 15).

(a) (b)

FigureFigure 15. 15.Bathymetric Bathymetric chart chart of of the the tombolo tombolo phenomenon phenomenon in Sopotin Sopot (a) ( anda) and a bathymetric a bathymetric map map based based on dataon data from from the 2011the 2011 electronic electronic navigational navigational chart chart (ENC) (ENC) (b). (b).

FigureFigure 15 15 clearly clearly showsshows thatthat thethe oofficialfficial 20112011 chartchart ofof thisthis regionregion didiffersffers significantly significantly fromfrom the the actualactual results results of bathymetricof bathymetric measurements measurements carried ca outrried in out 2018. in Having 2018. consideredHaving considered the map presented the map inpresented Figure 15 ina, itFigure should 15a, be notedit should that be the noted berthing that area the locatedberthing in area the easternlocated partin the of theeastern pier andpart inof thethe southernpier and partin the of southern the yacht part marina of the is not yacht suitable marina for is navigation not suitable due for to navigation the very shallow due to depthsthe very (1–1.5 shallow m) occurringdepths (1–1.5 there. m) It occurring should also there. be stressed It should that also the be authors stressed carried that the out author bathymetrics carried measurements out bathymetric of the areas in the vicinity of the Sopot pier using a manned vessel Navigator One AMG, which has one of the shallowest drafts possible (30 cm). Nevertheless, the vessel’s bottom repeatedly scraped the sand on the seafloor, which posed a risk of damage to both the vessel and the measuring equipment. These measurements clearly proved that sailing on such vessels (even with the shallowest draft possible) in ultra-shallow waters is risky and poses a hazard of damaging the measurement systems [50,51]. The shape of the isobaths from measurements of the tombolo phenomenon (Figure 15a) indicates that as a result of reduced sand transport along the coast, sand began to accumulate on the right side of the pier in Sopot. The result of this state is that the seafloor has significantly increased over the entire length of the waterbody connecting the marina with the shore. At the inner marina, the depths decreased by 1.5–2 m. Halfway between the marina and the shore, the seafloor rose by about 1 m. The width of the peninsula, which will be built in the future, can be estimated at about 90–100 m, at the water surface. The degree of advancement of the progressive tombolo phenomenon can also be estimated by comparing two digital terrain models (DTMs) [52] from 2011 and 2018 (measured). As a result of their comparison, the volume sand deposited over 8 years on the pier area in Sopot was determined. Figure 16 presents a map in which depth differences between the tombolo DTM models developed Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 19

measurements of the areas in the vicinity of the Sopot pier using a manned vessel Navigator One AMG, which has one of the shallowest drafts possible (30 cm). Nevertheless, the vessel’s bottom repeatedly scraped the sand on the seafloor, which posed a risk of damage to both the vessel and the measuring equipment. These measurements clearly proved that sailing on such vessels (even with the shallowest draft possible) in ultra-shallow waters is risky and poses a hazard of damaging the measurement systems [50–51]. The shape of the isobaths from measurements of the tombolo phenomenon (Figure 15a) indicates that as a result of reduced sand transport along the coast, sand began to accumulate on the right side of the pier in Sopot. The result of this state is that the seafloor has significantly increased over the entire length of the waterbody connecting the marina with the shore. At the inner marina, the depths decreased by 1.5–2 m. Halfway between the marina and the shore, the seafloor rose by about 1 m. The width of the peninsula, which will be built in the future, can be estimated at about 90–100 m, at the water surface. The degree of advancement of the progressive tombolo phenomenon can also be estimated by comparing two digital terrain models (DTMs) [52] from 2011 and 2018 (measured). As a result of their Remote Sens. 2020, 12, 737 14 of 18 comparison, the volume sand deposited over 8 years on the pier area in Sopot was determined. Figure 16 presents a map in which depth differences between the tombolo DTM models developed on the onbasis the basis of measurements of measurements and and the the ENC ENC data data are are sh shownown in in colour. TheThe volumevolume sand sand deposited deposited is is 31,963,45031,963,450 m 3m.3 This. This means means that that to to remove remove all all the the sand sand accumulated accumulated with with the the tombolo tombolo phenomenon, phenomenon, 26632663 large large dump dump trucks trucks with with a capacitya capacity of of 12 12 m 3mwould3 would have have to to be be used. used.

FigureFigure 16. 16.Map Map showing showing depth depth di differencesfferences between between tombolo tombolo digital digital terrain terrain models models (DTMs) (DTMs) developed developed onon the the basis basis of of measurements measurements and and electronic electronic navigational navigational chart chart (ENC) (ENC) data. data. The measurements and bathymetric charts made it possible to determine the areas of dredging The measurements and bathymetric charts made it possible to determine the areas of dredging works that are planned by the city of Sopot. It is noteworthy that the width of the depth changes due works that are planned by the city of Sopot. It is noteworthy that the width of the depth changes due to sand accumulation is about 300 m. to sand accumulation is about 300 m. 3.3. Additional Research Results 3.3. Additional Research Results In order to determine the scale of the tombolo phenomenon, it was decided to analyse the In order to determine the scale of the tombolo phenomenon, it was decided to analyse the coastline course in Sopot over the last 10 years. To this end, aerial photographs from the website coastline course in Sopot over the last 10 years. To this end, aerial photographs from the website https://mapa.trojmiasto.pl/ provided by the MGGP Aero (years 2008, 2011, 2014, and 2016) and satellite https://mapa.trojmiasto.pl/ provided by the MGGP Aero (years 2008, 2011, 2014, and 2016) and photographs from Google Earth Pro, a popular geographical photo and information sharing application, satellite photographs from Google Earth Pro, a popular geographical photo and information sharing were used. Vectorisation and cartographic adjustment of the photographs in terms of the inclination application, were used. Vectorisation and cartographic adjustment of the photographs in terms of the angle, rotation, and scale helped to identify considerable morphological changes in the coast in the inclination angle, rotation, and scale helped to identify considerable morphological changes in the vicinity of Sopot (Figure 17)[25]. coast in the vicinity of Sopot (Figure 17) [25]. Moreover, the conducted analyses enabled both the calculation of the Sopot beach area in selected years and the determination of its area variability degree over the years (Table3)[25].

Table 3. The increase in the beach surface in Sopot due to the tombolo phenomenon, developed on the basis of satellite images.

Month and Year Surface Area [m2] Increase in Surface Area [m2] May 2008 37 786 0 July 2010 41 834 4 048 April 2011 42 583 4 797 May 2012 40 600 2 814 June 2013 40 364 2 578 July 2014 48 059 10 273 October 2015 45 404 7 618 September 2016 47 977 10 191 August 2017 50 830 13 044 May 2018 50 848 13 062 Remote Sens. 2020, 12, 737 15 of 18 Remote Sens. 2020, 12, x FOR PEER REVIEW 15 of 19

Figure 17. The course of the Sopot beach coastline over the years 2008-2018.

4. DiscussionMoreover, the conducted analyses enabled both the calculation of the Sopot beach area in selectedAs ayears result and of the constructiondetermination of of the its marina area variability in Sopot, degree there was over a the change years in (Table the morphological 3) [25]. forms of the coast around the pier on a sandy beach of 800 m length. Accumulating sediment transport Table 3. The increase in the beach surface in Sopot due to the tombolo phenomenon, developed on along the beach has caused the seafloor to rise, which has lead to a tombolo in the proceeding years the basis of satellite images. (a narrow belt connecting the mainland with an island lying near the shore formed as a result of sand and gravel being depositedMonth and by yea sear currents). Surface area In addition[m2] Increase to coastline in surface changes, area [m this2] phenomenon has led to increased vegetationMay 2008 of this area, threatening 37 786 the city’s spa character. 0 This publication presents research results aimedJuly at integrating2010 measurement 41 834 methods and 4 systems048 of modern geodesy and hydrography for geospatialApril 2011 modeling of the 42 583 tombolo phenomenon. 4 797 May 2012 40 600 2 814 As part of the study, TLS and UAV beach measurements were carried out with an analysis of their June 2013 40 364 2 578 accuracy, coastline changes were determined based on the analysis of satellite images, bathymetric July 2014 48 059 10 273 measurement of theOctober waterbody 2015 adjacent 45 to 404 the sea was carried out, 7 618 and bathymetric charts of the tombolo phenomenonSeptember were made 2016 and they 47 were 977 compared with data 10 from191 the ENC. Finally, the volume and area of accumulatingAugust sand 2017 in the developing 50 830 tombolo was determined. 13 044 The analysis resultsMay can 2018 be used by the 50 local 848 government in Sopot 13 062 to solve this important problem for the town, as well as develop a systemic approach to sand removal that is very expensive. It is recommended4. Discussion that dredging works be carried out only in selected places and in such a way that restoresAs a sediment result of transportthe construction along theof the beach, marina because in Sopot, its completethere was cessation a change wouldin the morphological accelerate the processforms of of the tombolo coast development,around the pier as well on asa sandy have a beach significant of 800 impact m length. on the Accumulating waterbody’s vegetation. sediment transportBased along on Figure the 17beach, it can has be caused concluded the that seafloor the municipal to rise, beachwhich area has has lead been to steadilya tombolo increasing, in the andproceeding the coastline years (a to narrow the east belt of the connecting pier is increasingly the mainland approaching with an island the yachtlying near marina. the shore It can formed also be 2 seenas a result that in of the sand years and 2008–2018,gravel being the deposited coastal belt by sea area currents). in this place In addition increased to bycoastline approx. changes, 13,062 this m , andphenomenon the coastline has shiftedled to increased towards thevegetation sea by 53 of m this [25 area,]. This threatening has happened the city's mainly spa as character. a result of This the constructionpublication presents of an additional research breakwater results aimed protecting at integrating the pier againstmeasurement storms. methods Therefore, and it issystems necessary of tomodern conduct geodesy continuous and hydrography cartographic for monitoring geospatial in modeling this area. of the tombolo phenomenon. ItAs should part of also the bestudy, emphasized TLS and thatUAV hydrographic beach measurements measurements were carried with a out manned with vesselan analysis in such of shallowtheir accuracy, water are coastline difficult andchanges can oftenwere be determined unachievable based due toon the the small analysis depths of (below satellite 1 m). images, Hence, bathymetric measurement of the waterbody adjacent to the sea was carried out, and bathymetric charts of the tombolo phenomenon were made and they were compared with data from the ENC. Finally, the volume and area of accumulating sand in the developing tombolo was determined.

Remote Sens. 2020, 12, 737 16 of 18 future work has focused on the use of the hydrographic USV, which was successfully carried out in December 2019. The proposed research methodology involving integration of geodetic and hydrographic methods aimed at creating a 3D tombolo model can be successfully used to assess other phenomena of this type.

Author Contributions: Conceptualization, C.S., J.M., and R.M.; data curation, M.S., P.D., and A.M.; investigation, M.S., P.D., and A.M.; methodology, C.S., J.M., and R.M.; supervision, C.S., J.M., and R.M.; visualization, M.S., P.D., and A.M.; writing—original draft, C.S. and M.S.; writing—review & editing, J.M., P.D., R.M., and A.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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