Multi-Sensor 3D Modeling of Natural Heritage:

Example of the Lake Zmajevo Oko

Lovre Panđa

, Ante Šiljeg

and Ivan Marić

University of Zadar, Department of Geography, Trg kneza Višeslava 9, Zadar, Croatia

Keywords: Zmajevo Oko, Geospatial Technologies, UAV, WASSP S3, MBES, Virtual Walk.

Abstract: In recent decades geospatial technologies (GST) have been affected by a process of rapid development. One

of their applications involves the documentation of the protected areas reference state and the development of

high-quality models for their preservation and management. The research area of this paper is Lake Zmajevo

oko (Lake Dragon eye), near Rogoznica (Croatia). The research goals were to create a multisensor model of

the lake and present the application of these technologies for promotional purposes. The secondary objective

was to obtain morphometric data about the lake. The research methodology included performing the process

of UAV photogrammetry and bathymetry. In UAV photogrammetry, a Phantom 4 Pro was used. In the

bathymetric survey, an integrated system composed of WASSP S3 multibeam depth sounder (MBES) and the

Hemisphere V320 GNSS smart antenna was used. The data collected by both methods were combined and an

multisensor high-quality model of the lake was created. Ten underwater tunnels on the steep sides of the lake

were detected. From the derived models, the volume and surface area of the lake, as well as the length of the

lake shoreline were calculated. Furthermore, a virtual walk around the lake was made to promote this area. A

physical model of the lake, which can serve as a souvenir, was printed with the 3D printer Prusa i3 MK3. The

collected high-quality data can serve as the basis for future research, while derived models and a virtual walk

can be used for its promotion.

1 INTRODUCTION

The rapid development of geospatial technologies

(GST) in the last twenty years (Bodzin and Cirucci,

2009; Bishop et al., 2012; Šiljeg et al., 2018), enabled

the modeling of complex processes and objects in

different levels of detail (LoD), depending on the

purpose of the research (Kyriakaki et al., 2014). To

improve its quality, multisensor models are

developed. These models are made from data

collected by different methods and techniques (Fabris

et al., 2010; Abdalla, 2016, Erenoglu et al., 2017)

using different sensors (Hackett and Shah, 1990). In

the process of generating multisensor models, data

obtained by UAV photogrammetry and bathymetry

are often being combined (Fabris et al. 2010; Šiljeg et

al 2022). Great advances and the increasing

availability of modern technology have enabled 3D-

4D-5D documentation, conservation and, digital

https://orcid.org/0000-0003-4549-4481

https://orcid.org/0000-0001-6332-174X

https://orcid.org/0000-0002-9723-6778

promotion of protected areas and facilities

(Remondino and Rizzi, 2010; Ficarra, 2011,

Kyriakaki et al., 2014). 3D documentation involves

collecting, processing, reproduction, and presentation

of geospatial data by determining the position, shape,

and dimensions of an object or area in three-

dimensional space to preserve the current state (Marić

et al., 2019) of cultural (Manić et al., 2013) or natural

heritage (Leonov et al., 2011; Bishop et al., 2012).

UNESCO (1972) uses the term “natural heritage” for

physical, biological, and geological features,

formations, and sites of exceptional value from an

aesthetic or scientific point of view (Boehler et al.,

2001). The study of natural heritage largely depends

on conservation because global climate change,

natural disasters, mass tourism, terrorism, and human

negligence greatly affect changes in the landscape,

and thus on the protected areas (Marić et al., 2019).

Furthermore, the rapid development of virtual reality

138

Pan

da, L., Šiljeg, A. and Mari

c, I.

Multi-Sensor 3D Modeling of Natural Heritage: Example of the Lake Zmajevo Oko.

DOI: 10.5220/0011065300003185

In Proceedings of the 8th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2022), pages 138-146

ISBN: 978-989-758-571-5; ISSN: 2184-500X

technology (VR) offers the possibility of using GSTs

for tourism purposes (Bruno et al., 2010; Leonov et

al., 2011; Tussyadiah et al., 2018). Also, the use of

3D printers enables the production of attractive

souvenirs. Therefore, the promotion of protected

natural areas, using modern GSTs is necessary if

decision-makers and administration want to keep up

with the modern requirements of tourism. The object

of this research was Lake Zmajevo oko near

Rogoznica (Figure 1). It was recorded using UAV

photogrammetry and multibeam echosounder

(MBES). The research goals were to create an m

model of the lake and present the application of these

technologies for promotional purposes. Also, the

secondary objective was to obtain morphometric data

about the lake (surface area, volume, depth, and

shoreline length).

2 STUDY AREA

The bathymetric survey covered the area of Lake

Zmajevo oko (Figure 1), while the UAV

photogrammetric survey covered the wider coastal

area. The area is characterized by a large terrain

roughness. It is located on the Gradina peninsula,

which is connected to the mainland by a thin isthmus

and surrounded by the bays of Soline and Koprišće.

The lake is connected to the sea by underground

tunnels. This is indicated by the salinity of the water,

the presence of the mediolateral staircase, and the

change of tides (Bakran-Petricioli et al., 1998).

Physicochemical properties are specific. The lake is

characterized by sudden changes in temperature,

salinity, and density caused by poor water mixing,

geomorphological features, and meteorological

conditions. The lake is naturally sheltered from the

wind since it is located in a valley. This is one of the

main reasons for the constant chemical stratification

(Ciglenečki et al., 1996; Bakran-Petricioli et al.,

1998; Bura-Nakić et al., 2012). The bottom layer is

characterized by a thick layer of hydrogen sulfide

which makes the water cloudy and white. Anoxia is

also present (Bakran-Petricioli et al., 1998; Bura-

Nakić et al., 2007; Bura-Nakić et al., 2012).

3 MATERIALS AND METHODS

UAV photogrammetry was used for the recording of

the water body and the wider coastal area in order to

obtain data about the surface area and the shoreline

length as well as to develop high-resolution models

Figure 1: Geographical position of the Lake Zmajevo oko.

for visualization. An MBES survey was performed to

obtain data on the depth and volume of the lake as

well as to detect and map underwater tunnels.

3.1 UAV Photogrammetric Survey

A UAV photogrammetric survey of Lake Zmajevo

oko was performed on July 19, 2019. The Phantom 4

Pro (Figure 2A) was used for imagery acquisition

while RTK-GPS Stonex S10 (Figure 2B) was used to

measure ground control (GCP) and check points

(CP). The detailed specifications of the used UAV are

available (URL1) and GNSS receiver at (URL2). The

Stonex S10 was also used to monitor the water level

during data acquisition.

Figure 2: Phantom 4 Pro and Stonex S10.

A UAV photogrammetric data acquisition and

processing were performed in several steps. The first

was to mark and collect GCPs and CPs. A total of 9

points (6 GCPs and 3 CPs) were marked and collected

(Figure 3). On the east side of the lake, it was not

possible to collect any points because the area was

fenced (private property).

The second step was the development of an

optimal survey plan. That included the selection of

mission types concerning the morphology of the

terrain and the distribution of points. Multiple double

grid missions were performed at flight altitudes of 70

m with a camera angle of 90°. Also, one circular and

one free flight mission was conducted to collect

oblique images of the steep coastal parts of the lake.

The front and side overlap was set to 80%. Before

data acquisition, the vision positioning system,

inertial measurement unit (IMU), and compass were

calibrated. An image processing was performed in

Multi-Sensor 3D Modeling of Natural Heritage: Example of the Lake Zmajevo Oko

139

Figure 3: Spatial distribution of GCPs and CPs.

Agisoft Metashape Professional 1.5.1. The image

workflow process consisted of several steps as in

Marić et al. (2019). The camera calibration was done

automatically whilst aligning photos in Metashape.

The image alignment, the building of the dense cloud,

and the mesh were set to medium quality. After

positioning the GCPs and CPs, the accuracy of the

derived models were calculated mean absolute error

(MAE) and root mean square error (RMSE).

The values of MAE and RMSE in CPs are stated in

(Table 1). The MAE for the X-axis was 1.48, for the

Y-axis 3.26, and for the Z-axis 1.16 cm. The total MAE

of the CPs was 3.99 cm. The MAE in the image

coordinate system was 0.18 pixels. The RMSE for the

X-axis was 1.72, for the Y-axis 3.48, and for the Z-axis

1.36 cm. The total RMSE of the CPs was 4.13 cm. The

RMSE in the image coordinate system was 0.20 pixels.

The MAE and RMSE correspond to the values

achieved in similar studies and papers (Agüera-Vega et

al., 2017; Marić et al., 2019; Rogers et al., 2020). We

are aware that only three CPs are not enough to assess

the accuracy of the model, but due to the large terrain

roughness, very dense vegetation, and private property

on the east side, there were not many available places

where more points could be marked.

Table 1: The accuracy of CPs.

Label X error

(cm)

Y error

(cm)

Z error

(cm)

Total

error (cm)

Image

(pix)

CP1 2.45 -2.63 -1.73 3.99 0.30

CP2 1.67 -2.18 -0.15 2.75 0.16

CP3 -0.32 -4.96 1.59 5.22 0.08

MAE 1.48 3.26 1.16 3.99 0.18

RMSE 1.72 3.48 1.36 4.13 0.20

3.2 MBES Survey

The first step of the MBES survey was to obtain

secondary data that were necessary for optimal

bathymetric survey planning. The reference mean

water level was determined using geodetic

measurement (GNSS-RTK receiver Stonex S10). The

reference mean water level was recognized as the

point marked by wet conditions, a difference in color,

and algae formed on the steep side of the lake. The

MBES survey was performed with an integrated

measurement system (Figure 4) which included eight

main components: (a) WASSP S3 Multibeam

Wideband Sounder c/w DRX (Figure 4B); (b) WASSP

Sensor Box (Figure 4D) with integrated Spatial IMU

(Figure 4E); (c) Hemisphere V320 GNSS Smart

Antenna (Figure 4A); (d) WMB-160 probe (Figure

4C); (e) battery and power cord; (f) configuration

computer and cable; (g) configuration software; (h)

Data Management and Exporting Software (CDX).

Figure 4: Integrated measuring system components.

All components were connected and configured

according to the manufacturer's instructions and

international standards concerning the characteristics

of the boat. In addition to the above system

configuration, it was important to calibrate the probe

during the measurement and to calculate the optimal

speed of sound in water based on the output results of

the temperature and salinity of the water. The visual

effect of incorrect speed of sound is manifested by the

concave or convex curvature of the flat seabed (Dong

et al., 2007; Dong et al., 2011) (Figure 5A).

Therefore, the device was calibrated on the flattened

part of the lake to minimize measurement errors

(Figure 5B). Considering the characteristics of

temperature and salinity on the day of measurement,

the speed of sound was 1531.58 m/s.

Figure 5: A) An example of the inaccuracy of the lake

bottom display due to an incorrect speed of sound; B) An

GISTAM 2022 - 8th International Conference on Geographical Information Systems Theory, Applications and Management

140

set speed of sound during the bathymetric survey of the

lake.

The MBES survey (Figure 6) was performed four

times (Table 2) to collect as many points as possible

so that a more accurate bathymetric model can be

made after the point filtering process. The percentage

of overlap was 50% because the relatively small

research area allowed it. The operating frequency was

set to 160 kHz, while the operating beam width was

20°. The ping rate varied from 10 to 14 /sec. The

device collected and displayed real-time data of boat

position, speed, azimuth, plane deviation, and depth

based on predefined user-defined system settings.

The GPS antenna was connected to the CROPOS

(Croatian Positioning System) system via the GSM

network. That enabled a very high accuracy of the

collected data. The system collected data in the

WGS84_UTM_N33 projection.

Figure 6: The MBES survey of the Lake Zmajevo oko.

Table 2: Characteristics of collected unfiltered points.

Survey Min Z Max Z Z Range Number of

points

1 -0.91 -18.54 -17.63 1 397 243

2 -0.73 -19.82 -19.09 1 267 833

3 -1.07 -20.37 -19.30 1 487 819

4 -1.25 -16.91 -15.66 521 969

Total 4 674 864

The measured data were transferred to the desktop

computer for further processing and interpretation via

software packages for guidance (CDX) and data

management (Data Manager). Numerous authors

stated that one of the most demanding tasks in the

MBES process is data filtering (An et al., 2019; Šiljeg

et al. 2022). Following the Šiljeg et al. (2022) three

methods were used in the data filtering process: (a)

manual removal which is an extremely time-

consuming and demanding process; (b) automatic

method Label Connected Components in

CloudCompare software, and (c) SOR filter

(Statistical Outlier Removal).

A digital bathymetry model (DBM) with

continuous surfaces was produced from a filtered

dense point cloud using the interpolation method

Natural Neighbour. Spatial resolution was calculated

using the Grid calculator and the Point pattern

analysis method was chosen (Hengl, 2006). The

optimal spatial resolution of 5 cm was selected for the

multisensor model (DBM+DSM). It represents a

compromise between the sampling density of points

collected by UAV and MBES points and the research

area. The resolution was reduced to 10 cm for

visualization. The validation method of the generated

DBM included only the visual inspection of the

created model with the findings of an experienced

diver who knows the bottom of the lake in order to

make sure that the most conspicuous outliers were

removed. In the final phase of the processing, the

surface area, volume, and shoreline length were

calculated. The volume was calculated from a regular

grid obtained by interpolation. Three Newton-Cotes

formulas integrated within the Surfer software were

used: (a) Trapezoidal Rule; (b) Simpson's Rule; and

lake and the shoreline length were calculated in the

ArcMap software.

3.3 Virtual Walk

Using the trial version of Lumion, a short promotion

video and six 360° panoramas were created. First, the

created 3D models in .obj format were imported. The

high polygon count within the photogrammetry

process caused a slowdown. Therefore, the number of

it was reduced. The next step included a selection of

the six locations which represented the best model

(Figure 7). The final step was to set the parameters in

the render all 360° panoramas option. The output

quality was set to 4/5, the stereoscopic view was

turned on and the distance between the two eyes was

set to 64.0 mm to give the appearance of space depth.

The images were generated for computer or mobile

platforms. Panoramas were exported in resolution

4096x4096. To create a promotional video, 50

selected viewpoints were rendered. Video quality was

set to 5/5, and the number of frames per second to 30.

The video resolution was set to Full HD (1920x1080).

Multi-Sensor 3D Modeling of Natural Heritage: Example of the Lake Zmajevo Oko

141

Figure 7: Spatial distribution of the six 360° panoramas.

3.4 3D Print

A 3D printing of the Lake Zmajevo oko was

performed using a 3D printer Prusa i3 MK3. Creating

a 3D physical model consisted of several steps

(Figure 8). First, the data was converted to .stl format.

This mesh must be dense enough to achieve the

satisfactory quality of the physical model. However,

it should not be too complex to avoid overloading and

slowing down of the computer (Marić et al., 2019).

Then, the 3D printing parameters were set. The

orientation of the model and the type of filament were

adjusted. PLA (polylactide) plastic has proven to be

the best choice because of its compactness, strength,

and further processing.

The height of the filament layer was set at 0.15

mm. Then the model was exported in the G-code

format that is standard for most 3D printers and

industrial machines. In the final phase, the model was

transferred to a previously calibrated 3D printer with

a USB memory and 3D printing of the lake was

performed.

Figure 8: 3D printing process of the Lake Zmajevo oko.

4 RESULTS AND DISCUSSION

4.1 UAV Photogrammetric Survey

From the UAV photogrammetric survey conducted

on July 19, 2019, the following models of the Lake

Zmajevo oko were derived: (a) a high-resolution DOP

(Figure 9); (b) a high-resolution DSM (Figure 10);

and (c) a 3D model of the lake (Figure 11). A high-

resolution DOP of the Lake Zmajevo oko had a

spatial resolution of 1.56 cm (Figure 9).

Figure 9: A high-resolution DOP of the lake.

A DSM (GSD = 3.12 cm) (Figure 10) of the wider

coastal area of the Lake Zmajevo oko was generated

from a dense point cloud. The high value of resolution

is because the Quality parameter in Agisoft

Metashape was set to High.

Figure 10: A high-resolution DSM of the lake.

Finally, a 3D model of the wider coastal area of

Lake Zmajevo oko was generated (Figure 11). The

derived model can be used for promotional purposes

by creating a virtual walk. It is necessary to render

selected points around the lake to do that. However,

it is also possible to make a virtual walk in which the

user cannot manage and make decisions, but the

whole walk is pre-programmed. Panoramic 360°

shots and a short promotional video were made based

on this model.

GISTAM 2022 - 8th International Conference on Geographical Information Systems Theory, Applications and Management

142

Figure 11: A 3D model of the lake.

4.2 MBES Survey

A total of 4 674 864 points were collected. The

operating frequency of 160 kHz allowed mapping the

actual bottom of the lake despite the presence of a

thick layer of hydrogen sulfide in the bottom layer.

The minimum collected value of depth was -0.73 m,

and the maximum depth was -20.37 m (Table 1).

These maximum and minimum values are the results

of outliers present in the unfiltered point cloud. They

were removed following the point-cloud processing

approach presented in Šiljeg et al. (2022). Thanks to

the wide operating angle, the possibility of side-

scanning, and the selected operating frequency, ten

horizontal underwater tunnels (cracks) were detected

and marked (Figure 12).

Figure 12: Locations of underwater tunnels (cracks).

A DBM with a maximum depth of -13.45 m was

generated. The first multisensor model of the Lake

Zmajevo oko was created by merging a DBM with a

DSM created by UAV photogrammetry (Figure 13).

The volume of the Lake Zmajevo oko on the day

of the MBES survey was 90691.74 m

, the length of

the coastline was 513.15 m, and the surface area was

0.99 ha.

Figure 13: An multisensor model of the lake Zmajevo oko.

4.3 Virtual Walk

A promotional video called Zmajevo oko Virtual

Walk (Rogoznica, Croatia) was created (Figure 14).

It was published on the official website of the

Geospatial Analysis Laboratory (GAL).

Six viewpoints with panoramic 360

images were

also rendered (Figures 7 and 15). They cover the

researched area and give the potential visitor a quality

visual impression about this protected area. Videos

and panoramic images can be used to promote the

lake online.

Figure 14: A clip from a promotional video Zmajevo oko

Virtual Walk (Rogoznica, Croatia).

Panoramic 360

images of the lake can be found

on the 360Cities open-source platform. The

recordings can be viewed at the following links:

P1: https://cutt.ly/KInIJbQ;

P2: https://cutt.ly/GInI2ye;

P3: https://cutt.ly/gInI7Gy;

P4: https://cutt.ly/NInOtJP;

P5: https://cutt.ly/lInOp89;

P6: https://cutt.ly/KInOh64.

Multi-Sensor 3D Modeling of Natural Heritage: Example of the Lake Zmajevo Oko

143

Figure 15: Panoramic 360

images of the lake Zmajevo oko

from six viewpoints.

4.4 3D Print

The physical model (12 x 9 cm) of the Lake Zmajevo

oko (Figure 16) was generated. This model can be

used as a souvenir, and if desired, it can be

personalized. The filament is made of completely

workable material and can be painted as desired.

Anastasiadou and Vettese, (2019) examined visitor

preferences and managers' views on 3D printed

souvenirs and concluded that there is a tourist interest

in this type of souvenir and that this is the future of

tourism.

Figure 16: A physical model of the Lake Zmajevo oko.

5 CONCLUSIONS

In this research, we presented an application of

modern GSTs and their use for promotional purposes.

High-quality DSM and DBM have been created. A

multisensor model of this natural phenomenon was

made. The accuracy of the UAV images calculated by

the MAE is 3.99 cm and for the RMSE 4.13 cm in

CPs. The biggest challenge was the choice of the

operating frequency of the MBES considering the

physio-chemical characteristics of the lake. The

operating frequency of 160 kHz allowed mapping the

actual bottom of the lake despite the presence of a

thick layer of hydrogen sulfide in the bottom layer.

All four measurements were made with the same

frequency and dense cloud points were successfully

collected.

The integrated measuring system enabled the

detection of underwater tunnels. The distribution and

characteristics of the detected underwater tunnels will

be discussed in future research. Based on the DOP

and DSM, exact volume, surface area and shoreline

length data were derived. Based on 3D model of the

lake the virtual walk was generated. Finally, using a

modern 3D printer, a physical model of the lake with

a wider coastal area was made. That model can be

used as a souvenir to promote the Lake Zmajevo oko.

The documentation of this natural heritage can enable

further monitoring of changes and it is the starting

point for future research in this area.

ACKNOWLEDGEMENTS

This research was performed within the project

Bathymetric and geodetic survey of the lake Zmajevo

oko as a part of economic cooperation between the

University of Zadar and the Public Institution for the

Management of Protected Areas and Other Protected

Parts of Nature of Šibenik-Knin County and

supported by Croatian Science Foundation under the

project UIP-2017-05-2694.

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