3d models as didactic resources.pdf

Arias et al. Heritage Science (2022) 10:112 https://doi.org/10.1186/s40494-022-00738-x RESEARCH Open Access Use of 3D models as a didactic resource in...

Arias et al. Heritage Science (2022) 10:112 https://doi.org/10.1186/s40494-022-00738-x RESEARCH Open Access Use of 3D models as a didactic resource in archaeology. A case study analysis Francisco Arias1 , Carlos Enríquez2 , Juan Manuel Jurado3 , Lidia Ortega4 , Antonio Romero‑Manchado2   and Juan José Cubillas5*    Abstract The generation of 3D models through Terrestrial Laser Scanning has proved to be valuable tools for the study, docu‑ mentation and recreation of archaeological remains. In this context, it is described how to generate a physical model to provide not only to researchers, but also as teaching material for teachers for university students, facilitating their access and study. As a practical case, this article describes the acquisition, processing and management of archaeo‑ logical data in the archaeological site of Cástulo, Jaén, in South Spain. We expound how to get the 3D-printed model of the Muslim tower, showing how it is possible to generate a scale and very reliable reproduction of the structure, being also an useful and tangible material in the teaching of cultural heritage. Keywords: Teaching archaeology, 3D-model, 3D-printing, Tangible resources Introduction memory of the content dealt with. Therefore, it is impor- In education there are many environments where three- tant to have didactic material to offer students real prob- dimensional elements are used for learning content. lems and situations with which they interact and have to Tangible objects are common tools in fields such as engi- analyse and provide answers. neering and architecture, in geography or even in anat- If tangible objects are important in education, virtual omy classes, where models, relief maps or organ replicas learning environments are a digital solution with similar are used. In fact, several studies show that content is didactic characteristics for students in different fields of learned more quickly through the manipulation of tangi- science. Such is the case, for example, of the use of artifi- ble objects. This is due to the fact that, traditionally, cial intelligence as an educational resource. study and teaching have been limited to the one-sided- In the field of archaeology, the incorporation of new ness of information and the two-dimensionality of paper virtual environment technologies is particularly inter- or the screen, forgetting that a large part of the subjects esting. Consider that the traditional techniques used dealt with are representations of events and elements to preserve and document this information are draw- that belong to a 3D (three-dimensional) universe. In this ing, an inherently subjective process, and photography. sense, there are students and teachers who, although they However, these data collection procedures are tedious have mastered their subjects, do not manage to form a and its degree of detail and accuracy are not enough for mental image of the subject because they do not know today’s researchers and conservators needs. If metric the physical elements to relate them to, making it difficult measurements are needed, photogrammetric techniques to receive and retain the message, causing a provisional and a 3D realistic model are needed in order to docu- ment, manage and analyse the shape and dimension of the represented objects with a high degree of accuracy *Correspondence: [email protected] and resolution for each archaeological record and. 5 Dept Tecnologías de La Información Y Comunicación Aplicadas a La 3D models can also be obtained from other techniques Educación, Universidad Internacional de La Rioja, Logroño, Spain Full list of author information is available at the end of the article such as laser technology. In this sense, although 3D © The Author(s) 2022. 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The Creative Commons Public Domain Dedication waiver (http://​creat​iveco​ mmons.​org/​publi​cdoma​in/​zero/1.​0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Arias et al. Heritage Science (2022) 10:112 Page 2 of 15 laser technology was developed during the last half of three-dimensional metric model at very high resolu- the twentieth century, it was not until the mid-nineties tion. With the help of a RTK-GNSS, it is possible to when they become available to general researches. georeference the data to establish a spatial correlation Ten years ago and due to different reasons, among them between the different findings belonging to the same the high cost of 3D equipment , its systematic use for archaeological site, and their localization within the documentation or surveying archaeological sites was not appropriate spatial framework. This allows researchers established. to share and analyse, almost in real time, the current Terrestrial Laser Scanners (TLS) has been benefited and previous state of the archaeological records. from the development of technology. Its irruption has Considering that 3D models can be used to gener- meant a new revolution in the generation of 3D models, ate educational resources, either in the classroom or since the model is obtained at real scale, while the model even for presentations in visitor centres, museums and generated with photogrammetric techniques needs to be through the media to improve accessibility, engagement scaled to get real dimensions. Therefore, TLS can be used and understanding. Thus, and in favour of further to get a detailed record of complete or portions of a site, dissemination and value of these findings, the model which may be lost or changed, such as an archaeological is 3D printed. In the field of archaeology and cultural excavation or a site at risk. In addition, as an active sen- heritage, this provides a more holistic appreciation of sor, it does not require environmental light to generate objects, although it makes it necessary to develop basic the model, being very practical in environments with lit- guidelines for 3D printed models. Thus, 3D printing is tle or no lighting: caves, galleries, etc. not only vital in the field of object reconstruction, but Both techniques are non-invasive and not only they also for research, documentation, preservation and do not damage the archaeological artefacts but provide educational purposes, and has the potential to serve metric information and its 3D visualization as well. These these purposes in an accessible and inclusive way. geomatic techniques have been used in a complemen- The use of 3D printing in different fields of science is tary way, and their integration have been applied at the becoming more and more widespread, in fact there is process of study, planning and execution of the restora- already a specific regulation to regulate the whole pro- tion. Nevertheless, while digital photogrammetry cess of additive manufacturing. is a passive method, based on recording light reflected The main aim of this paper is to demonstrate the from illuminated surfaces by natural or artificial light, 3D model as an educational resource in archaeology, laser scanning is an active method, which records its for this purpose a description of how is it generated a own transmitted light reflected back from its target. The scaled and very reliable reproduction of any structure, selection of the method depends on several factors as regardless of its original size, can be generated in a sim- the user’s previous experience, the investigated object or ple and inexpensive way. For this case study, TLS tech- area, and the available budget and time. In this sense, nology has been selected instead of photogrammetric the advantages and disadvantages of using TLS or SfM SfM. The main reason was that, following the literature photogrammetry, it is worth mentioning the study car- review above, TLS gives very good results in terms of ried out in 2012 in which TLS was compared with terres- the metric quality of the products generated. This rea- trial photogrammetry in archaeological applications. son, together with the availability of this instrumenta- However, in recent building structure modelling appli- tion, led us to choose this technology. In this work, we cations , a comparison between the two techniques describe the process of acquisition, processing, man- finally favours TLS for reasons such as the need for fewer agement of archaeological data and the generation of a GCP (Ground Control Points) than SfM photogramme- tower of a Muslim castle in the archaeological site of try, and with reference to the accuracy of measurements Cástulo (Jaén). on the object geometry, work such as favour TLS Cástulo was an Ibero-Roman city in Baetica, near Lin- over photogrammetry. More recently, these two geo- ares (Jaén), in southern Spain. It was inhabited from the matics techniques have been compared for the study of end of the Neolithic period until the end of the fourteenth a Roman mosaic, in this case although both technologies century, and with more than four thousand years behind offer very good results the porosity of the material pro- it, it is a good example of one of the most common prob- duced a slight delay in the reflection of the signal emitted lems archaeologists face in their work: excavation is by the scanner. a destructive process and there is no way to achieve its In short, in the field of archaeology and cultural archaeological record without destroying later records. heritage, these techniques and instrumentation Consequently, the availability of 3D virtual environments allow massive captures of 3D data of the environ- of the different scenes at each moment of the excavation ment through a TLS, capable of generating a textured is a suitable teaching tool for monitoring the excavation Arias et al. Heritage Science (2022) 10:112 Page 3 of 15 process of this site, without losing any descriptive or geo- it possible. Nowadays, there are massive data collection metric detail. systems, which allow you to obtain 3D models with a The paper is structured to provide a full vision of the high degree of accuracy and 3D printers that can mate- whole process: from the data acquisition until the 3D rialize these models at low cost. Figure 1 shows the flow printed process. Firstly, and once the research aims are diagram of the whole process. presented, we present a basic review of the technology The objective of this work is to show in detail these and material resources that were available for the project. phases: data collection, generation of the model, and the We also describe the techniques that have been used. 3D printing. Secondly, we describe the archaeological place of Cástulo and detail the data acquisition procedure as well as how Review of technology to tackle with data processing. Thirdly, we explain how to In this part, we will give a brief overview of the tech- get the 3D-printed model of the Muslim tower obtained nics and instruments used in our work. A deep analysis by TLS techniques. Finally, we discuss the results and the in the employment of close range photogrammetry and perspectives of these techniques as educational tools. TLS can be found in , where the workflow phases for different case studies are described: From the aspects to Research aims be considered in survey planning, field operation, data Having a Muslim tower of the thirteenth century in our acquisition, preparation, processing to quality control of hands was something unthinkable until a few years ago, the results. The topic of georeferencing terrestrial laser but today the development of new technologies has made scanning or photogrammetric data is widely exposed in Fig. 1 Workflow diagram Arias et al. Heritage Science (2022) 10:112 Page 4 of 15 , here different georeferencing methods are described the two first types can be found in. We focus our concluding topographic instrumentation is the most attention in the latter one, for describing the operating accurate compared to other methods based on virtual scheme of our TLS. models with tie points or the use of low-cost GPS. Finally, In this method, the TLS emits continuously a peri- how the technologies are used for solid printing in Cul- odical signal of moderate intensity and compares the tural Heritage are described in , which concludes that phase of the laser source with the same when radiation in 3D models printed, the metric characteristics of pre- comes back. As only the phase-shifting is measured, cision and accuracy must be evaluated in relation to the the number of full wavelengths between the transmit- precision of the instrumentation used in the data acquisi- ter and the receiver at the instant of the measurement tion of the object. is an unknown value, known as ambiguity, Fig. 2. The problem is that there is no way to fix it if we only use a Terrestrial laser scanning (TLS) single frequency length because, as we can see in Fig. 2, The Terrestrial Laser Scanning is the name that LIDAR all the red lines, separated by the wavelenght L1, rep- technology receives when the sensor is placed on the resent possible solutions. However, adding another one Earth’s surface. It uses light in the form of a pulse radar L2 reduces the number of solutions to five and adding a to measure ranges. The 3-D point clouds acquired is third one, L3, reduces it just to one (A). matched with digital images taken of the scanned area to This is why several frequencies are used (multi-fre- create geometric 3-D models. This is a non-contact, non- quency-ranging or MF), being the higher used to cal- destructive and active technology, which allows taking a culate range and the lower used to eliminate ambiguity huge amount of data in absence of external light.. Another capability of some scanners is capturing The TLS might be classified according to its range of the colour of measured points. In consequence, point measurements or its principle of operation: triangula- clouds are much more representative of the scanned tion, time-of-flight or phase-based. The description of objects. Fig. 2 Fundamentals of phase-shifthing measurement Arias et al. Heritage Science (2022) 10:112 Page 5 of 15 When comparing TLS and close range photogram- long-term monitoring or structural analysis and the geo- metry, we found that a TLS is less versatile than a cam- referencing of found artefacts, barrows and structures era. Scanning might take a minimum of 15 min or over that can lead to studies of their spatial relationship and an hour for each scan if higher resolutions and qualities more extensive archaeological analysis. are required, while a camera records a scene in a few sec- It also provides, when working with TLS, a network of onds. There is still more issues: cost, a TLS weights much accurate points so that the point clouds can be success- more than a camera, transportation problems, and the fully unified into a common coordinate system and gives complexity of post-processing. In brief, TLS are not as certainty and, with some redundancy in the network, an versatile or flexible as cameras with regard to capturing estimate of the overall accuracy. Finally, it can provide the data and it might be unnecessary for the level of deliver- basis for long-term monitoring or structural analysis. able output required. However, equipments are improv- The control network was built by widening a previously ing in performance and portability, the 12 kg of weight existing one, with a series of stakes driven onto the earth. of the Leica HDS3000 have become the 5.2 kg of the Faro Once the stakes were driven, a metal washer of 18 mm Focus, and a handheld mobile scanner is even lighter. in diameter was screwed into them, so that the spheres A complete guide of TLS theory and practice can be could be fixed thanks to the magnetic support that it found in and a description of the services and stand- bears, Fig. 3. ards required for the supply of various types of metric To get the coordinates of the control network, we use survey in cultural heritage, including a study of laser a single Topcon GR5 RTK-GNSS, with centimetre accu- scanner accuracies in. racy linked to the Andalusian Positioning Network (Red Both TLS technology and digital photogrammetry are Andaluza de Posicionamiento, RAP), an active geodetic techniques used and equally valid for cultural heritage reference frame materialized by 22 permanent refer- conservation. However, there are differences between ence stations. RAP-NRTK solution provides a precision them that make one or the other more appropriate between 0.004 m and 0.030 m in the East component, depending on the main objective of the work. In this between 0.004 m and 0.059 m in the North component sense there are some research which specifically focuses and between 0.007 m and 0.094 m in the Up component on the need for geometrically accurate documents of. The coordinates are in ETRS89, the map projection cultural heritage sites rather than generating 3D models used is UTM, zone 30, and the heights are referred to the for visualization. In these cases, a comparison between mean sea level at Alicante (Spain), that is they are ortho- the results obtain from TLS & digital photogrammetry metric heights, Table 1. Therefore, we get coordinates of result in better performance of the TLS by obtaining bet- the centre of the spheres with a centimetre accuracy. ter accuracy in systematic errors in the position of the control points. Our TLS is a Focus 3D X 130 HDR Printed 3D‑model (High Dynamic Range), which uses phase shift technol- 3D printing has become a highlighted technology in the ogy. Its range varies from 0.6 m up to 130 m, and it can archaeological area in order to generate three-dimen- capture millions of 3D measurements at up to 976,000 sional models for multiple purposes. Traditionally, the points/second, with a ranging error of ± 2 mm. In addi- tion, it has a built-in 8 mega-pixel, HDR-colour camera which captures detailed imagery easily while providing a natural colour overlay, up to 165 megapixel colour, to the scan data in extreme lighting conditions. The point cloud was referenced in the topographic reference system by using the set of the 145 mm diameter spheres supplied by the manufacturer. We used target-based registration with spheres because in planar views a circle is always visible, no matter the angle of view. Data was processed with the software Scene 7.0 and Cloud Compare was used to display some models. Georeferencing An important aspect of the data collection process is determining a control network which georeferences the whole area to a national grid and height datum. On Fig. 3 Sphere attached into a stake with a metal washer archaeological excavations, it provides the basis for Arias et al. Heritage Science (2022) 10:112 Page 6 of 15 Table 1 Coordinates of the spheres - Reference System: UTM zone 30 (ETRS89) ID_Point X (m) Y (m) H (m) P_01 445,231.567 4,209,561.140 304.751 P_02 445,252.984 4,209,569.550 305.033 P_03 445,271.503 4,209,560.821 304.598 P_04 445,267.313 4,209,538.216 304.157 P_05 445,240.459 4,209,530.924 302.830 P_06 445,251.575 4,209,557.726 306.164 P_07 445,215.497 4,209,571.763 299.135 P_08 445,240.741 4,209,590.468 298.958 P_09 445,273.549 4,209,592.124 296.452 difficult access to artefacts in the archaeological sites has supposed a negative impact on research. Likewise, han- dling original artefacts may be risky, as many items are Fig. 4 Overview of process for 3D printing easily damaged. The use of 3D printing in archaeology may be a support for a wide range of research activities involving that archaeological discoveries become more Once the 3D model is prepared to be printed, we have accessible to educators and the researchers worldwide. used an open-source printing software for the slicing However, scanned models are not usually water light so process (Fig. 5). In this stage, all printing settings are they have many holes in its geometry and non-manifold defined to generate the G-code file that contains com- edges. Consequently, apart from the capturing method mands to move the extruder and all parts within the 3D such as LIDAR or photogrammetry, the resulting models printer. In this regard, the correct configuration of the need to be repaired, and therefore printed. printing properties influences the quality of the result- Pre-processing techniques commonly focus on error ing 3D model. These are always depending on the input correction, decimation of surfaces, cutting the model, model so there is not a single valid setting for printing and scaling and positioning on the (virtual) printer artefacts. ground plane. In this work, a 3D printer based on FDM Other relevant issue for 3D printing is the distribu- (Fused Deposited Modeling) technology generates print- tion of the support area. In this case, we try to preserve ing models. Thereby, according to the limits of this 3D as much as possible the details of the artefact surfaces. printing technology, we have determined two main goals: For this purpose, the option to insert the support mate- (1) the removal of holes in the geometry and (2) the mini- rial everywhere has to be disabled. It is just allowed mization of support material. The workflow is shown in from the build plate to the model. According to this con- Fig. 4. straint, we try to minimize the required support quan- The resulting geometry which has been captured from tity. Our method is capable of testing automatically the a scanned method usually presents complex triangular input model with multiple poses. Thus, a transformation meshes with many errors and lack the flat base for 3D matrix is computed to estimate the most adequate orien- printing. Therefore, the input model needs to be repaired tation and translation of the 3D model. Finally, after all and this task is carried out in the remeshing stage. Firstly, these stages are concluded, the output G-code file is gen- we are going to eliminate non-manifold geometry. Non- erated for the 3D printing. manifold geometry is defined as any edge shared by To get the printed model we used a Prusa MK3 3D more than two faces. It ends up confusing for 3D print- printer, based on FDM technology, able to build a model ing software. The next task is checking surface normal. with a resolution of up to one millimetre of thickness per The normal vector has to be perpendicular to the surface layer. This family of 3D printers is very versatile and fre- and facing outward. If the surface normal is reversed, quently used for the generation of prototypes in many the model is not correctly loaded for the slicing. Finally, professional sectors. In archaeology, 3D printing is a sig- the 3D model must be a hollow model. According to the nificant advance since it makes possible the rapid fabri- normal vectors, every hole is detected and fixed in the cation of a faithful replica from the previously digitized geometry. archaeological finding. Arias et al. Heritage Science (2022) 10:112 Page 7 of 15 Fig. 5 Slicing method Methodology found both on the slopes and on a walled plateau located The archaeological place of Cástulo. in the hills of Plaza de Armas and La Muela, at about The oppidum, or fortified city, of Cástulo, was the most 300 m above sea level, being an important junction, con- important population hub of the Iberian Oretania. Its trolling an extensive visual field that dominates the fertile archaeological site covers about 70 ha and is placed about plain of the river and the entrance to the mines of Sierra 5 km south of Linares (Jaén), on one of the terraces on Morena. the right bank of the River Guadalimar (Fig. 6). This com- During the Second Punic War, Cástulo was allied to plex stratigraphic and temporary sequence stars in the the Carthaginian cause, in fact, Himilice, Hannibal’s wife, Cooper Age, and it is possible to identify different peri- was from Cástulo, but as the tide of the war changed Cás- ods of occupation between the ­3rd millennium BCE until tulo finally allied with Rome, avoiding its destruction. the fourteenth century. Vestiges of the settlement are After the fall of the Roman Empire, begun its decline and, Fig. 6 Location and aerial view of the archaeological site of Cástulo (ETRS89/UTM Zone: 30) Arias et al. Heritage Science (2022) 10:112 Page 8 of 15 at the beginning of the thirteenth century, under Islamic However, it is possible to get additional shots to capture rule, its walls were destroyed, and the town was depopu- hidden zones, but always observing the fore mentioned lated shortly afterwards. Therefore, it is possible to find restrictions. For example, in Fig. 7 (right), we find the remains from at least eight cultures: Copper Age, Bronze places where the scanner was situated and, in blue color, Age, Iberian, Carthaginian, Roman, Visigoth, Jews, and the zones where the scanner should not be placed due to Islamic. the geometrical restriction. However, we can see that the The tower is the only vestige that remains of the Mus- position TLS_08 is located in a restricted are. It had to be lim castle of Santa Eufemia, and is located in a small done like that, due to the complexity of the zone that is watchtower at the southern end of the Archaeological delimited by remains of walls about half metre in height. Site of Cástulo, being a construction isolated from the At the right, we can see the possible positions for the rest of the archaeological site. It emerged as a remote photographic shots divided into four rings: a general one, facility in the south of the city, dominating the River Gua- to give a view of the whole, and three around the struc- dalimar enclosed the most recent remains of the Cástulo tures, to capture the details. history. Planning and data acquisition Point cloud processing Our first goal was to ensure the full coverage of the zone. The result of the scanner is an unprocessed point cloud, Our prerequisites were that the first set of scans should which can provide useful information by displaying the include a good distribution of control points while the project through static scenes or cross sections. This raw second set may be matched sufficiently by cloud-to-cloud file can be used to extract information in a Computer- registration. In addition, it was desirable a significant Aided Design (CAD) or Geographic Information System amount of overlap among scans, and to get an orthogonal (GIS) system. Nonetheless, in most cases, as we want to view of the subject to be scanned. Finally, our last con- get as much information as possible, the scan data must dition was that, as long as possible, the last scan should be processed. overlap the first one. Figure 8 shows the point cloud resulting from the scan Under these assumptions, we made a first selection in grey levels, whose value depends on the value of the of suitable TLS setup points and the five spheres were reflectance obtained when the laser pulse reaches the located with these conditions: object. To colour the point cloud, we use the images taken with the scanner camera and the use of the HDR to At least, there must be three common spheres improve the quality of the images obtained. between each surveying. Point clouds are recorded and saved in a local coordi- These spheres must not be aligned. nate system whose origin is the position where the laser Their disposition must be regular around the TLS meets the mirror. Therefore, if you have two or more setup point. scans taken at different places, as each one is relative to The maximum distance between the TLS and the the scanner, each scan will only know its own coordinate spheres should be less than 30 m (Europe Laserscan- system. Hence, it is necessary to determine the spatial ing). relationship between them, to express all the scanners in the same coordinate system. This process is called Reg- Data were taken with a quarter of the maximum reso- istering the Scan and the step from the scan coordinate lution and a quality of 4x, so each point is measured four system into the overall coordinate system is called trans- times, to reduce the noise of the measurement, looking formation. Subsequent processing steps to provide deliv- for a compromise of accuracy and time of scanning, so erable products include: cleaning, filtering, segmentation, that it took about 20 min to perform a complete 360º classification, sectioning, meshing, rendering (texturing), scan with HDR colour. With this resolution and at work- tracing CAD or Building Information Modeling (BIM) ing distance of 30 m, the expected accuracy is about detail (vectorisation), image-based output, animation, 2 mm. and visualisation. In the design phase, it is necessary to specify the sensor, To remove undesired scan data, like people or vegeta- TLS or camera, its characteristics as well as to define the tion, data must be cleaned and filtered. Besides, these optimal zones to place the sensor, to get a good geom- processes reduce the size of the dataset and should make etry of the shot. That implies that the incidence angle registration more efficient. Even more, spurious data can between the ray and the surface must be greater than be removed after registration, if they may prove useful for 30º. This geometrical restrain establishes a restricted area registration in the absence of good overlaps, or if there is around the target where you cannot place the sensor. little ground control. Also filtering is used to remove the Arias et al. Heritage Science (2022) 10:112 Page 9 of 15 Fig. 7 Planning of the scanning around the Muslim tower Fig. 8 Grey point cloud generated by the Scene noise generated by poor signal return or to select only the the filters that we have applied to remove or correct inac- first return obtained when the laser dot strikes an edge. curate points. Filters can be applied simultaneously or Data was processed using SCENE 7.0, the software after the point cloud generation. The workflow process is provided by the manufacturer. Table 2 lists the values of shown in Fig. 9. Arias et al. Heritage Science (2022) 10:112 Page 10 of 15 Table 2 Applied filters Filter Definition Dark Scan Point It removes points based on a reflectance valuve Reflectance threshold 200 It is used to remove scan points with too much noise Distance It removes all scan points which are outside of a certain distance range Maximum distance: 30 m Stray Point It checks if the 2D grid cell of a scan point contains a sufficient percentage of points Grid size: 3px with a distance similar to the scan point itself Distance threshold: 0.02 m Allocation threshold 50% Edge Artifact It is especially useful to remove artefacts at the edges of obejects Yes Smooth It is used to minimize noise on surfaces No We applied the filters simultaneously to the point the Smooth Filter to assure that important characteristics cloud generation to obtain it as clean as possible. The were not deleted and the Distance Filter was set to 30 m. Stray Point Filter is useful to remove scan points result- Prior to Registering the Scan, it is convenient enter ing from hitting two objects with the laser spot or by hit- the UTM coordinates of each of the spheres. To do ting no object at all, for example the sky. We do not use this we create a text file with *.csv format in which each line contains: the sphere name, its UTM coordinates, and the height; with all the fields separated by com- mas. Next, in Import Objects, we import the file and a new folder called References with the uploaded points is created. Now, when registering, we check the option of ‘Forcing Correspondences by names of objectives’ to make sure we are taking into account all the common references, in our case the spheres. Although register- ing makes all scans form a single model, it has the dis- advantage that errors in the transformation must be added to measurement errors. Finally, the point cloud is generated and the final file is exported. An aerial view of the Castle scanning generated by the software is shown in Fig. 10. In this image, it is pos- sible to appreciate, in form of white spots, the places where the TLS was placed. Generation of the 3D model In this section, we describe the 3D printing process, once the 3D point cloud has been generated from the 3D reconstruction software. The printer used is based on the FDM technology and it is able to build strong, dura- ble and dimensionally stable models with a high accuracy and repeatability. The use of 3D printing in Archaeology is an enhancement for many applications. One of the key issues is the access to archaeological remains which are located some distance away and the direct view becomes costly and time-consuming to travel and view in-situ. Likewise, the use of 3D printing for a faithful replication of the target piece provides many advantages such as the risk decreasing by not handling original artefacts, which may be easily damaged, a full 3D observation and manip- ulation, and an accurate and detailed measurement. Fig. 9 Point cloud processing workflow Arias et al. Heritage Science (2022) 10:112 Page 11 of 15 Fig. 10 Scanning of the Castle. Observe the white spots, where the TLS was placed to summarize the obtained results, in Table 3 we show the number of scans and the size of the point cloud generated Table 3 Summary of the obtained results distribution of support areas is a requirement for every View points Maximum error Recording Scanning time printing. It supposes a worse quality in the object sur- (mm) points/million (days) face, which is below the support material and a tedious work to support removing. We propose two multi- 14 18.1 179 2 material solutions with very similar results to over- come the previous problem. The first one is the use of polylactic acid (PLA) for model printing and polyvi- The output point cloud from the LIDAR scanning is tri- nyl alcohol (PVA) for printing support. Both materials angulated for the mesh generation by applying the Pois- share the same melting point 200º so the same extruder son surface reconstruction method. However, the may deposit them. The second solution is based on the resulting mesh presents noise and many holes, which fusion of acrylonitrile butadiene styrene (ABS) and high need to be repaired in order to be properly printed. For impact polystyrene (HIPS) for printing model and sup- this purpose, the geometry of this model is edited with port respectively. In this last solution, the melting point Blender, an open source 3D creation suite. Several cus- of both materials is a little higher in the range from tom tools are applied for filling holes, smoothing surface 210º to 249º. These multi-material printing provide us and making planar faces. As a result, the mesh is now the capability of using a specific solvent to remove the without holes and the lower part of the model is removed support material as the water for PVA and limonene for to make one side flat, so it can be adhered to a research HIPS. poster or exhibition panel. Finally, we must just set the printing parameters in The next stage is the selection of the most adequate order to achieve the expected quality. In this sense, printing material. A detailed survey is out of the scope our model was printed with a high resolution so the of this paper, and we briefly review some key mate- thickness of each layer is one millimetre. In addition, rials in this section. In general, the reconstruction to ensure a greater resistance in the borders, the wall of archaeological objects produces 3D models with width has been increased 0.5 cm. The infill density is many concave and convex surfaces. Consequently, the reduced until 30% to add less plastic on the inside of the print and thus cut the time printing. Arias et al. Heritage Science (2022) 10:112 Page 12 of 15 Results The below image, Fig. 11, shows the resulting 3D print- The Muslim castle ing which has an optimal appearance and a detailed The model obtained from the Muslim tower, Fig. 10, model of the scanned environment. is highly detailed and shows, not only the construc- tive characteristics with absolute fidelity but also the Printing of 3D models remains of another two small towers. Having very sharp The availability of a scale model of the castle thanks to edges, you can see the edge effect on the point cloud, 3D printing is a resource available to any educational which is not a problem when obtaining a 3D printed centre, Fig. 12. From a didactic point of view, it allows model. the student to appreciate more directly general aspects Fig. 11 The Muslim castle generated by the Scene Fig. 12. 3D printed model Arias et al. Heritage Science (2022) 10:112 Page 13 of 15 of geometry and more concretely information about the were a large amount of rocks and debris in the ground proportions of the castle. Comparing this model with causing many shaded areas. other scenarios printed at the same scale facilitates, for Neither can we ignore that to handle the huge amount example, a comparative analysis of the sizes and dimen- of data generated by a TLS, it is necessary to have a state- sions of specific parts. This kind of analysis is more dif- of-the-art computer. Still, the times of processing and ficult to perform on a projected image on a screen, generation of the point cloud are high so that it is con- especially for students with poor spatial vision. venient to arm yourself with a good dose of patience. 3D printing also aids in the dissemination of the site However, this one is the minor problems, since it is fore- under study for audiences with different levels of exper- seeable that in a few years the development of the hard- tise. In this case it is an archaeological model, which con- ware will facilitate computers with higher performances tributes to the better preservation of the remains found that facilitate us even more the work. and even, for a specialised public, to test reconstructive Despite all these issues, it was clear that the possibili- hypotheses based on the construction of the model itself. ties of the TLS from the point of view of research, edu- The model helps to provide an overview, spatial refer- cation and dissemination of heritage are enormous, ences and sometimes conjecture about the form, location although its high price plays against it. However, this and relationship of the original buildings. The quality of point should be qualified, since on the equipment is the techniques used in the generation of the 3D model becoming more affordable and there is the possibility of means that the print faithfully reflects aspects to be iden- renting the equipment. tified by a specialised public without losing the attraction Another advantage of the TLS techniques is once the for a public with a lower level of archaeological knowl- scanners are done, the excavation site can be protected edge. As discussed in the introduction section, numer- through covering with soil, and the zones can be analysed ous studies highlight the importance of technology in subsequently without direct access to the surfaces. Even archaeological education [24, 25] among others already more, when the 3D model is generated, it can be upload mentioned. However, although some research has been to a public domain repository, allowing anyone to down- conducted [26, 27], the integration of 3D visualisation load and to reproduce it, in its own home. As the gen- systems with successful pedagogical theories that pro- eration of 3D models has become more affordable, its mote deep learning approaches in field archaeology applications will boost in the next few years. Although we courses is lacking. have only applied our results to a printed model, with a In the case of the Tower of Castulo, the use of the 3D little effort it can be also used for virtual reality applica- model brings the students, in a very realistic way, closer tions. For those applications that does not require mil- to the current state of the Tower. In this way, with the limetre accuracy, low cost close range photogrammetry use of specific 3D model processing software, such as 3D should be an interesting option. Builder, Blender or others , a virtual reintegration can To sum up, 3D printing is begging to show its capabili- be generated and a visual reconstruction can be obtained ties to reproduce in a quite accurate way any structure, in which the decoration and texture and even the conti- no matter its original size was. This opens up a set of very nuity of the Tower are reproduced using non-invasive interesting applications that can range from documenta- techniques. tion to allow enable visually impaired the recognition of From the didactic point of view, the student can directly any monument. It is pretty obvious its potential as a tool analyse the exploded view of the element by studying for educational purpose at any level, from primary to uni- separately the elevation, the ground plan, etc.… even in versity levels. inaccessible areas, such as the highest part of the Tower. Finally, the 3D models ensure the integrity and reli- In short, 3D models are offered as an attractive and ability of the heritage documentation, as the information motivating tool for students by approaching the stud- recorded is timely, relevant and accurate. It provides a ied scenario in a different way. Not only does it allow clear understanding of the condition and materials of the for visualisation and manipulation but, being a prod- studied environment, as well as the chronology of modi- uct with metric characteristics, it facilitates the assimi- fications and alterations to the property throughout its lation of proportions, volumes and other geometric excavation. Therefore, documenting and recording these characteristics. aspects, and being able to use and display them in such an accessible and easy to manipulate way. In this sense Conclusion 3D models are an essential teaching tool in the field of As the castle was in an isolated place, we had the advan- archaeology, being an attractive and motivating tool for tages that there were not many visitors and the survey students as they approach the scenario under study in can be done at different seasons. As a drawback, there a different way. Not only does it allow visualisation and Arias et al. Heritage Science (2022) 10:112 Page 14 of 15 manipulation, but also, as it is a product with metric 2. Ramirez EBR, Diaz FJ, Arias GAG, Villamizar NI. Impresión 3D como Herramienta Didáctica para la Enseñanza de Algunos Conceptos de characteristics, it facilitates the assimilation of propor- Ingeniería y Diseño. Ingeniería. 2018;23:70–83. https://​doi.​org/​10.​14483/​ tions, volumes and other geometric characteristics. 23448​393.​12248. 3. Zawacki-Richter O, Marín VI, Bond M, Gouverneur F. Systematic review of research on artificial intelligence applications in higher education— Abbreviations where are the educators? Int J Educ Technol High Educ. 2019;16:39. ABS: Acrylonitrile butadiene styrene; BCE: Before Current Era; BIM: Building https://​doi.​org/​10.​1186/​s41239-​019-​0171-0. Information Modeling; CAD: Computer-aided design; ETRS89: Earth Terrestrial 4. Lerma JL, Navarro S, Cabrelles M, Villaverde V. Terrestrial laser scanning Reference System 1989.; FDM: Fused Deposited Modeling; GIS: Geographic and close range photogrammetry for 3D archaeological documentation: Information System; Ha: Hectare; HDR: High Dynamic Range; HIPS: High the upper palaeolithic cave of Parpalló as a case study. J Archaeol Sci. impact polystyrene; LIDAR: Light Detection and Ranging; M: Meters; MF: Multi- 2010;37:499–507. https://​doi.​org/​10.​1016/j.​jas.​2009.​10.​011. frequency-ranging; Mm: Milimeters; NRTK: Network Real Time Kinematic; PLA: 5. Marín-Buzón C, Pérez-Romero A, López-Castro JL, Ben Jerbania I, Polylactic acid; PVA: Polyvinyl alcohol; RAP: Red Andaluza de Posicionamiento; Manzano-Agugliaro F. Photogrammetry as a new scientific tool in archae‑ RTK-GNSS: Real Time Kinematic – Global Navigation Satellite System; TLS: Ter‑ ology: worldwide research trends. Sustainability. 2021;13:5319. https://​ restrial Laser Scanners; UTM: Universal Transverse Mercator. doi.​org/​10.​3390/​su130​95319. 6. Marín-Buzón C, Pérez-Romero AM, León-Bonillo MJ, Martínez-Álvarez Acknowledgements R, Mejías-García JC, Manzano-Agugliaro F. Photogrammetry (SfM) vs. We would like to thank the support given by D. Marcelo Castro, the director of terrestrial laser scanning (tls) for archaeological excavations: mosaic of the Conjunto Arqueológico de Cástulo, the Tourism Area of the City of Linares, cantillana (Spain) as a case study. Appl Sci. 2021. https://​doi.​org/​10.​3390/​ and Ph. D. Justin St P Walsh, from the Chapman University. 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Geomatics supporting knowledge of cultural herit‑ eters in order to obtained and the desired photogrammetric products. JMJ age aimed at recovery and restoration. 2020. Int Arch Photogramm has supervised the planning of the flight and has also contributed with the Remote Sens Spatial Inf Sci. https://​doi.​org/​10.​5194/​isprs-​archi​ execution of the drone flights. LO has contributed in the supervision of the ves-​XLIII-​B2-​2022-​909-​2022. analysis of the products resulting from the processing of the images. AR has 10. Fabris M, Achilli V, Artese G, Bragagnolo D, Menin A. High resolution contributed to the quality analysis of the cartographic products generated. survey of Phaistos palace (Crete) by tls and terrestrial photogrammetry. JJC has contributed to the general supervision of the paper giving it a didactic 2012. 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