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Laser scanning in archaeology and cultural heritage documentation
The remote sensors play a significant role in field archaeology as they offer non-invasive means for collecting data related to the physical and chemical properties of objects from space based, aerial and terrestrial platforms. |
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Indian heritage and culture are vast and vivid, encompassing both tangible and intangible elements. With numerous archaeological sites spread across the whole country, a tool like remote sensing and geographic information system has high potential for exploring these, especially the built heritage. These tools and methods help in establishing certain facts about the sites that may not be possible to achieve from other conventional methods. The remote sensors play a significant role in field archaeology as they offer non-invasive means for collecting data related to the physical and chemical properties of objects from space based, aerial and terrestrial platforms. (Parack,2017)
The archaeologists and historians, involved in various activities connected to heritage preservation and research would find the technology greatly beneficial since digital tools such as 3D scanning are providing increasingly detailed and accurate information within a relatively shorter time span.(Bassier,2018)
Lidar, which stands for Light Detection and Ranging, is a remote sensing method for determining ranges by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. Each laser pulse can produce multiple consecutive measurements from reflections off several surfaces in its path. It can also be used to make digital 3-D representations of areas on the earth’s surface and ocean bottom by varying the wavelength of light, in other words the technique can generate accurate 3D information about the earth surface and the target object.
LiDAR systems are most commonly used for surveying tasks i.e. their ability to collect three dimensional measurements, surveying the built environment (such as buildings, road networks and railways), creating digital terrain (DTM) and elevation models (DEMs) of specific landscapes. However the technique can be used for a myriad of applications in civil engineering, corridor applications(power utility, railway etc), archaeology, forestry, mining, environmental research and disaster management work; to name a few.
The laser scanners are preferred mode for 3D data acquisition due to advantages like high data density, fast data acquisition with minimum human dependence, high range of accuracy, independent of illumination and weather conditions to a great extent, ability to penetrate vegetation canopy unlike photogrammetry. These characteristics make laser scanners especially useful for application in cultural heritage documentation and archaeological studies.
Cultural heritage refers to the physical and cultural assets, both tangible and nontangible, passed on to us by our previous generations. Historical structures are one of the most significant elements of cultural heritage. They reflect history, lifestyle and tradition of a country and society. In the current scenario cultural heritage is threatened by various factors such as natural hazards, vandalism, development of cities, and natural aging. Built heritage is an essential aspect of cultural heritage. The built heritage also referred as monuments, have suffered significant damage especially in India, majorly due to human activity and population pressure. Hence, proper mapping, damage detection and digital documentation of cultural heritage sites thus becomes essential for their preservation and protection (Remodino et al., 2007)
Approaches for comprehensive digital documentation of cultural heritage need to be developed; employing techniques and instruments which are capable of providing multi-dimensional information in quick and easy manner. This would enable effective monitoring and conservation of monuments and heritage sites (Prasanna et al., 2012)
New technologies and a growing need to document and preserve information and objects related to cultural heritage is expanding at a rapid rate. 3D documentation, multiscale database of monument and digital blueprinting of heritage structures provides the necessary details and information for constructing heritage inventories, assessing damages/ risk and management and monitoring the built heritage sites. However, despite all these potential applications, a systematic and targeted use of 3D surveying and modelling in the cultural heritage field is still not yet employed in our country. Many studies have proved that terrestrial laser scanner is a powerful tool towards recording of objects and sites for heritage preservation purposes, scientific research and built environment applications. However, widespread practical application by users is lacking hence in order to utilise the technology for practical operational work, a set of standard operating procedure needs to be developed. This will enable easy understanding of the complex technique and instrumentation by nonexperts. The objective is to demonstrate how technically advanced ground based instruments e.g. TLS, GPS etc can help to improve the conventional standards of heritage recording and documentation and utilize geospatial technology for digital documentation of cultural sites.
It is seen that terrestrial laser scanning (TLS) along with close range photogrammetry has immense potential in documentation of heritage monuments. Once a detailed digital model is available, a standard operating procedure for digital documentation and automated damage assessment of built heritage sites can be developed.
Documentation process using TLS primarily aims to create geometric and photorealistic 3D models for both precise reconstruction and visualization (Besl and McKay, 1992). Over the last two decades, the documentation of cultural heritage using terrestrial laser scanning (TLS) technique has significantly increased (Remondino, and Campana, 2014). This fact is mainly due to the wide availability of laser scanning technology and its ability to provide dense surface models in a short period of time with accuracy and reliability.
The capability of capturing dense, ultrahigh resolution points with associated x,y,z coordinates and RGB values makes the technique suitable for acquiring the finer details of artefacts/architecture.
A few UNESCO World heritage sites of northern India viz. Humanyun’s tomb, Qutub Complex (Delhi) and Nalanda Mahavihar (Bihar) have been taken up to demonstrate the potential of 3D data and models for historical site documentation. The utility of using remote sensors to carry out a comprehensive (internal and external) multiscale documentation of heritage monuments, which would enable digital blue printing and damage detection at very high resolutions is explored. The case example of the Qutub Complex (Delhi) is described here since the Monuments of Qutub Complex display an interesting blend of variety and complexity of structure e.g. a vertical tapering tower with considerable height, extensively damaged structures and wide expanse of the site. Figure 1 displays the map location of the Qutub Complex, the complex boundary is also overlaid on satellite image along with field photo and laser scan data of the Minar.
Procedural workflow
i. Data acquisition, Integration and Generation of 3D model: Since the monuments have complex shape, vast expanse and height and tapering structures, calculating the amount of overlap and the number of scans required to cover the structure in its entirety becomes complicated and needs careful planning. The shape of the structures is generalized and multiple scans are acquired with a higher overlap percentage of 90% to capture the details of structure and its engravings.
The TLS point cloud was filtered, and individual scans were co-registered to generate 3D point cloud. Point clouds obtained from different scans, covering the entire site were merged and aligned to get the final merged point cloud.
ii. Extraction of Architectural Elements and Digital Blue Printing: Architectural elements are the unique details and component parts that, together, form the architectural style of the monuments. To extract the architectural elements the coloured and textured point cloud was smoothed by means of a smoothing filter and was then subjected to edge extraction technique, which resulted in extraction of significant edges. The geometrical inaccuracies in the preliminary boundaries obtained from above approach needs to be corrected and refined before the drawing of individual architectural elements can be created. Image processing based algorithms were applied on the preliminary boundaries. This approach was successful in refining the major edges; however blue printing requires minute detailing also. Hence manual editing was required in certain portions, for proper representation of the architectural elements. Figure 3 shows the entire procedure applied on a section of Tomb of Imam Zamin within the Qutub complex.
The procedure was applied to other sections of the point cloud representing different architectural elements, which were extracted using the automated method. The individual elements were integrated to result in a digital blue print of the monument (Figure 4).
The digital blueprints so generated are the preliminary step to deal with aspects of heritage documentation and management. These blueprints provide a very diverse range of data (quantitative and qualitative) which can be investigated to produce an accurate digital representation of the building. It can also play a key role in preparing a Heritage Building Information Modelling (H-BIM) system for effective management of the different aspects of dealing with heritage buildings. Examples of ultra-high resolution 3D models of some structures within the Qutub Heritage Complex are shown in figure 5
Data acquisition and analysis methods utilising non-invasive technology e.g. LiDAR are also ideal for applications such as structural health assessment of monuments. Such data when combined with advanced image processingtechniques help in identifying damages, such as cracks, broken parts, discolorations etc, virtual reconstruction of broken patterns or artefacts and conjectural reconstruction. These realistic three dimensional models with associated attributes can also be used as primary data to create digital museums and other interactive virtual environments.
Therefore the 3D models and metric products are useful for creating a detailed multiscale database of monument and a precise digital blueprint of built heritage structures. It provides a tool for damage detection using point clouds and multiscale representation of structure for monitoring, assessment and reconstruction.
In conclusion, this is not only an example of effective and operational use of latest digital surveying techniques especially terrestrial laser scanners for digital documentation, 3D visualization, damage detection and monitoring of built structures; heritage monuments in particular but also represents the prospect of enabling preventive measures for maintenance and decisionmaking and heritage management .
References
Bassier, M.; An Overview of Innovative Heritage Deliverables Based on Remote Sensing Techniques. Remote Sens. 2018, 10(10), 1607; https:// doi.org/10.3390/rs10101607.
Besl, Paul J. and McKay, Neil D. “Method for registration of 3-D shapes”, Proc. SPIE 1611, Sensor Fusion IV: Control Paradigms and Data Structures, (30 April 1992); https://doi.org/10.1117/12.57955
Parcak, S.H; GIS, Remote Sensing, and Landscape Archaeology.(2017) DOI: 10.1093/ oxfordhb/9780199935413.013.11 (Accessed March 30, 2022).
Prasanna, P. , Dana K., Gucunski, N. , and Basily B.(2012) "Computer-vision based crack detection and analysis", Proc. SPIE 8345, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2012, 834542; https://doi.org/10.1117/12.915384
Remondino, F. and Campana, S., 2014. 3D recording and modelling in archaeology and cultural heritage. BAR international series, 2598, pp.111-127.
Remondino, F., Rizzi, A. Realitybased 3D documentation of natural and cultural heritage sites—techniques, problems, and examples. Appl Geomat 2, 85–100 (2010). https://doi. org/10.1007/s12518-010-0025-x
Sevara, C., M. Pregesbauer, M. Doneus, G. Verhoeven, and I. Trinks (2016). Pixel Versus Object—A Comparison of Strategies for the Semi-Automated Mapping of Archaeological Features Using Airborne Laser Scanning Data.” Journal of Archaeological Science 5: 485– 498.
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