APPLICATION OF LASER SCANNING IN EXAMINING AND PRESERVING HISTORICAL MONUMENTS

ABSTRACT

This article analyzes the advantages and some disadvantages of using laser scanning in assessing the technical condition of buildings and structures. The practical significance of using laser scanning was demonstrated by the example of assessing the technical condition of the Kok Gumbaz mosque, located nearby in the city of Shakhrisabz, Kashkadarya region. Its technical effectiveness has been demonstrated.

АННОТАЦИЯ

В данной статье анализируются преимущества и некоторые недостатки использования лазерного сканирования при оценке технического состояния зданий и сооружений. Практическая значимость применения лазерного сканирования была продемонстрирована на примере оценки технического состояния мечети «Кок Гумбаз», расположенной вблизи в городе Шахрисабз Кашкадарьинской области. Была показана его техническая эффективность.

Ключевые слова: лазерное сканирование, техническое состояние, исторические памятники, оценка, результаты, объект, мониторинг.

Keywords: laser scanning, technical condition, historical monuments, assessment, results, object, monitoring.

Introduction. It should be noted that the technical condition assessment of buildings and structures is primarily based on two stages, which consist of preliminary inspection and subsequent instrumental tools to determine the real state of stress and deformation [1,2]. To improve the accuracy of the results obtained from the conducted inspections, modern high-tech tools are used because their reliability is the basis for the effectiveness of the project documentation. Among them, laser scanning can be highlighted as an example [3, 4, 5, 6, 7].

The use of laser scanning provides significant opportunities in the following areas:

• In production facilities;
• In architecture and construction;
• In road construction and repair;
• In mining operations;
• In the monitoring of buildings and structures, and others.

Here, we will provide some information about the laser scanning method.

The purpose and tasks of laser scanning

Laser scanning (profiling) is a highly automated process that is based on the following various tasks:

• Creating a three-dimensional cadaster of immovable objects.
• In topographic surveys and the design of industrial buildings and structures, as well as infrastructure facilities.
• Measuring the external and internal facades of buildings.
• Obtaining comprehensive information about memory heritage.
• Organizing walls, partitions, and other elements of buildings and structures with information about the vertical alignment.
• Creating floor plans of buildings.
• Arranging rooms with complex geometry, arches, niches, and other elements.
• Locating defects and deformations in constructed or operational objects.
• Collecting information for the repair, reconstruction, or construction of buildings.

The use of laser technology reduces human error and ensures the safety and accuracy of the work being carried out.

Characteristics of the method

The working principles of laser scanners involve directing high-frequency light towards an object being scanned and then capturing and processing the reflected light. A 3D scanner accomplishes this and measures precise distances and angles, creating a point cloud (a collection of points). One significant advantage of this technology is its ability to acquire information from a large volume in a relatively short time.

Laser classification is considered a contactless and fully automated method. The measurement portion of the device automatically aligns the results in vertical and horizontal directions. All measurements are performed with high accuracy from a fixed point.

Types of 3D Scanning

Based on the technical characteristics, dimensions, and complexity of buildings and structures, the following 3D classifications are used:

• Laser scanning of buildings;
• Mobile scanning;
• Aerial scanning of buildings.

Each type allows obtaining phase results connected to a unified coordinate system for each element of the building. Laser scanning is a versatile method that proves to be highly effective in technical inspection and research of complex geometric parts of buildings and structures, using straightforward means.

Laser scanning devices rapidly and efficiently address various tasks in all orientations:

• Supervision of construction and building design;
• Creation of precise 3D models for interior design purposes;
• Verification of the technical condition of buildings and structures;
• Identification of engineering communications in specific buildings and structures;
• Classification of historical monuments and building facades;
• Execution of surveys for repair, reconstruction, or design of buildings;
• Quick completion of topographic surveys and engineering search operations;
• Creation of 3D models of archaeological sites.

Advantages

• High precision of scans is ensured;
• Completeness of obtained data;
• Quick acquisition of results on three dimensions;
• Safe and efficient;
• Fully automated without human intervention.

Disadvantages

• Multiple models may only function optimally in favorable temperature conditions;
• Handling complex memory from objects with intricate geometry can be computationally intensive.

3D laser scanning is a traditional measurement method that uses conventional measurement techniques and is helpful for measuring and inspecting complex surfaces and geometries. The laser scanner used for research purposes in the example is the FARO FOCUS M70 (see Image 1).

Figure 1. The appearance of the FARO FOCUS M70 laser scanner

The FARO FOCUS M70 laser scanner has the capability to capture objects at a distance of up to 70 meters. It boasts a scanning speed of 488,000 points per second, ensuring fast data acquisition and high-quality results. Its compact size and lightweight design contribute to its ease of use and portability. To view, manage, and edit the collected point cloud files, it comes with a license for the Faro Scene software.

The advantages of the FARO FOCUS M70 laser scanner include adjustable scanning speed, ±3 mm accuracy, a scanning range of 360°/300°, the presence of a navigation sensor, HDR image capture capability, touchscreen operation, and IP54 protection.

The FARO FOCUS M70 3D scanner covers a measurement range from 60 cm to 70 meters, making it suitable for capturing both small objects and large-scale structures quickly. Its accuracy for distance measurements is ±3 mm, and its field of view is 190 degrees.

The key advantage of the laser scanner lies in its dimensions, with a size of 230 x 183 x 103 mm, and its weight, which is only 4.2 kg including the battery. It is easy to set up, convenient for storage, and its compact design is highly portable. The maximum scanning speed is 488,000 points per second, and it can be reduced to 122,000 or 244,000 points per second if needed. The camera is capable of capturing images with up to 165 megapixels, and the High Dynamic Range (HDR) technology ensures uniform exposure throughout the entire image, enhancing image quality and visibility.

The FARO FOCUS M70 laser scanner comes with additional features. It utilizes the GPS and GLONASS navigation systems through the installed navigation sensor, simplifying georeferencing. An additional port extends the scanner's operational duration, and it allows for easy attachment of new modules when new functions and capabilities become available. The system automatically recognizes connected accessories.

Data is stored on a 32 GB SD card, and the scanner can be wirelessly connected to a data controller, including data transfer in HTML 5 format via Wi-Fi when needed.

Laser Scanner Specifications. The operating temperature range is from +5°C to +40°C. It boasts an IP54 rating, indicating high resistance to dust and moisture, making it well-suited for challenging conditions, including humid environments and exposure to rain. These combined features allow the FARO FOCUS M70 scanner to operate in harsh conditions without causing harm to its electronics and optics. Its Li-Ion battery has a capacity of 6750 mAh per hour, ensuring uninterrupted operation for up to 4.5 hours. The charging port is integrated into the system.

The mosque building was scanned from 24 points (as shown in the 2nd image). Scanning was conducted from various angles, covering both the exterior of the mosque and the main hall, including the galleries attached to the mosque's exterior and the primary interior areas of the mosque building.

Figure 2. Results of measuring an object using the FARO FOCUS M70 laser scanner

This mosque is a grand example of Islamic architecture and was constructed during the rule of the eminent Uzbek leader in the year 1437. Its architectural design takes the form of a square shape, with dimensions measuring 21x24 meters. The inner dimensions of the prayer hall are 12.5x12.5 meters. All the walls have dimensions of 4.6x4.6 meters, and the depth is approximately 3.0 meters (as shown in the 3rd image). The entrance to the mosque is elaborately designed in the style of a "peshtak," with a total height of 22.38 meters. The mosque features a unique architectural style known as "kushgumbazli," which means it has both inner and outer domes. The inner dome has a diameter of 13.5 meters, while the outer dome has a diameter of 16.92 meters. The overall height of the dome's pinnacle, including the outer dome, reaches 31.65 meters [8].

Figure 3. The plan view of the Kuk Gumbaz Mosque

The main entrance of the mosque is adorned with an inner arch. The mosque is designed with an entrance arch to the east and additional arches on the north and south sides to access the balconies located on the east side of the mosque. The construction of the mosque's main dome is supported by an arch system, and within its inner dome, there are 16 primary and auxiliary arches. These arches serve a fundamental role in supporting the outer dome (drum). The top dome, with a diameter of 16.92 meters, is constructed on top of the inner dome (as shown in the 4th image). The outer side of the dome is adorned with decorative elements in vibrant colors, and the junction points with the dome (drum) are embellished with intricately designed “muqarnas”, following the style of the Eastern method.     

Figure 4. The section of the Kok Gumbaz Mosque

The mosque's central courtyard is surrounded by four arches. The eastern arch serves as the main entrance to the mosque. The northern and southern arches lead to the mosque's gallery. Inside the northern and southern arches, smaller arched openings are found, which also serve as entrances to the mosque. These entrances are designed to be relatively low, creating a sense of humility when entering the mosque. They are situated in such a way that the mosque can be entered from all three sides, making the mosque's layout tripartite.

Above these entrances, there are galleries and alcoves, the tops of which are adorned with decorative arches. These arches are placed in a perpendicular manner and have small windows in the upper part of the walls. They are positioned at the points where the walls of the mosque are thickened.

Currently, the main building (maqsura) is situated, with a wide courtyard facing the street, forming a separate prayer hall. The primary volume of the maqsura is closer to a cube in shape. The square prayer chamber with four domes is built separately from the inner and outer domes. The inner dome is based on the central eight-cornered plan and has rhombus-shaped pendentives with muqarnas decoration, while the outer dome is in the form of a flattened dome. The building's walls have deep niches that intersect with the layout of the mosque. Wide arches pass through the galleries' walls. These arches have been recently reconstructed.

The thick brick walls have four flat pylons that extend to the base of the mosque's galleries. They are connected to the gallery's thickness and form part of the gallery's volume. The large convex niches in the walls of the building extend up to the level of the galleries. The galleries are traversed by wide arches. The thick brick walls have four flat pylons that extend to the base of the mosque's galleries. They are connected to the gallery's thickness and form part of the gallery's volume. The large convex niches in the walls of the building extend up to the level of the galleries. The galleries are traversed by wide arches. The large, projecting arches on the building's walls are adorned with four tiers of blind niches, and they continue up to the galleries' height. The outer dome, which appears large from the outside, is supported by an inner dome with a tambour [9].

Conclusion

 Laser scanning offers significant advantages in assessing the technical condition of buildings and structures, beyond just creating three-dimensional models of objects. It can be particularly valuable in various complex tasks in this field.

One of the key advantages of laser scanning is its ability to monitor and document the technical condition of historical sites and buildings. With laser scanning, it becomes possible to accurately capture the internal dimensions of buildings and detect any changes or deformations over time. This monitoring, often referred to as "structural health monitoring," can be invaluable in the preservation and restoration of historical buildings. The data collected through laser scanning can serve as the basis for renovation and restoration projects and the creation of technical documentation.

Laser scanning technology is widely used in the assessment of historical events and structures due to its versatility. It enables not only the creation of 3D models but also the detection of minute changes and deformations in buildings over time. This makes it an essential tool in architectural and historical research.

In conclusion, laser scanning is a highly valuable technology in various technical assessment tasks related to buildings and structures, including historical ones. Its ability to capture precise data and monitor changes over time makes it an indispensable tool in preservation, renovation, and restoration projects. The application of laser scanning technology in the analysis of the Kuk Gumbaz Mosque demonstrates its effectiveness in such contexts.

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