GIS Based Studies in the Humanities and Social Sciences - Chpater 3

3 A Laser-Scanner System for Acquiring Archaeological Data: Case of the Tyre Remains Ryosuke Shibasaki, Takura Izumi, Hiroya Tanaka, Masafumi Nakagawa, Yosinori Iwamoto, Hidetomo Fujiwara, and Dinesh Manandhar CONTENTS 3.1 Introduction..................................................................................................36 3.2 A New, Three-Dimensional Measurement Device: Laser Scanner ...............................................................................................37 3.3. Architecture of a System for Collecting and Organizing Archaeological Remain Data (Archae-Collector) ...................................38 3.3.1 Data Types and Objects..................................................................38 3.3.1.1 Archaeological Remains ..................................................39 3.3.1.2 Archaeological Relics .......................................................40 3.3.1.3 Other Documents..............................................................40 3.3.2 Associations Among Data .............................................................41 3.3.3 Architecture of a System for Collecting and Organizing Archaeological Information (Archae-Collector).........................42 3.4 Three-Dimensional Data Acquisition and Model Development with a Laser Scanner.....................................................44 3.4.1 Types of Laser Scanners and Their Combinations....................44 3.4.2 Geometric Registration of Laser-Scanner Data..........................45 3.4.3 Reconstruction of Three-Dimensional Shapes from Laser-Scanner Data .........................................................................47 3.4.4 Visualization by Combining Laser-Scanner Data and Digital-Camera Images...................................................................49 3.5 Implementation of an Example of Archae-Collector.............................51 3.6 Conclusions and Future Prospects ...........................................................53 Acknowledgment..................................................................................................54 References...............................................................................................................54 35 Copyright © 2006 Taylor & Francis Group, LLC 36 GIS-based Studies in the Humanities and Social Sciences 3.1 Introduction Excavation in archaeology is conducted to acquire and collect information on archaeological remains and relics in a systematic way using limited time and human resources. Data to be collected are so diversified. The data ranges from overall structure of archaeological remains and relations of strata, details of individual parts of archaeological remains, and informa-tion on each relic, such as its classification, location, and strata of unearthed position, its three-dimensional shape, and photos. These voluminous and diversified pieces of information should be efficiently collected, acquired, and organized in such a manner that the relationships among them can be easily retrieved. In recent years, digital camera, laser scanners, spatial-database manage-ment system, such as Geographic Information Systems (GIS), and three-dimensional drawing and modeling tools, such as Computer Aided Design (CAD), have made very rapid progress. The advances make it so easy to acquire digital data on archaeological remains and relics. At the same time, it also provides a possibility of developing new types of prod-ucts, such as three-dimensional models. In addition, using the Internet, the digital data can be easily shared among archaeologists. Through shar-ing digital archaeological data among larger numbers of researchers, com-parative studies and analysis from more diversified viewpoints can be promoted, which will eventually result in greater contribution to the advances in archaeology. To actually realize more efficient acquisition and collection of informa-tion and sharing in archaeological excavation, how to use and combine advanced sensors, devices, and software has to be discussed and devised. Sensors, data-measurement devices, and software are tools. They require know-how and ideas to effectively apply, just like carpenter tools alone are not enough to build a good house if no skills and know-how are combined with them. Good “design” on how and in which aspects to use, combine them for excavation, and subsequent organizing and analyzing works is really a key. Good design may also reveal some missing links, i.e., a kind of software and devices to be developed especially for archae-ological excavation. This section reports an example of “good design” on how to better use three-dimensional measurement tools, such as laser scanners and data-management tools, such as GIS, including newly developed software and know-how to fill gaps between advanced technologies and the demand in archaeological excavation, through a case study of Tyre remains, Leb-anon. Copyright © 2006 Taylor & Francis Group, LLC A Laser-Scanner System for Acquiring Archaeological Data 37 FIGURE 3.1 Laser scanner in Tyre, Lebanon. 3.2 A New, Three-Dimensional Measurement Device: Laser Scanner For the past four or five years, laser scanners for three-dimensional measure-ment have become drastically cheaper and smaller, and therefore, much more popular (Figure 3.1). Laser scanners acquire three-dimensional shape data on an object in the following process. At first, as shown in Figure 3.2, a laser scanner emits a laser beam and measures the return time of the beam reflected on the surface of the object. From the travel time of the laser beam, the exact distance between the laser scanner and the object is measured. In parallel, beam angle, i.e., horizontal and vertical angles, are measured. By combining the distance, the horizontal and vertical angle, the three-dimensional coordi-nate values relative to the laser scanner can be computed. By repeating this process with an incremental change in angles several thousand to several hundred thousand times per second, a very large amount of three-dimensional points are generated. The three-dimensional point data acquired in this man-ner is called “point-cloud” data. With the three-dimensional point-cloud data, the shape of the object surface is represented. The measurement accuracy usually ranges from several millimeters to several centimeters. Another method of three-dimensional measurement employs photographs and images. A typical example is photogrammetry. By taking pictures of an object from different viewing angles and measuring the location of the object in the pho-tographs or images, three-dimensional location of the object can be estimated. But this measurement process requires exact estimation of position and atti-tude of a camera or an imaging device in image data acquisition. The estima- Copyright © 2006 Taylor & Francis Group, LLC 38 GIS-based Studies in the Humanities and Social Sciences Vertical scanning angle Laser beam Horizontal scanning angle Laser Spot Laser range scanner FIGURE 3.2 3D measurement with a ground-based laser scanner. tion of the position and attitude also require the measurement of the image coordinates of ground control points, or GCPs, with exact ground coordinate values measured in advance. In addition, stereoscopic observation for the three-dimensional measures needs some training. Although cameras them-selves have become digital devices that are very easily operated, the prepara-tion and skill needed for three-dimensional measurement make the digital photogrammetry a bit difficult for the ordinary archaeologist. On the other hand, laser scanners, though still quite expensive, make it possible to automate the three-dimensional data acquisition. Automation in measurement is a great advantage of laser scanners over the other measurement devices. 3.3. Architecture of a System for Collecting and Organizing Archaeological Remains Data (Archae-Collector) 3.3.1 Data Types and Objects Major types of data collected and generated through excavation include drawings, documents, and photos, not limited to three-dimensional mea-surement data with laser scanners. This chapter proposes the architecture of a system for collecting and organizing data from archaeological excavations, before describing three-dimensional measurement and modeling of archae-ological sites. Objects for data collection and generation are classified as follows: Copyright © 2006 Taylor & Francis Group, LLC A Laser-Scanner System for Acquiring Archaeological Data 39 1. Archaeological remains 2. Archaeological relics 3. Excavation work records such as schedule 3.3.1.1 Archaeological Remains Archaeological remains are mainly represented by a series of drawings, ranging from relatively macroscopic ones of the overall configuration to more microscopic ones on three-dimensional details of individual parts. Drawings reflect the results of judgments on what are important enough to record, as well as the geometric properties of the remains. In this sense, drawings are regarded as a unique form of representation, rather than a symbolic representation of geometric properties. However, because what is considered to be trivial in making drawings may be found to be impor-tant afterwards, it is necessary to record source data, such as three-dimen-sional measurement data and photo data. For the acquisition of three-dimensional data, considering the diversity of archaeological remains in size and required accuracy, the combination of other three-dimensional measurement methods, such as aerial photogrammetry, ground-based pho-togrammetry, and ground-based survey, rather than laser-scanning devices, should be considered. Moreover, sketches and photos are also important as complementary data to the drawings and three-dimensional measurement data. Especially, photo data can record colors and texture. They can apply to any locations where laser scanners are difficult to apply. In addition, they are effective to let archaeologists easily record with short FIGURE 3.3 Example drawings of archaeological remains. Copyright © 2006 Taylor & Francis Group, LLC ... - tailieumienphi.vn