# A Framework of Wearable Sensor-System Development for Urban 3D Modeling
## 1 Introduction
### Purpose of Study  
The purpose of this study is to propose a framework for building a wearable system that generates accurate RGB-D data. More specifically, the objectives of this study are: (1) to suggest guidelines for the design of a wearable multi-sensor system, (2) to propose a framework that can accurately conduct calibration of a wearable sensor system, and (3) to propose an efficient system calibration method.  
In this study, criteria for sensor placement of wearable systems are suggested for efficient and accurate RGB-D data generation. For efficient data acquisition and accurate data fusion, sensor placement and system design should be determined carefully. However, most previous studies have not expanded upon the details of the sensor placement for wearable systems. Therefore, this study suggests the criteria to be considered in the sensor arrangement of a wearable sensor system.  
This study proposed a calibration framework optimized for wearable sensor systems. Because the number of mounted sensors in a wearable system is limited, the FOV overlap between the same modal sensors is generally kept small for efficient data acquisition. Therefore, previous calibration methods for sensors with high FOV overlap are not suitable for wearable systems calibration. The proposed calibration framework is optimized for wearable sensor systems and can be applied to various sensor systems. Sensor-bundle configuration proceeds based on interpretation of FOV overlap between sensors, and subsequently conducts system calibrations for each bundle.  
Finally, in this study, a calibration method based on point-plane correspondence is proposed for more efficient system calibration. Previous calibration methods for multi-sensor systems require (i) the coordinates of GCPs or the length of control lines, or (ii) processes to extract points or lines on the edge of the calibration target of the test-bed. On the other hand, the proposed method does not require the pre-processed coordinates of GCPs for point or line extraction on the edge of the calibration target. Therefore, the proposed method contributes to efficient system calibration. Furthermore, the proposed method has high utility because it is applicable to sensors with low FOV overlap. It is based on bundle adjustment (BA) using a co-linearity condition, which is commonly used in optical camera calibration.  
This paper describes the research in the following order. Section 2 presents the sensor arrangement method for the backpack-based wearable system used in this study, and describes the actual fabricated sensor system. Section 3 provides a detailed description of the proposed calibration framework. The contents of self-calibration for the optical camera and LiDAR sensor (mounted in the wearable system) in the previous study are briefly introduced, and the details of the proposed system calibration framework are described. Section 4 evaluates the proposed framework through the calibration of a wearable sensor system and the analysis of data fusion results. Finally, Section 5 considers the significance of this study and presents suggestions for future research.