An attempt of SfM photogrammetry to narrow and dark underground building heritage

The 3D models creating by SfM (Structure-from-Motion) photogrammetry became one of the important and convenient methods for any kinds of objects on geomorphology, geoheritage, or geoarchaeological fields. These objects are landforms, monuments, buildings, relics and so on. In order to evaluate these objects, it is necessary to collect morphological characteristics, and then proceeding to decide investigating points or areas of these materials.

The progress of this methods developed significantly, however, there have been still remained difficulties depending on the objects. For example, it is difficult to create 3D models that the object is too flat, too dark, and/or any restricts of combination of target size and focusing distances. The present study attempts to these difficulties by targeting to narrow and dark underground space. The investigating object is an archaeological man-made cave, called Taya Cave, in central Japan. It was excavated in 13 century originally and used as study areas for Buddhists by making Buddhism bas-reliefs. The cave has a total length of 570 m underground passage with a three-layer structure. The cave also has several domes connected by narrow paths. The present study tried to make a 3D model of this complicated, dark and narrow cave by SfM photogrammetry. In order to concur to make 3D models for the whole area of the cave, it is useful making chunks; separating several areas of simple morphology and then compiled. When facing narrow path, it is better to take photographs not by perpendicularly but by inclinedly. Furthermore, it is important to use strong light with attach to camera. After obtained the image data of the whole cave, the accuracy of the created model was evaluated. The results were that the accuracy of horizontal distances are higher than that of vertical distances.