Characterizing Strategies of Fixing Full-Scale Models in Construction Photogrammetric Surveying

In construction photogrammetric surveying, it is imperative to convert the relative scale of a 3D model into absolute measurements so that geometric measurements can be taken. Previous work suggests this can be done through the determination of a reference line in absolute units, but the position and quantity of reference lines has not been investigated. This paper attempts to characterize the behavior of different strategies in selecting reference lines. Factors including relative position of the reference line, the number of reference lines, and spatial relationships of the reference lines are considered. Three infrastructure objects serve as the test beds, and experiments using various layouts of scale bars (calibrated reference lines) are conducted on each object. The surveying results are then compared with ground truth data, which allows for accuracy determination and aids in determining the best strategy in selecting reference lines for scale fixing. The results from these experiments show that one horizontal reference line in the middle of the object performed with consistent accuracy; but, if a specific area on the object needs more accurate measurements, it is best to select a reference line in that area. Using three reference lines is ideal from a time and accuracy improvement perspective.