Computer Vision and Pattern Recognition Technologies for Construction - Mobile and Pervasive Computing in Construction - page 200

transmitting laser/infrared light to measure the distance to an object point. Since
the speed of this light is known, the travel distance, which is twice the distance of the
object point to the sensor, can be found by timing the round-trip time of the
outgoing and reflected light signal. These sensors create a highly accurate 3D image
of a surface. The significant cost of the equipment for laser scanning and the short
measurement range of 3D range video cameras, however, preclude their use for
most construction projects. On the other hand, visual triangulation-based sensors,
such as digital cameras, make use of the natural light in the environment to capture
the surroundings information in the form of visual data. Photogrammetry prin-
ciples are then used to extract spatial coordinates of each object point in different
images. Photogrammetry is defined as the process of measuring 3D coordinates of
real-world objects from digital images based on triangulation, whereby a point in
space is reconstructed from two mathematically converging lines from 2D locations
of the target point in two images (Figure 10.6).
Figure 10.6 Visual triangulation: P - an object point; P l and P r - mathematical
converging lines; and p l and p r - 2D locations of the object point in two images (i.e.,
corresponding points)