EXPERIENCE IN MOBILE LASER SCANNING BY MEANS OF LYNX SYSTEM IN L’AQUILA CITY

Abstract

The terrestrial laser scanner is an efficient topographical instrumentation used to acquire a redundant number of points distributed over a physical surface. The goal of laser scanning is the definition of very accurate models of the studied areas. In this way, deformations or changes can be monitored by means of repeated surveys in different epochs [Pesci et al., 2005; 2007]. The laser signal is characterized by highly collimated, monochromatic, and coherent radiation that is well suitable for very short impulse generation in the nanosecond scale. The operating methodology of a time-of-flight laser scanner is similar to a laser range-finder, measuring the time it takes a laser pulse to travel from a transmitter to the surface surveyed, and back to a detector device. The range d is computed using the relation d = ct / 2, where t is the time of flight and c is the speed of light. The advantage of this instruments is the laser beam deflection over a very accurate angular grid, that can be obtained by oscillating and rotating mirrors, thus providing a wide coverage area between adjacent points. Each point is collected into a local reference system consisting of the origin at the instrument sensor, well-known angular parameters, and very accurate measurements of range. Together with point coordinates (x, y, z) , radiometric values related to the surveyed object’s reflectivity can be calculated from returned signal energy. The maximum measurable range depends on the illuminated material roughness and color, and the laser wavelength [Fidera et al. 2004, Pesci and Teza, 2008]. Divergence values for new generation long-range scanners are extremely reduced, illuminating very small surface elements for each shot. The spot dimension increases linearly with the distance, and is always greater than the lower limit of the instantaneous field of view (IFOV) due to physical diffraction. Effective laser scanner characteristics are defined by a set of parameters, including: range resolution (depending on telemeter efficiency), single point measurement accuracy (depending on the internal electronic device, signal-to-noise ratio and critical time needed for pulse recognition), beam divergence (which defines the IFOV, depending on laser wavelength), and minimum angular step (depending on the internal mirrors calibrated system) [Wehr and Lohr 1999]. Overlap is the laser scanning strategy that can reduce errors, because redundant points are acquired belonging to the same illuminated area. A common overlap is obtained by fixing the ratio between spot dimension (the area illuminated by a single pulse with a given divergence) and angular step so that a given point is measured 10 times. For instance, if the divergence is 3 mrad and angular variation about 0.3 mrad, at 100 m distance, an element included in a 3 cm area is observed 10 times. The final result of a laser scanner application is a very dense point cloud, with radiometric reflectivity data for each point.