Survey system for roadway topography

An experimental trailer-mounted instrumentation system which surveys and records roadway topography data while being towed at normal highway speeds has been developed and tested. Non-contacting electro-optical sensors developed for this system are used to measure the relative height of the roadway surface along 12 paths which are spaced at selected intervals across the 12-ft. width of a normal traffic lane. The cross slope and longitudinal slope of the surface and the direction of travel are obtained from a gyro-type inertial reference unit while compensation for vehicular bounce is achieved by double integration of the signal from a vertical accelerometer. At intervals of one (or two) feet along the roadway a solid-state data acquisition system samples the output from each height sensor; the roll, pitch and azimuth signals from the inertial reference system; the output from the accelerometer and the double integrated accelerometer signal. This data is digitized to 10-bit resolution and recorded on a digital cassette type magnetic tape. The taped data, covering up to 2-miles of roadway on each cassette, is computer processed for analysis and graphical display. The computer graphics show the cross profile of the roadway surface as well as the longitudinal profiles along each sensor path, the cross and longitudinal slope angles, the vertical acceleration and the vehicular heading. The non-contacting electro-optical height sensors use a high power (200 mw.) gallium arsenide infrared LED to illuminate a nominal 4-inch (10 cm) diameter area or spot on the pavement surface. This spot is detected by a pair of dual photocells in a configuration that allows the distance from the instrument to the average height of the surface within the illuminated area to be accurately measured while compensating for effects of non-uniform surface reflectivity. The infrared LED is square-wave modulated at 5 kHz and synchronous detection, along with IR optical bandpass filtering, is used to eliminate the interfering effects of sunlight or other ambient light sources. Special attention has been given to the lens design to maximize the utilization of the IR emitted by the hemispherical source. This sensor could be of value in other highway applications where capability to measure the distance from a vehicle to reflecting or scattering surfaces of imperfect quality is needed for survey, guidance or control purposes. Preliminary tests of the system have indicated a height sensing accuracy of better than 0.01-in. (0.25mm) and a slope measurement accuracy of 0.2°. The height sensor has been found to be usable on concrete as well as asphaltic types of roadway surfaces. The height measurement range of ±2-inches about a nominal height of 15-inches could be modified to meet the requirements of other applications.