Quantifying Roughness of Unpaved Roads by Terrestrial Laser Scanning

Unpaved roads represent a major component of the roadway network in Iowa and require regular surface grading to control roughness (e.g., corrugation). This paper presents a study that shows how terrestrial laser scanning can characterize road roughness in relation to (a) a spatially analyzed international roughness index (IRI), (b) a fast Fourier transform (FFT) spectral analysis, and (c) surface texture characterization that uses statistical analysis. Algorithms are presented for each of the roughness calculations. The spatial nature of terrestrial laser scanning makes possible the determination of IRI values along any selected path within the scan area. In this study, median IRI values were used to characterize overall road roughness. To characterize the nature of road roughness, FFT was used to decompose the laser scan height spectrum. Examples are presented for unpaved road sections with a relatively smooth surface, a smooth surface with corrugations, an unsystematically rough surface, and an unsystematically rough surface with corrugations. On the basis of the observation that the surface aggregate materials were segregated, selected small regions were studied with a scan with spacing of 0.5-mm points. These scans were filtered and statistically analyzed for variations in height by using root mean square, skewness, and kurtosis parameters. These parameters were studied in relation to the values for particle size index for the surface materials of the three distinct gradations. The algorithms presented will be of value to terrestrial laser scanning users interested in characterizing unpaved roadway conditions.