Quantifying the effects of terrestrial laser scanner settings and survey configuration on land surface roughness measurement

Abstract

The application of terrestrial laser scanning (TLS) for measuring Earth surface features is increasing. However, TLS surveys require users to choose and specify certain properties of the scan (i.e., resolution, height, distance, number of scan positions), often with limited understanding of how these properties affect the accuracy of the data. This paper presents results from an experiment that quantifi es the effects of different scan settings and survey confi gurations on the measurement of centimeter-scale surface roughness. The main goal is to provide quantitative evidence to help guide and optimize fi eld-based surface roughness measurements involving TLS data. The experiment involved an array of artifi cial roughness elements placed on an asphalt surface, similar to the approach of using inverted buckets in boundary layer experiments to simulate a rocky or sparsely vegetated surface with smooth interspaces. The independent variables consisted of laser point spacing, number of scan positions, and the height and distance of the scanner relative to the roughness array. The dependent variables were roughness element height, data occlusion, relative vertical accuracy, the root mean square height of the cup array, and the relative roughness of the asphalt surface. Two roughness patterns were tested, isotropic and anisotropic. Results show that when the laser point spacing was greater than the size of the individual roughness elements, their calculated height was between 32% and 73% below their actual height, but with a smaller spacing the calculated height was either equivalent to their actual height or only slight lower. Therefore, before a TLS survey is undertaken, manual measurements of roughness elements should be used to determine the size of the smallest roughness elements of interest, thus guiding the selection of laser point spacing. Larger point spacing also decreased the vertical accuracy of surfaces interpolated from the point clouds compared to global positioning system points. By combining point clouds from three scan stations arranged in a triangular network around the roughness array, the proportion of data occlusion decreased to as little as 0.24%, but due to error associated with point cloud registration, the accuracy of roughness element height decreased and the roughness of the asphalt surface artifi cially increased. For the most accurate measurements of the roughness element height and interspace roughness our results suggest that high-resolution point clouds obtained from one vantage point should be used. In regard to scanner height and distance, we measured a doubling of the occluded area when the scanner height decreased and distance to the array increased, thus increasing the angle of incidence. Reducing the angle of incidence decreases occlusion but also limits the areal coverage of each TLS scan and increases the number of scan stations at different vantage points required to cover large areas, which ultimately affects the accuracy of other roughness metrics. Overall, this case study de...

Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

“…The scanner was mounted on a small tripod (2 m) to maximize the angle of incidence and minimize occlusion (Brown and Hugenholtz, 2013; Figure 1(c)). An area of approximately 7 m ×7 m was scanned a total of 50 times collectively during the three monitoring periods.…”

“…The scanned area was upwind and adjacent to the location where measurements of velocity profiles and saltation activity were obtained. The scanner was mounted on a small tripod (2 m) to maximize the angle of incidence and minimize occlusion (Brown and Hugenholtz, 2013; Figure 1(c)). The Leica C10 has an inherent surface-model accuracy of approximately 2 mm (Hodge, 2010).…”

Predicting the roughness length of turbulent flows over landscapes with multi-scale microtopography

“…Each study site was an area of at least 30 m×30 m with relatively uniform roughness, as judged visually and by analysis of the TLS data. The minimum fetch required for an equilibrium boundary-layer flow is typically assumed to be 1000 times the height of the dominant roughness elements (Counehan, 1971). Based on this criterion, 30 m was adequate fetch for 7 of the 10 sites, i.e., all except for the three Death Valley sites, where roughness elements were up to 300 mm; hence, the area of homogeneous roughness was verified to a distance of only ∼ 100 times the height of the dominant roughness elements.…”