A digital close range photogrammetric observation system for measuring soil surface morphology during ongoing rainfall

Soil erosion is a hydro-geomorphic process occurring over the earth' surface. The observation of soil surface morphology during ongoing rainfall at fine spatial and temporal scales are critical for the study of soil erosion. A digital close range photogrammetric observation system based on wireless networking technology was explored and established in this study. Through wireless networking, multi-cameras were concurrently controlled, solving the problems of synchronous acquisition of underlying digital image, big data transmission and storage. Image sensors collected data in parallel based on network command, and noise on the images such as raindrops was eliminated based on the K-means clustering algorithm by serialization technology during data acquisition. After parallel preprocessing, high density 3D point clouds and digital elevation models (DEMs) were reconstructed. The evolution of soil surface topography was dynamically monitored by instantaneous image acquisition at different time intervals during ongoing rainfall. The results showed that the measurement precision of the established system could reach a sub-millimeter level with the minimum single measurement error being 0.0069 mm. The maximum error between check point coordinates generated by photogrammetry and those generated by a Total station was 5.7 mm on the horizontal axes and 9.6 mm on the vertical axis. The temporal and spatial resolutions of the observation system were 1 min and 2 mm, respectively. Compared with a runoff sediment collection method, the average relative error of the system for estimating soil loss was 5.63%, and the accuracy of single observation was up to 99.58%. The observation methods explored in our study provided a reliable way to monitor soil erosion processes, which was also helpful to analyze the role and influence mechanism of soil erosion in surface hydrological process.