Abstract
Soil roughness represents fine-scale surface geometry which figures in many geophysical models. While static photogrammetric techniques (terrestrial images and laser scanning) have been recently proposed as a new source for deriving roughness heights, there is still need to overcome acquisition scale and viewing geometry issues. By contrast to the static techniques, images taken from unmanned aerial vehicles (UAV) can maintain near-nadir looking geometry over scales of several agricultural fields. This paper presents a pilot study on high-resolution, soil roughness reconstruction and assessment from UAV images over an agricultural plot. As a reference method, terrestrial laser scanning (TLS) was applied on a 10 m x 1.5 m subplot. The UAV images were self-calibrated and oriented within a bundle adjustment, and processed further up to a dense-matched digital surface model (DSM). The analysis of the UAV- and TLS-DSMs were performed in the spatial domain based on the surface autocorrelation function and the correlation length, and in the frequency domain based on the roughness spectrum and the surface fractal dimension (spectral slope). The TLS- and UAV-DSM differences were found to be under ±1 cm, while the UAV DSM showed a systematic pattern below this scale, which was explained by weakly tied sub-blocks of the bundle block. The results also confirmed that the existing TLS methods leads to roughness assessment up to 5 mm resolution. However, for our UAV data, this was not possible to achieve, though it was shown that for spatial scales of 12 cm and larger, both methods appear to be usable. Additionally, this paper suggests a method to propagate measurement errors to the correlation length.
Highlights
Roughness is a property of surfaces, required to understand and model interaction at these surfaces, e.g. in hydraulics, radar remote sensing, or soil erosion
Two elevation-difference rasters were prepared to report on the co-registration accuracy: one to check the co-registration of the individual terrestrial laser scanning (TLS) scans, and another to check the co-registration of the TLS digital surface model (DSM) and unmanned aerial vehicles (UAV) DSM
The co-registration of the individual TLS scans was checked with the minimum-maximum elevation difference for each pixel of a set of six overlapping DSMs interpolated from the individual TLS scans
Summary
Roughness is a property of surfaces, required to understand and model interaction at these surfaces, e.g. in hydraulics, radar remote sensing, or soil erosion. The assessment of roughness has been traditionally performed by mechanical profiling (Mattia et al, 2003), but this is naturally restricted by the length of the ruler and the effort to place it at different locations. The range envelope for which roughness should be quantified depends on the application. But likewise in the optical domain, the backscattering behavior depends on the roughness in relation to the wavelength (Ulaby et al, 1986). The roughness between a few mm and up to several decimeters should be modeled
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