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Surface Roughness Calculation

Definition

Surface roughness quantifies small-scale height variability that affects wind drag, radar backscatter, habitat, and hydrology. Metrics include standard deviation of elevations, root-mean-square slope, fractal dimension, and aerodynamic roughness length (z0). Roughness can be computed from DEMs, lidar point clouds, or imagery-derived textures, using windowed statistics at scales relevant to the process of interest.

Application

Meteorologists set z0 for boundary-layer models; radar interpreters distinguish surfaces (water vs vegetation); ecologists study microhabitats; and engineers analyze pavement skid resistance and erosion thresholds.

FAQ

How does window size influence roughness estimates?

Small windows capture micro-texture but are noisy; larger windows smooth variability and may miss critical microtopography. Match the window to the physical scale affecting your process.

What’s the relation between aerodynamic roughness and land cover?

Vegetation height and density largely control z0; crops, forests, and urban canopies have distinct ranges that change seasonally.

How do sensor artifacts masquerade as roughness?

Striping, interpolation seams, and co-registration errors inflate roughness metrics. Preprocess with de-striping and alignment checks before calculating statistics.

When is spectral texture a good proxy for roughness?

For surfaces where tonal variation correlates with microrelief (e.g., gravel bars), texture indices from imagery can substitute when elevation data are lacking—validated with samples.