Terrain-neglected radiometric distortion remains a major challenge in quantitative remote sensing over slope terrain. While numerous topographic correction models have been developed and validated in specific regions or simulated settings, a systematic and quantitative evaluation of the satellite-derived reflectance bias introduced by terrain neglect is still lacking. A global whole-year multi-angular reflectance data set was produced for >15000 homogeneous 500 m single-slopes by coupling MODIS daily products with a physically based radiative-transfer model, including both terrain-neglected and terrain-corrected surface reflectance. The relative percent bias of terrain-neglected reflectance to terrain-corrected reflectance (rBias) was evaluated across four slope groups (10°–15°, 15°–20°, 20°–30°, ≥30°). The results show that rBias is primarily governed by slope-driven variation in relative solar zenith angles (rSZA). Positive and negative values of rBias flips with rSZA: terrain-neglected reflectance is totally overestimated at small rSZA (~0°–15°) while totally underestimated at high rSZA (~69°–90°), and the whole proportion of underestimation increases with slope, from 58.4% at 10°–15° to 71.5% at ≥30°. Magnitude of rBias escalates non-linearly with both slope and rSZA, as median rBias ranges from −29.5% to 4.2% at 10°–15° slopes and from −61.2% to 11.9% at slopes ≥30°. Also, with the rBias extrema expand from (−55.2%, 24.8%) at 10°–15° slopes to (−88.3%, 55.0%) at slopes ≥30°. These findings quantify terrain-neglected satellite-derived reflectance bias and highlight the need for topographic correction under steep slopes and high rSZA.
Chen et al. (Thu,) studied this question.