Geometry effect on the estimation of band reflectance in an urban area

Reflectance of the urban surface is an important factor for urban climate studies and can be assessed using standard remote sensing applications. However, no application considers the three-dimensional structure of the city surface and its resulting shading patterns or the inclined roof surfaces. To determine the effect of these factors on the estimation of urban surface reflectance, a high-resolution raster-based city-surface model was used to estimate the spatial solar irradiance in an example city, namely Basel in Switzerland. Eight times daily for 1 year, the solar irradiance was calculated using MODTRAN and the illumination geometry of the city. Subsequently, the spatial distribution of the solar irradiance, as well as the error in assumed reflectance values were analysed. The error in estimation of reflectance increased with lower solar-elevation angle, so its maxima were found in winter. Higher visibility of the assumed atmosphere also increased the estimated error due to the lower proportion of diffuse irradiance. The error decreased with coarser spatial resolution of the pixel.