Topographic correction, or topographic effects correction, aims to adjust the spectral values of an image according to effects of solar illumination differences due to the irregular shape of the terrain. Topographic slope and aspect introduce radiometric distortion of the recorded signal. Further, terrain shadow dramatically affects the brightness values of the covered pixels in an image. Topographic effects of illumination and shadow are particularly relevant in mountainous regions and in regions towards the higher latitudes of the southern and northern hemisphere. The effects appear pronounced during the winter season. Together with sensor calibration and atmospheric correction, topographic correction is part of the radiometric correction process to obtain true reflectance values from sensor radiance. This process is necessary when using EO data for obtaining geophysical measurements. It can also benefit the accuracy of image classifications by reducing the internal variability of vegetation types, since the corrected reflectance relates better to the geometrical or biological properties of the plant than to the original reflectance. Methods for the removal of topographic effects from remotely sensed images can simply be based on band ratios that do not require additional input. Alternatively, they use digital elevation models (DEMs) as an additional input and apply sophisticated modelling of the illumination conditions. The illumination model describes various aspects of the relationship between the sensor measurement, the sun illumination, the ground reflectance and the diffuse irradiance at the surface. The model incorporates the angles between the sun position, the ground position (described by slope and aspect from the DEM), and the sensor position. Among these methods are lambertian methods and non-lambertian methods such as the bidirectional reflectance distribution function (BRDF). The BRDF, which is more suitable to the non-Lambertian properties of the observed surfaces, describes how the reflectance varies in each cover considering the angles of incidence and observation. If achieved with a high quality, the resulting topographically corrected image appears to be illuminated evenly as if all its pixels would be part of a flat surface without the presence of any terrain differences. However, the much larger benefit than the improved appearance is the availability of pixel values that are closest to the true reflectance when compared to TOA, BOA and DN values.
Produce a surface corrected version of image values from BOA reflectance that removes topographic effects based on an input DSM and equations representing the relationship between sun incidence angle relative to terrain surface orientation
In progress