Many transposition models have been proposed in the literature to convert solar irradiance on the horizontal plane to that on a tilted plane. The inverse process, i.e., the conversion from tilted to horizontal is investigated here based on six months of in-plane global solar irradiance measurements recorded on the roof of the Royal Meteorological Institute of Belgium's radiation tower in Uccle (Latitude 50.79 degrees N, Longitude 435 degrees E). Up to three pyranometers mounted on inclined planes of different tilts and orientations were involved in the inverse transposition process. Our results indicate that (1) the tilt to horizontal irradiance conversion is improved by using a multi-pyranometers approach and (2) the improvement from using an isotropic model to anisotropic models in the inverse transposition problem is not significant. (C) 2017 Elsevier Ltd. All rights reserved.