Research and acquisition of historical aerial photos and satellite images from public and private entities and on-line; interpretation of the collected remote sensing data in order to perform a multi-temporal analysis aimed at the recognition of variations over time in the studied locations. This methodology is specifically effective when there is little and uncertain direct information and where the activities and the ownership of the site have changed over the years. In some cases, where remote sensing imagery is lacking, historical maps and plans of the plants are also used, if reliable and dated. In other cases, when a precise volumetric evaluation of any ground movement is required, aerial photogrammetric restitution can be performed.
Usually this kind of analysis allows retracing the evolution of processing plants, waste deposits, the individuation of settling tanks and related outlets, the presence of earthworks for digging or backfill (potential pollution sources), which are presently hidden or whose memory has been lost.