An algorithm for monitoring informal constructions : an application in coastal areas
Psaltis, Christodoulos and Charalabos Ionnidis
An efficient environmental protection of coastal areas is of high priority, but it can only be achieved if a multidisciplinary team of experts provide the necessary knowledge, tools and strategies to analyse the current situation, detect possible risks and act to prevent them. Informal settlements is one of the dominant problems faced, which surveyors can help solving. Modern photogrammetric techniques enable automatic monitoring, thus reducing the cost and time required for such procedures while keeping a high level of accuracy. This paper introduces a simple and straightforward technique and discusses the first results of an application test carried out in a coastal area of Eastern Attica in Greece. The proposed strategy is based in comparing two digital surface models (DSM) of the same area in two different time periods and by facilitating change between them to assert possible cases of informal constructions. The DSMs used are very dense in order for at least a few points to lay on each rooftop and they are produced from large scale aerial image stereopairs oriented in the same reference system. These DSMs are transformed to raster format with the same cell size and georeference and then the reference DSM is subtracted from the new one. Next, the resulting raster is transformed to binary by an algorithm using a uniform threshold leaving out points with a high probability of being erroneous. The remaining areas are filtered further with size and shape criteria. Those areas that passed filtering are considered as changed.
Event: 6th FIG Regional Conference 2007 : Strategic Integration of Surveying Services
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