to etc.) has appropriate database formats. Each database pixels, and for each pixel a geometrical or electromag- format is associated to appropriate propagation models. netic parameter is stored, then a raster database is Therefore mixed environments pose a number of problems obtained. Otherwise, if the environment is decomposed to both database handling and field prediction modelling. into basic geometrical elements (triangles, polygons, The different types of data must be harmonised by means segments, etc.) then a vectorial database is obtained. of proper conversion, interpolation, over- or undersampling Vectorial databases are usually more compact and ac- and/or the different propagation models must be somehow curate than raster databases and are therefore adopted combined. Although few authors within COST 273 by default in the present section, except where other- worked on database issues, it is evident that problems such wise stated. In vectorial databases, buildings are usually as high cost, low reliability and high complexity of input represented as polygonal prisms with flat tops. In all databases (and the corresponding high handling and pre- cases, the intrinsic cost of urban databases is high, diction time) probably represent the main limits to the ranging from some hundred to some thousand euros per widespread adoption of deterministic propagation models. square kilometre. Moreover, the attainable accuracy is Studies related to database extraction, handling and sim- necessarily limited. The standard deviation of the error in plification carried out within COST 273 are briefly urban databases is of the order of 0.5 m for horizontal reported in the remaining part of this subsection. coordinates and much higher for vertical coordinates. Vertical dimensions are often unavailable or particularly subject to errors, due to deviations in rooftops from flat Electromagnetic parameters determination polygons, with an evident impact on the reliability of over- In [ScWi03], RT simulation of wave propagation in hos-