## 1 Introduction
### State of the Art on HBIM-GIS integration: IFC and CityGML Standards  
The research interest on the topic of BIM and GIS integration and harmonization has been increasing in the last years. The multitude of studies in the recent literature is due to the necessity of integration between these two different domains in order to express and communicate information on territory, urban areas and buildings at different scales. However, even if the main incompatibilities between BIM and GIS systems have been partially solved, most of the interoperability issues arise when it comes to data conversion between their standards and, most of all, when dealing with HBIM models [3]. In fact, these historical 3D digital models are mainly structured on non-standard categories and classes that have to be able to describe the complex geometries and a variety of attributes and peculiar elements typical of the CH domain. In this case, different solutions could be undertaken from the operator when structuring and defining a model or a project, not allowing a generalization of the methodology. In general, when facing BIM-GIS integration, one of the main interests highlighted by the state-of-art is to study the various factors related to new buildings, which influence the context and the surrounding urban area [4; 5] as shadow analyses [6] or flood damage that can occur [7]. In addition to investigating how to use the models to improve the environment, for example reducing energy consumption at a district level [8], supporting the sustainable urban development [9] or establishing networks for emergency response [10; 11]. Despite numerous research over years having attempted to bridge the gap among BIM and GIS formats and standards in different ways [12; 13; 14], only a few studies have been focused on the integration of HBIM [15; 16; 17; 18] in the GIS environment. When considering the binomial HBIM/GIS, the studies usually try to manage the object-oriented model into the GIS environment in order to offer system support for planning and building, simplifying thematic analysis and synthetic elaborations, technical-economical evaluations, virtual restoration hypothesis and so on [19; 20; 21; 22]. It has also to be considered that, as in this case we are facing with an existing building, it is fundamental not only to think about the integration between the standards, but also the geo-referencing accuracy of the existing building in a 3D GIS environment. This topic has been solved lately through different solutions as, for example, the \"FME (Feature Manipulation Engine) Exporter for Revit\" plugin [17; 21], which allows to keep the spatial reference when exporting from Revit to ArcMap, ArcScene or QGIS, placing the building in the correct position. However, even if the issue of the insertion of the HBIM model in the GIS environment has been solved, the problem of how to create an effective dialogue between the two standards, and the structuring of a model that can integrate a large amount of data describing a complex system such as a city or territory, with an adequate level of detail that goes up to the architectural scale for CH, has not been fully developed yet. To fix these interoperability issues it is fundamental to consider the adoption of standards in order to allow an effective dialogue. CityGML was developed in 2002 by the Special Interest Group 3D (SIG3D) of the \"Geodata Infrastructure\" initiative in North Rhine-Westphalia, Germany. In 2008 it was adopted by the Open Geospatial Consortium (OGC) as the official standard for modelling and exchanging 3D models of cities and landscapes and was quickly adopted internationally. It is structured to represent cities and buildings features with a different level of detail (LoD) (Table 1). LoD 0 is for regional and landscape level, LoD 1 for regional level, LoD 2 for city district and urban context, LoD 3 for the exterior architectural models and landmarks and finally LoD 4 for the interior architectural model [23].