## 2 Research Background and Related Work
### Data Conversion from BIM to GIS  
Data conversion between BIM and GIS can be generally categorized into two groups: within-industry conversion and cross-industry conversion [24]. Within-industry conversion refers to data conversion that takes place between different applications within the same industry, for example, between different BIM systems. Such a conversion is easier, as similar data standards and conversion methods are usually being adopted by different applications in a specific industry. For example, most systems in BIM use IFC for data exchange [29]. Cross-industry conversion, as suggested by its title, refers to data conversion that takes place between different applications in different domains, for example, between BIM and GIS. A comparison with within-industry conversion shows that cross-industry conversion faces many more challenges, including: (1) conversion between different types of representations; (2) coordinate system transformation; and (3) other problems caused by the use of different receiving data formats, such as CityGML and shapefile. A good example that demonstrates the difference between within-industry and cross-industry conversion is the study conducted by [PERSON] et al. [30]. In their study, the geometry conversion was realized between MicroStation DGN file and the SketchUp file using Bentley BIM extensions (within-industry conversion), whereas problems arose unexpectedly in their attempts to convert IFC files into a shapefile format using Feature Manipulation Engine (FME).  
While geometry conversion between different representation types, for example, CSG, B-Rep, and swept solid, has been well studied within the computer-aided design (CAD) and BIM area [31; 32], it still remains a challenging problem for conversion between BIM and GIS [19; 33; 34; 35], especially the conversion of clipping. In a study which aimed to implement building information models in geospatial context, [PERSON] et al. developed methods for converting IFC data into shapefile and geodatabase formats [35]. However, this study had several limits, such as incorrect spatial location of transformed models, as well as a lengthy processing time and incompleteness in geometry conversion. In terms of geometry conversion, their methods did not support walls defined by four or more clippings and walls defined by three clippings could contain errors after conversion. In addition, the converted top faces of walls were presented as a set of unordered points in their study which impaired the usefulness of these models. In another study, although the geometry conversion problem was not pointed out explicitly [36], it could be inferred from the figures they presented, in the paper, that the walls represented using clipping were not successfully converted. In some studies, the conversion of clipping was even neglected. [PERSON] et al. developed methods to convert IFC geometry, however, they failed to consider the clipping method [34]. [PERSON] et al. developed the open-source Approach (OSA) for converting IFC dataset into a shapefile dataset [24], however, the conversion process for clipping was not fully automatic and human intervention was needed.  
Some commercial products, such as Data Interoperability Extension for ArcGIS (DIA) [37] and the aforementioned FME [38], also provide the possibility to convert IFC datasets to GIS datasets. However, these products are not sufficiently reliable for geometry conversion and can crash during operation [15]. [PERSON] et al. investigated this issue and found it was caused by the inability of FME in converting CSG and clipping to B-Rep [30]. Since DIA uses FME as the geometry conversion engine, it has the same problem with FME. All in all, currently, methods for converting clipping are not reliable or efficient.