# City Information Modeling: State of the Art
## 3 CIM Research Areas
### Data Integration  
CIM data exist in large volumes and are obtained from various sources, making it challenging to integrate the data. Therefore, effective frameworks and methods for the data integration of CIM are necessary. This paper presents a summary of previous research on the above frameworks and methods.  
Integrated Framework  
Various definitions of CIM lead to different frameworks. [PERSON] et al. [32] opine that CIM is an extension of BIM, in which urban infrastructure and geographic information are combined based on BIM architecture. [PERSON] et al. [33] classified the CIM framework into three modules (that is, land, facilities, and water bodies) based on flood analysis. By comparison, the framework proposed by [PERSON] et al. has a more comprehensive classification, including architecture, transportation, water bodies, mechanical, electrical, plumbing, and infrastructure [5].  
In September 2020, the Ministry of Housing and Urban-Rural Development (MHURD) of China published the Technical Guidelines on the Basic Platform of City Information Modeling [34]. The framework of the CIM basic platform recommended in the guideline is depicted in Figure 4, showing five layers (device, data, service, application, and user). This framework can integrate multisource data and serve different application scenarios. The above review indicates that most existing CIM frameworks are oriented toward specific applications. Therefore, the framework developed by the MHURD of China is recommended.  
Data integration is considered a critical factor in CIM. City geographic information is typically stored using GIS technology, whereas most internal building information is managed using BIM and computer-aided design (CAD) technology. Thus, the data integration method can be divided into two categories: GIS-CAD and GIS-BIM.  
CAD-based models exclude detailed information about buildings; therefore, the CIM generated using CAD and GIS has limitations, such as information retrieval and format conversion [35; 36]. In contrast, BIM-based models have high-fidelity 3D geometries and include sufficient details about buildings, leading to better advantages for integration with GIS.  
Currently, the integration of BIM and GIS has become a widely used modeling method for CIM [37; 38]. IFC and CityGML are international data standards for BIM and GIS, respectively. Based on these two standards, the integration methods of BIM and GIS can be divided into four categories [5; 39], that is, direct conversion, mapping, plug-in, and semantic web technology. The advantages and disadvantages of these methods are listed in Table 2. Generally, current BIM-GIS integration methods are imperfect, and further studies are required.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Advantages** & **Disadvantages** \\ \hline Direct conversion from IFC & It is convenient and easy & Data is lost because of \\ to CityGML & for users. & extensive conversion. \\ Mapping between IFC & It has high efficiency in & It is challenging to develop a \\ and CityGML & data retrieval. & complete mapping between \\ Intermediate plug-in on BIM & It is convenient & There is no universal interface \\ platform to import GIS data & for operations. & for all data types. \\ Semantic web technology for IFC & Bilateral conversion and & The corresponding algorithms \\ and CityGML & good compatibility exist & and models are limited. \\ \hline \hline \end{tabular}
\end{table}
Table 2: BIM-GIS integration methods based on IFC and CityGML [5; 39].  
Figure 4: Framework of CIM basic platform [34].