*3.2. 3D Underground Utility Data Model for Land Administration*

Current work focuses on the conceptual design of a 3D underground utility data model and connects it to land administration. In order to understand the demands of underground utility data users, a workshop was organized to learn the work process and needs of land administration in Singapore. This studying includes four application domains: land acquisition and purchase, planning and coordination, land transfer and sale, and land leasing. Currently, the existing data sources are the hardcopy of the utility network, 2D CAD and 2D geospatial information. There is an urgent demand of 3D geospatial information of underground utility and space to evaluate underground environment and support reallocation, land sales and the other applications. Therefore, the 3D underground utility data model includes three packages to organize the basic information and structure of utility networks, utility survey information, and the land administration information (Figure 8). In order to connect the 3D underground utility data model to the information of land administration, these three packages inherit from the Singapore cadastral data model and LADM (ISO 19152). Meanwhile, the geometric and spatial definition are inherited from the spatial schema data model ([41]).

**Figure 8.** The overview of packages of the 3D underground utility data model.

The *Utility Networks* package describes the basic information of utility networks, which includes geometric, spatial and physical information. Based on the partonomy (part–whole) relationships, this work defines the hierarchy of utility networks in three levels (Figure 9). The macro-level is the whole utility networks, which is described by the *UtilityNetwork* class with the basic information of utility networks, such as the type, and material of utility networks. The meso-level is the surface of the utility networks, which is the part of the utility networks. The surface could be the tunnel, duck, manhole and the other types of space in the utility networks. Hence, the aims of

*UtilityNetworkSurface* class are to describe the types and 3D geometric information (e.g., diameter) of surface. The micro-level is the basic elements of utility networks, which includes nodes and segments of utilities. The node is a connection point in the network, which is defined by the *UtilityNetworkNode* class. The segment is the line segment of the utility, which is defined by the *UtilityNetworkSegment* class. The relationship between micro and meso level helps to transform 2D to 3D data as well. Figure 10 shows the relationships of different classes in the *Utility Network* package and basic attributes of each class. The values of utility networks type inherit from LA\_LegalSpaceUtilityNetwork in the LADM (ISO 19152) [19].

The *LA\_UtilityNetworks* class aims to describe the land administration information of utilities. On one side, it connects to the utility network surface in order to identify the land administration information of different parts of utility networks. On the other side, it connects to the cadastral parcel from the Singapore cadastral data model and LADM [19]. The spatial relationship is used to describe the relationship of cadastral parcels and utilities, which includes contain, cross and touch. This class could support ownership management of utilities and land administration management.

The *Utility Survey* class aims to organize utility survey information. It could help to manage survey status and accuracy of data. The *Utility Survey* class inherits attributes of the survey from the Singapore cadastral data model. Furthermore, the ground conditions and survey methods are related to the accuracy of data directly. Hence, the *Utility Survey* class integrates information from Standard and Specification for Utility Survey in Singapore [42]. Meanwhile, the *Utility Survey* class builds the connection between utility networks and *LA\_Point*, *LA\_BoundaryFace* and *LA\_SpatialSource* in the *Surveying and Representation* package. The *Evaluate* attribute describes the method to check the accuracy of surveying data. If the accuracy of the data is unknown, the value of *Evaluate* is null. In future work, the accuracy level should be defined to be based on the depth level, soil condition and survey method.

**Figure 9.** Multilevel structure of utility networks.

**Figure 10.** The classes diagram of utility networks.
