Due to the needs of modern society, cadastral systems should be designed to support three dimensional (3D) spatial data. One of many possible approaches for implementing a 3D cadastre, in countries such as Croatia, is to establish a Building Register as a transitional register between 2D and 3D cadastres, where data about buildings and infrastructure would be collected, gradually adapted to the data model of 3D cadastre, and finally migrated to the 3D cadastre database. Sources for establishing the Building Register can be based on the records of state surveys, the register of administrative units, the land book, data managed by local and regional self-government units, data from construction documentation according to special regulations in the field of spatial planning, data managed by the building and infrastructure managers, as well as from other sources. A significant element of the 3D cadastre would be the inclusion of representations of buildings and units of use, as well as public utility infrastructure and complex spatial real-life entities (e.g., bridges, tunnels, overpasses, underpasses, overlapping of constructed objects with natural facilities, large shopping malls with more underground and overhead floors etc.). This paper presents a conceptual model of a 3D cadastre in Croatia by establishing the Building Register with focus on unit of use of real properties, namely apartments and office spaces. The paper also summarizes the current situation regarding the Croatian Land Administration System (LAS) and proposes a conceptual model for modelling unit of use of real properties. Additionally, a proposal is made herein to assign unique identifiers to buildings and their parts in a logical manner which would be intuitive and clear to citizens of Croatia, citizens of European Union and citizens of Croatia neighboring countries. The proposed methodology of determining unique identifiers could provide the means for easier navigation in 3D space and better understanding of spatial information by lay citizens, by institutions or emergency services.

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As argued in many earlier FIG publications, the potential benefits of 3D Cadastre are huge. This is especially true in the Caribbean setting with quite dense urban areas and large and important marine areas, where rights (restrictions and responsibilities; RRRs) in 3D is a prerequisite for the blue economy. The Caribbean with a plethora of 3D use cases present, limited legacy data and systems, and a relative small size, is an excellent site for establishing an integrated 2D/3D Cadastral registration. Such a future system will support sustainable development of the Caribbean in the economic sense, while protecting the environmental values and human values (based on legislation and tools to support this). This paper will present an initial inventory of 11 different relevant 3D Cadastre use cases in the Caribbean (from Aruba, Bonaire and Trinidad&Tobago). This initial inventory gives an indication of the multi-disciplinary nature and importance of the topic, but will also be used for scoping the future development.@en