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.

This paper explores the laws and other legal acts related to the Croatian 3D cadastre with an emphasis on those which relate to interests in strata, spatial planning, and other regulations that are valid or were valid on Croatian territory. The effects of the application of these regulations on the present situation of registration in cadastre and land register were considered. This paper also explores current legal, institutional, and technical solutions implemented in the Croatian Land Administration System and the possibilities for its upgrade to 3D cadastre. Implementation of any technological option to establish a 3D cadastre is tightly related to legislation. Hence, legislation and technological options are considered to find solutions that will be possible to implement. One suggestion presented in this paper was to use other sources of 3D data such as topographic signs or symbols used to represent topographic objects on 2D maps. In combination with other geodetic and cartographic products, useful information can be obtained, often quite relevant to provide a reference context for a 3D cadastre. Topographic signs on topographic maps and on other geodetic products provide a representation of complex real-world situations (tunnels, bridges, overpasses etc.) that are not usually presented on cadastral maps. This paper presents the possibility of utilizing those topographic signs to achieve the first steps towards establishing a 3D cadastre. Furthermore, this study proposes the establishment of a 3D Multipurpose Land Administration System as the most efficient system of land administration in a time when spatial information is easier to obtain than ever before and traditional real estate registers are subject to frequent and demanding changes.