In this section an approach to achieving a 3D digital cadastre is identified with rationale provided as to why it was selected. This leads into discussion around the application of the approach at the (reasonably high) technical level.
3.1. Identifying an Approach
Solutions need to consider how the extents of 3D property rights, restrictions and responsibilities are legally defined in a digital environment and also how the related digital data is incorporated into and managed within the system. As noted earlier New Zealand’s legislative framework already supports the definition of property rights, restrictions and responsibilities in 3D and does not inhibit the development of the cadastral survey system to cater for 3D digital data. It is at the technical level where modification is required.
Gulliver (2015) reviewed the literature associated with the development of 3D cadastres, particularly stemming from the research of Stoter and van Oosterom [16
], where three fundamental interpretations of 3D cadastre are presented: fully 3D cadastre, hybrid solution and 3D tags. The option being pursued for New Zealand is based on a variation on the concept of ‘hybrid cadastre’. Under this approach, 3D property rights, restrictions and responsibilities can be integrated into the digital cadastre and subsequently maintained. In situations where the upper and lower height limits of property rights, restrictions and responsibilities are defined, a full 3D spatial depiction would be used. Otherwise ‘2D’ parcels would be maintained as a default.
The development of a 3D digital cadastre using a variation of Stoter’s hybrid approach is deemed to be the most appropriate solution to enhance New Zealand’s cadastral survey system. This approach builds on the existing robust 2D digital cadastre by allowing 3D data to be digitally captured, validated, maintained and made available for reuse as and where necessary. Importantly the approach also allows New Zealand’s monument-based legal cadastre (discussed in Section 2.3
and Section 2.6
above) to be preserved as the foundation of the digital cadastre.
3.2. Spatial Objects to Represent 3D Parcels
The concept of a ‘spatial object’ (Figure 7
) is being pursued to allow parcels defined in 3D to be submitted and integrated into the digital cadastre and subsequently maintained. Spatial object modelling is a seasoned tool used in GIS (Geographic Information System) applications [17
] and its relevance to advancing cadastral systems was foreseen in Cadastre 2014: A vision for a future cadastre system [18
]. In the context of this article, a spatial object describes (within specified accuracy standards) the size, shape and extent of property rights, restrictions and responsibilities as a ‘watertight’ 3D volume. In a GIS context, the spatial object is a coordinated 3D volume—defined in terms of x
. Modern spatial technologies, including GIS and BIM, have functionality for creating, manipulating, viewing, and managing such spatial objects.
3.3. Establishing the Fundamental Requirements
The requirements considered to be fundamental for handling 3D cadastral survey datasets in a 3D digital cadastre are considered in terms of the Cycle of Digital Cadastral Survey Data, as presented in Figure 8
In the Cycle of Digital Cadastral Survey Data a cadastral survey typically commences with a surveyor searching the cadastral survey database for records that relate to the area of interest. Relevant information is obtained and spatial data is extracted and uploaded into the surveyor’s survey software where it is combined with new data from their field survey. A dataset is prepared in the surveyor’s software and checked for initial compliance against Rules and system requirements via the validation service. The surveyor then sends their dataset to the LINZ staging environment. Here the surveyor finalises the cadastral survey dataset and checks it for accuracy and completeness, again through the validation service. The surveyor then certifies the cadastral survey dataset and submits it to LINZ for approval and integration into the cadastral survey database.
From the Cycle of Digital Cadastral Survey Data, the fundamental requirements of a 3D digital cadastre are identified (refer to Table 1
) and then discussed in further detail in the following subsections.
3.3.1. Search, Visualise & Retrieve
A search function would need to have the ability to visualise, interrogate and extract digital 3D survey and boundary information. Specifically, users of the system should be able to:
Existing 3D parcels can be changed (e.g., subdivided, boundaries shifted) just as ordinary ‘2D’ parcels can be altered. In this case, surveyors would obtain existing 3D survey and parcel data from the system and then upload it into their third-party software. The surveyor would then combine new survey work with the existing data as they create a new cadastral survey dataset.
Exports of 3D parcels for use in other software would be dependent on the user’s requirement of the data. Two different options for extracting a 3D parcel would be available:
as it was lodged, certified and approved in the cadastral survey dataset, as that is the authoritative record of the legal position of the boundary; and;
as transformed to fit the digital cadastre, recognising that positions change over time due to improved data and geodetic shifts.
In the former case the future surveyor could then transform that data to fit marks found on the new survey to accurately determine the location on the ground at that time.
3D parcels as spatial objects would be able to be integrated into GIS or other spatial information systems and overlaid with other datasets, whether in 2D or 3D. Being coordinated in terms of the official geodetic datum would enable the boundaries of the primary and secondary parcels (position of the spatial object) to be readily determined on the ground, especially through the use of positioning technologies.
It will be important for users to have the ability to become informed of the location and spatial extents of all property rights, restrictions and responsibilities through visual interrogation and analysis of the 3D digital cadastre. 3D parcels need to be suitably displayed in spatial views in terms of the underlying primary parcel fabric. In addition to 3D perspective views, 2D plan views could permit a quick assessment of the ‘footprint’ of all property rights, restrictions and responsibilities in relation to underlying primary parcels (refer to Figure 9
and Figure 10
). The user would be alerted to situations where a ‘footprint’ represents a right defined in 3D. Views in 3D could then be explored if further understanding was required by the user.
3.3.2. Creation of 3D Parcels
A 3D parcel would be based on data collected and verified as correct by the surveyor responsible for certifying and lodging the cadastral survey dataset. These data could be obtained through a variety of sources, including digital architectural and engineering designs, BIM data, and via direct survey measurements made in the field. It is expected that the latter would also be used to ground-truth each of the preceding scenarios.
As discussed in Section 3.2
above, every 3D parcel represented as a spatial object is by definition coordinated in terms of x, y, and z coordinates. In order to have a defined relationship between a 3D parcel and its underlying primary parcel, there would be a need to link the 3D parcel to the underlying parcel and nearby permanent reference marks (refer to Figure 11
From a survey definition perspective, the 3D parcel (a parcel with defined height limits) must be defined in relation to its underlying primary parcel. This ensures that the boundaries of the 3D parcel could be identified by firstly relocating boundary points on the underlying primary parcel which will have been fully defined by monuments.
Similarly, in the cadastral survey dataset, the relationship between the 3D parcel and its underlying primary parcel needs to be explicitly defined. This can be achieved by including in the dataset, horizontal coordinates that correspond to boundary points previously defined on its underlying primary parcel. Wherever the boundaries of the 3D parcel and the underlying parcel are coincident, the horizontal coordinates of both ends of the underlying boundary would also need to be included in the dataset. The accuracy between those coordinated points on the underlying parcel and the vertices of the spatial object would have to meet the relevant standard in the Rules. These requirements would also ensure that the spatial object for the 3D parcel could be maintained in alignment with the primary parcel network.
The vertical position of the 3D parcel needs to be capable of being re-established in the future, and also be reflected spatially in the digital cadastre. Therefore, the ‘z’ vertices inherent in the 3D parcel would need to be related to ‘z’ coordinates on existing boundary corners of the underlying primary parcel and/or permanent reference marks, all in terms of the official vertical datum.
3D Parcel Represented by Permanent Structure Boundaries
The Rules for Cadastral Survey 2010 allow certain types of secondary parcels to be defined by permanent structure boundaries. These boundaries are described in relation to a physical feature (e.g., the outer face of a wall, or an offset to the feature). The licensed cadastral surveyor who certifies the cadastral survey dataset is responsible for defining the position and accuracy of the permanent structure boundary in relation to the permanent structure.
Two options have been identified for recording this relationship:
Three-dimensional parcel representation of the permanent structure boundary only (as presented in Figure 7
). The parcel and its boundaries would be defined by a 3D spatial object, along with a description of the physical structure to which it is related and the relationship (e.g., ‘boundary through centre of wall’ or ‘boundary follows centre of concrete floor’). The description of the relationship between the permanent structure boundaries and the permanent structure is of great importance as it defines the legal position of the boundary.
Spatial object representation of the permanent structure boundary and the permanent structure (e.g., the physical structure of the apartment complex associated with the 3D parcels depicted in Figure 7
). The 3D parcel and its boundaries would be defined by a spatial object, as would the permanent structure itself (i.e., two layers of data would be provided). A description of the relationship between the two would not necessarily be required as this would be able to be determined from the spatial objects using a measurement tool in spatial software. This approach, in which legal spaces are associated with the physical elements to which they relate, was raised by Aien et al. [19
] and is being further promoted in recent research, by Atazadeh, Rajabifard and Kalantari [20
Common property, being land or a building that is for the use of all the property owners, would not necessarily be defined by a 3D parcel. It could be that part of the space remaining after the 3D parcels have been excluded, and would be viewable using suitable software that extruded the 2D underlying parcel boundaries as appropriate.
The system should be capable of receiving 3D cadastral survey datasets (that may include both 2D and 3D information) as digitally certified by the surveyor (i.e., without change). Cadastral survey datasets would be prepared by surveyors in third-party software. The contents of a 3D cadastral survey dataset would include information that describes the size, shape and spatial position of each of its 3D parcels in relation to:
the ‘2D’ underlying primary parcel, or the 3D parcel where it is being subdivided or redefined;
any permanent structure to which it is referenced (e.g., wall, floor and ceiling of an apartment complex).
A 3D cadastral survey dataset may also include ordinary (‘2D’) parcels and related survey information. For example, in a unit title development the surveyor may choose (or be required) to re-survey the ‘2D’ primary parcel to which new 3D parcels of an apartment block are referenced.
As is the situation for ordinary parcels, each 3D parcel will need to be identified through a unique appellation (label). This is required for the management of the cadastre and to enable tenure system managers to register (create) the associated right, restriction or responsibility.
Data being submitted into the cadastre needs to be validated. The primary purpose of validation is to ensure that the data complies with the Rules and that it is able to be integrated into the digital cadastre. Automated business rules should check that:
new 3D parcel boundaries do not overlap underlying parcel boundaries to which they relate;
new 3D parcels in a 3D cadastral survey dataset do not illegitimately overlap (some types of overlaps are permitted) other 3D parcels (new and existing);
3D cadastral survey datasets are in terms of the official national vertical and horizontal datums;
the 3D volume of any existing 3D parcel that is being subdivided (e.g., a unit redevelopment of an apartment complex) is completely taken into account.
Validation through automated business rules is a key part of the system, and this is emphasised in research by Thompson and van Oosterom [21
] and Karki et al. [22
]. It is likely that surveyors will be required to obtain and carry through links to existing data (e.g., underlying parcels, points) held in the system to enable automated validation. For maximum effect, automated validation should be available to surveyors as a cadastral survey dataset is being prepared in third-party software prior to certification and lodgement. LINZ staff are also required to perform validation prior to the approval of the certified cadastral survey dataset.
Three-dimensional clash detection routines found in current surveying, engineering, GIS and BIM software indicate that the validation of 3D parcels ought to be readily achieved. Despite any validation procedures, the responsibility for the correctness of certified data will continue to be the obligation of the surveyor responsible for the cadastral survey dataset.
The vision is for a digital cadastre where 3D property rights, restrictions and responsibilities are represented digitally in 2D or 3D as appropriate in a single integrated and seamless system. To achieve this, the data contained in 3D cadastral survey datasets, as provided by the surveyor, will need to be integrated into the digital cadastre (i.e., combined with existing data). The integration process will store the components of the cadastral survey dataset in the database in a similar manner to other cadastral survey datasets and will be completed by LINZ. To enable automated integration processes, there is likely to be a need for surveyors to obtain and carry through links to existing data held in the system (as for validation, discussed above). Once a cadastral survey dataset is approved, it would be adjusted into the digital parcel network and the representation of 3D parcels would be repositioned relative to their underlying parcel and the permanent reference marks in terms of the official coordinate datum and projection.
Currently primary and secondary parcels defined by nodes and lines are fully integrated into the boundary network. This means they are managed topologically and coordinates are generated through least-squares adjustment of the vector (bearing and distance) data. However, under this proposal 3D parcels would be managed through a different process. Topology would not be directly managed, and alignment would be maintained by applying a transformation to the 3D parcels, using the connection points to the cadastral network as the ‘control’ (i.e., the boundary points on the underlying parcel and the permanent reference marks, as recorded in the cadastral survey dataset).
3.3.6. Maintaining Spatial Alignment
LINZ needs to be able to maintain and update the spatial location of 3D parcels in the network over time. Any movement of geodetic marks (such as that caused by tectonic movements) that affect cadastral marks may have an effect on the underlying parcels which in turn has an effect on any associated 3D parcel. The processes for adjusting the 2D parcel network will need to be extended to also adjust any 3D parcel to keep them in the correct relationship. The requirement for connections that meet the accuracy standards in the Rules is intended to ensure that the transformation would result in negligible distortion of the 3D parcel during future alignment processes. Similarly, the height values of 3D parcels will need to be adjustable as the height values of geodetic control marks change over time.