Property boundaries on LISTmap
Digital cadastral boundaries : the spatial cadastre
Property, or cadastral, boundaries are a key component of digital spatial information and the importance of the digital representation of cadastral parcels in our society is reflected in the Australian and New Zealand Cadastre 2034: Powering Land and Real Property strategy.
A recent Australian and New Zealand research project examined the primary functions and purposes of the digital representation of parcels, which was termed the "spatial cadastre", identifying users ranging from spatial professionals like surveyors through utility companies and government agencies to the general public. In response the authors proposed that for the spatial cadastre the optimal "positional accuracy" that would support the majority of users should be :
0.1 - 0.2 metres in urban areas
0.3 - 0.5 metres in rural areas, and
up to 1 metre in remote areas.
Positional accuracy, referred to as Positional Uncertainty in the report and sometimes referred to as the spatial cadastre accuracy level, is a concept whereby the accuracy of the individual coordinates is described with respect to the national geodetic datum. That is, it is an absolute measure of position accuracy able to be applied to all datasets to support the appropriate use of data in an era of global positioning systems (GPS).
The report also highlighted the need for a clear method to indicate the estimated accuracy of the boundaries being displayed to support better risk-based decision making, in part recognising that boundary identification on the ground is primarily a legal process and only a qualified land surveyor is authorised to confirm the physical location of a title boundary.
Spatial cadastre on LISTMap and its accuracy
LISTmap displays many layers of adminstrative and property boundary information derived from the LIST Cadastral Parcels dataset. However, it is extremely important to understand that these layers are effectively only a digital version of a cadastral index map and the lineage and estimated positional accuracy of the boundaries is essential knowledge to apply when using the information to support land use / administration decisions.
The cadastral parcel boundaries shown on LISTmap were initially derived from hard-copy Tasmanian maps ranging from 1:5000 scale (1 mm on the map = 5m on the ground) to 1:25000 scale (1mm on the map = 25 metres on the ground) or sourced from other parties like local government. Commensurate with the uses of this data in the pre-GPS era, which was primarily to provide spatial relationship context, the positional accuracy of these lines wasn't a primary focus and a large range of positional accuracy was exhibited. In urban area's errors of 10 metres weren't uncommon whilst in rural areas errors could be 100 metres or even more.
Portion of LISTmap screenshot showing a rural example of the Cadastral Parcels layer (thin black line) originally derived from the 1:25000 map series with a plot of the boundaries measured by a modern field survey (green lines) indicating that large, variable and inconsistent position errors in the LISTMap spatial cadastre are possible.
In 2008, it was estimated that only 2% of the entire Tasmanian spatial cadastre was capable of being classified as having 0.1 metre positional accuracy and around 10% as being of 1 metre positional accuracy or better. Significant work has been undertaken to improve these figures. All new subdivisions are directly connected to the Tasmanian geodetic survey control network and have been plotted from the surveyed measurements when they are accepted by the Recorder of Titles. Land Tasmania has also collobarated with various local government authorities to undertake a spatial cadastre accuracy upgrade program involving backcapture of survey measurement information from older survey plans in selected areas, augmented by new field surveys connecting to the geodetic network where necessary.
Yellow lines showing spatial cadastre location in portion of Midway Point before upgrade in 2013
At 30 September 2022 approximately 51% of the spatial cadastre parcels were estimated as being plotted within 1 metre of their correct position with around 29% being represented within 0.5 metres and 10% within 0.2 metres.
Green lines showing spatial cadastre location after accuracy upgrade
Boundary Lines with Accuracy layer
Whilst the LIST Cadastral Parcels layer displays parcel property boundaries and delivers the land tenure information it should always be used in conjunction with the Boundary Lines with Accuracy layer where the position of the parcel boundary in relation to other mapped features is of interest. The portion of a LISTmap screenshot below shows the two layers and their legend entries - the black lines of the Cadastral Parcel layer are just visible inside the coloured lines of the Boundary Lines with Accuracy layer, with the colour coded categories that show the source of the linework providing a general indication of the positional accuracy - dark green is from modern survey measurements, light green is from the backcapture program, blue indicates information provided by other sources (e.g. local government), pink being "mapped" data not derived from mathematical survey measurements and brown being mapped topographic features e.g. mean high water mark.
Portion of a LISTmap screenshot showing the Cadastral Parcels layer in conjunction with the Boundary Lines with Accuracy layer.
The estimated horizontal positional accuracy of each individual boundary segment is one of the attributes of the Boundary Lines with Accuracy layer - users simply need to select or click on an individual line and the Identify Results window, pictured below, displays selected attributes including the estimated Horizontal Accuracy.
Query result from Boundary Lines with Accuracy layer
Attention is drawn to the layer specific warning advised in the feature note that users "..must not rely on this data for the on-ground location of boundaries" as not only is the estimated boundary position not 100% reliable the positions of any of the topographic features represented on LISTmap, including the underlying aerial imagery and other basemaps derived from it - are subject to unknown and inconsistent position errors that may become noticeable when a user "zooms in" to look at information in high detail.
Remember: boundary definition is primarily a legal process and only a land surveyor listed in the Register of Surveyors is authorised to confirm the physical location of a title boundary in Tasmania.