Improving positional accuracy of the digital cadastral database

Accuracy of the spatial cadastral data

The Queensland spatial cadastre was developed by manually digitising the best available cadastral maps at the time. In 1992 the digital capture was completed to the agreed standard.

Positional accuracy of the spatial cadastral data was derived from the source map scale and the precision of the digitising. Consequently, this positional accuracy initially varied throughout Queensland from +/-1 metre to +/-250 metres.

A continuing program to upgrade the positional accuracy of the spatial cadastral data due to the importance of the data in all the state's land information systems. The program has seen gradual improvement, to the extent that the worst positional accuracy in Queensland is currently ±63 metres, with these areas identified and prioritised for upgrade.

This map of Queensland depicts the current positional accuracy.

Partnerships with local governments

In line with our program to improve the positional accuracy of the spatial cadastral data, we regularly work with local governments who are planning to upgrade their local government areas.

Our preference is to upgrade the whole of local government areas, therefore the department enters into formal data enhancement agreements with local governments embarking on upgrades. The agreement sets out expectations of both parties, defines time frames and deliverables, and clarifies intellectual property issues.

Benefits to local governments

The spatial cadastral data is widely used by local governments in the administration of their rates databases and asset management environments.

Local government requirements for urban accuracy are in the order of 0.1 to 0.2 metres which provides the opportunity to accurately link utility data such as water, sewerage, communications, electricity, etc. to the spatial cadastral data and the improved ability to locate these utilities on the ground.

The department contributes the upgrade projects through providing survey plans, plan history linkages, permanent mark sketch plans and records from the survey control database (SCDB), once a data enhancement agreement has been entered into. This data is provided at no cost to the local government.

Upon receiving the data from the local government, the department then upgrades the spatial cadastre at no charge, at a high priority.

Upgrade processes using local government data

Methods vary between local governments, either using in-house capability or external surveying consultants depending on their capabilities.

The upgrade process may include full capture of bearing and distance from survey plans and/or employ some form of numerical or graphical adjustment.

A typical upgrade process undertaken by the local government may include:

• Establishment of a network of coordinated ground control connected to the cadastre (known as cadastrally connected). Generally this is: 1 control point per 40 land parcels in urban areas; 1 point per 80 land parcels in rural areas.
• Entry of bearing and distance from survey plans. This may also require a closure adjustment (e.g. Bowditch adjustment). Bowditch adjustment – the extent to which a closed traverse fails to meet is an indication of survey error. The misclosure can be distributed throughout the traverse to compensate for the accumulation of errors. The Bowditch adjustment assumes that the misclosure is proportional to the length of the perimeter. Corrections are distributed amongst the individual survey legs using calculations based on the lengths of each leg as a ratio of the perimeter.
• Some form of least squares adjustment to fit the ground control. Least squares adjustment – a least squares adjustment may often be performed to fit the spatial cadastral data to the ground control based on their positional uncertainty, age of data and distance from control.