Improving positional accuracy of the digital cadastral database

Accuracy of the digital cadastral database

The Queensland digital cadastral database (DCDB) 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 DCDB 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.

Because DCDB data is used in all of the state's land information systems, we have since developed a continuing program to upgrade its positional accuracy. This has seen gradual improvement, to the extent that the worst positional accuracy in Queensland is currently ±63 metres. These areas have been identified and prioritised for upgrade.

This map of Queensland depicts the current positional accuracy of the DCDB.

Partnerships with local governments

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

We prefer to deal with whole of local government area upgrades. If a local government is embarking on an upgrade of their area, we will enter into a formal data enhancement agreement. The agreement sets out expectations of both parties, defines time frames and deliverables, and clarifies intellectual property issues.

Benefits to local governments

The DCDB 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. This gives the opportunity to accurately link utility data such as water, sewerage, communications, electricity, etc. to the DCDB and the improved ability to locate such utilities on the ground.

Once a data enhancement agreement has been entered into, we can provide contributions to the project, including survey plans, plan history linkages, permanent mark sketch plans and records from the survey control database (SCDB). This data is provided at no cost to the local government.

Upon receiving the data from the local government, we then upgrade the DCDB at no charge, on a high priority schedule.

DCDB upgrade processes using local government data

Methods vary between local governments. Some have in-house capability, while others use external surveying consultants.

Their 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 cadastrally connected ground control. 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 DCDB to the ground control based on their positional uncertainty, age of data and distance from control.