Improving positional accuracy of the DCDB

Accuracy of the DCDB

The Queensland Digital Cadastral Database (DCDB) was developed by manually digitising the best available cadastral maps. In 1992, the digital capture was completed at the agreed standard. The outcome was a seamless database of the cadastral network compiled to a range of accuracy ratings.

Its positional accuracy was derived as a factor of the map scale and the precision of the digitising. As a consequence, the positional accuracy of the DCDB throughout Queensland varies from 0.1 metre to 250 metres.

Data in the DCDB consists of two parts, namely spatial and attribute components. The spatial component is used to depict the position and boundaries of parcels and the attribute information contains details about each parcel.

Through upgrade projects, the spatial component of the parcel is adjusted to a greater accuracy while the unchanged attribute data moves with the adjusted parcel. The integrity of the database is maintained.

Today, the need for improving the spatial component's accuracy is increasing. The DCDB has evolved to become the essential graphical data source which underpins all land information systems in the state.

This map of Queensland (PDF, 525K)* depicts the current positional accuracy of the DCDB (May 2008), highlighting upgrades via departmental initiatives explained below. This map will be updated regularly.

Partnerships with local governments

To achieve a desirable level of upgrade, the department undertakes projects as a co-operative venture with local governments.

The projects are driven by the needs of local government for improved positional accuracy. However, it is a fact that any form of positional upgrade requires the investment of substantial dollars. The extent of the upgrade projects are limited by the amount of funding available.

The department prefers to deal with whole of local government area upgrades. If a local government is embarking on an upgrade of their area, the department will enter into a formal agreement with Council. The agreement sets out expectations of both parties and defines timeframes and deliverables by both parties. It also serves to clarify intellectual property issues.

A typical data enhancement agreement (draft copy) (PDF, 158K)* between a council and the department.

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 is 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, the department can provide contributions to the project in the form of raster images of survey plans, plan history linkages and permanent mark sketch plans in conjunction with records from the Survey Control Database (SCDB). This data is provided at no cost to the local government.

Upon receiving the updated data from the local government, the Cadastral Information Services area of the department will upgrade the DCDB at no charge, on a high priority schedule.

Methods and processes of Upgrade

Methods of facilitating the project vary between local governments. Some local governments may have qualified people to enable them to do an 'in-house' upgrade project, while others may need to go to a competitive public tender to secure an authorised surveying company. Local governments need to effect negotiations if a consultant is to be used, however the department can offer some advice.

The upgrade process may include full re-entry of bearing and distance from survey plans or employ some form of numerical or graphical adjustment. Whatever the method used, it is essential that an adequate network of ground control is in place to control horizontal XY position and eliminate distortion of linework.

A typical upgrade process, implemented by the local authority, may include:

Capture bulk cadastrally-connected ground control with GPS; generally, 1 point to 40 parcels in urban areas, 1 point to 80 parcels in rural areas.
entry of bearing and distance and closure (perhaps Bowditch Adjustment) from Survey Plans.
some form of least squares adjustment to fit control.

This department doesn't have the monetary or human resources to assist with this process and plays no part in the third party negotiations. We are therefore unable to advise on associated costs.

The department will then move the existing DCDB to match the new data and subsequently enter the positional change into the DCDB.

The Queensland Interchange Format (QIF) standard makes provision for an accuracy indicator of upgraded parcels to reside in the DCDB and this metadata can be carried through to the local government's GIS system.

Local governments that have entered into an Upgrade agreement

This diagram of Queensland (PDF, 335K)* , shows the current extent of Local Government involvement in upgrading their jurisdictions.

Other upgrade methods

The DCDB Graphical Relativity Project is an initiative of the department which will progressively upgrade the positional accuracy of the DCDB in the rural and remote areas of the state. Currently these areas have an accuracy indicator ranging from +/- 251 metres to +/- 63 metres.

The upgrade is achieved by comparison of existing DCDB with current orthorectified satellite imagery or, where available, 1:40 000 orthophoto images.

Projected accuracy using satellite imagery is +/- 25 metres, while +/- 5 metres is sought using orthophoto imagery.

This diagram of Queensland (PDF, 401K)* depicts the current status of the Graphical Relativity Project.

The desired outcome for the department is to have a DCDB with improved positional accuracies which better meets the contemporary needs for vegetation monitoring and other natural resource management needs.

As outlined, the needs of local governments may differ from these requirements. Consequently, improved positional accuracy can be achieved through a co-operative upgrade process.

Glossary of terms

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.
Least squares adjustment: The method of least squares assumes that the best-fit traverse path is the path that has the minimal sum of the residuals squared, calculated from a given set of equally reliable measured values.

Further information

For further enquiries into progressing a cadastral upgrade agreement in your local authority, email Cadastral Information Services.

* Requires Acrobat Reader

Last updated: 13 May 2009