We are upgrading the spatial accuracy of all parcel boundaries across Victoria by using back captured cadastral plan and survey information.
A rigorous calculation procedure (based on least squares adjustment) is then being applied.
This will establish the most reliable parcel corner and reference mark positions. At the same time, we will be able to assess the quality (or uncertainty) of those positions.
The procedure will verify the quality of the back captured measurements and the computed results to ensure the highest possible integrity in the digital cadastre.
With more than 160 years of cadastral plans and survey data to go through, the procedure will also account for any inconsistencies.
These could include:
- variations in measurement precision with advances in cadastral surveying equipment;
- variations in survey standards and practices;
- rounding in angles and distances (applied to satisfy plan presentation standards);
- human error made in cadastral survey or documentation;
- Distribution and density of survey control network marks;
- inconsistencies between the physical evidence witnessed by previous surveyors and what evidence is present today, which could include occupation (i.e. fencing, buildings), reference marks, and natural boundaries like rivers;
- misalignments (i.e. “gaps” and “slithers”) in the cadastre arising from cadastral survey differences and amendments to title dimensions
When all issues are taken into account, the quality of the upgraded digital cadastre will vary across the state. Some areas will have a high level of spatial accuracy; other areas will not.
Below is an example of early thinking on how the variable spatial accuracy of the digital cadastre might be displayed.
The size of the red circle at each parcel corner and the darkness of each parcel indicate the level of quality achieved from the available input data.
Above Image: Example of how node and parcel uncertainty may end up being presented within the new digital cadastre.
What do we mean by adjustment and why is an adjustment necessary?
Least squares adjustment is a mathematical technique for estimating the most probable values of unknown parameters from a system of weighted measurements. In the context of the digital cadastre, the unknown parameters are the parcel corner positions, and the measurements are the title dimensions and cadastral surveying measurements back-captured from the registered plans and cadastral surveys. The measurement weights are the uncertainties (or quality indicators) assigned to those measurements and reflect the amount of contribution each measurement has on the estimated results.
Today, our land information data varies in quality due to variations in surveying equipment, cadastral standards, surveying practices and transcription errors spanning the 160-years of land management work. The purpose of least squares adjustment is to provide a robust, repeatable and statistically reliable way for estimating the most probable location of parcel boundaries. It also provides a reliable way of estimating how good the location of each calculated boundary is.
Experience in a number of places around the world has shown that the use least squares adjustment to calculate the position of parcel boundaries from the original plans and surveys provides the highest level of confidence and reliability.
This stage involves capturing and checking information from plans and surveys, and storing this information in a consistent digital format.
This stage will calculate position and uncertainty of each parcel boundary from the back-captured data and Victoria’s survey control network.
This stage will cascade the changes in parcel boundary positions through other VicmapTM layers and publish the upgraded digital cadastre.
Page last updated: 27/02/20