Data Quality

Positional Accuracy

Positional accuracy is an assessment of the closeness of the location of the spatial objects in relation to their true positions on the earth’s surface.

Positional accuracy includes:

  • Horizontal accuracy assessment, and

  • Vertical accuracy assessment.

The horizontal and vertical positional accuracy are the assessed accuracy after all transformations have been carried out.

Relative spatial accuracy of the Localities dataset used in the construction of postcodes reflects that of the source data.

Coordinates Referencing the GDA 2020 Datum

From the November 2019 publication, spatial features are available referencing the GDA 2020 datum. These coordinates are produced using a coordinate transformation from GDA 94 using the following parameters.

  • shift_x = 0.06155,

  • shift_y = -0.01087,

  • shift_z = -0.04019,

  • rotate_x = -0.0394924,

  • rotate_y = -0.0327221,

  • rotate_z = -0.0328979,

  • scale_adjust = -0.009994

Attribute accuracy

Attribute accuracy is an assessment of the reliability of values assigned to features in the dataset in relation to their true ‘real world’ values.

Key attributes (postcode and the unique identifier) have a high degree of accuracy in the order of 99.09% and are reflective of the operational needs of Australia Post. Other attributes derived from the processing of supplied data may have a lower degree of accuracy but less than previously released data. The postcode allocated to an area may not be the same postcode as published by Australia Post through their website or other sources, as the postcode is some cases is based on the most commonly used postcode for addresses in a given area. All attribute accuracies are dependent on the data accuracy supplied to Geoscape Australia.

For this product, feature and attribute accuracy is a measure of the degree to which the features and attribute values of spatial objects agree with the information on the source material. The allowable error in attribute accuracy was previously up to 5%.

A precise attribute accuracy assessment may not always be possible. In these cases an intuitive estimate of the expected attribute accuracy or the likely maximum error based on previous experience is acceptable.

The postcode allocated to a gazetted suburb/locality is the most commonly used postcode, in a small number of cases the postcode is different to the postcode published by Australia Post on their website. These cases are due to changes to locality boundary or locality name, where the postcode remains the same for delivery purposes.

Logical consistency

Logical consistency is a measure of the degree to which data complies with the technical specification. The allowable error in logical consistency previously ranged from 3% to 5%. The test procedures are a mixture of software scripts and onscreen visual checks.

The data structure has been tested for conformance with the data model. The following have been tested and confirmed to conform:

  • File names

  • Attribute names

  • Attribute lengths

  • Attribute types

  • Attribute domains

  • Attribute order in file

  • Object type

  • Compulsory attributes populated

Topological consistency

Topological consistency is the measure of how features spatially relate to other features within and across themes. Topological inconsistencies are identified using a combination of automated rules, and visual analysis. Where topological inconsistencies are identified they are notified back to the supplier organisation for remediation at source. Some minor topological inconsistencies are corrected during product processing using automated rules. The level of topological consistency is dependent on the data supplied to Geoscape.

During product processing there is no attempt to enforce topological consistency across State and Territory borders. Cross border topological consistency is a complex issue and Geoscape continues to engage the Federal, State, and Territory governments of Australia to improve the topological consistency of spatial datasets across these borders.

Completeness

Completeness is an assessment of the extent and range of the dataset with regard to completeness of coverage, completeness of classification and completeness of verification.

Dataset, theme, and layer coverage

National (based on coverage of suburbs/localities from the Localities product).

Attribute completeness

All attributes for each object are populated.

Temporal accuracy is applicable to most of the current release.

Quality scope

Polygon and point geometry accuracy and attribute accuracy for all included areas.