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4. Data quality

Lineage
Positional accuracy
Attribute accuracy
Logical consistency
Completeness

Spatial data quality elements provide information on the fitness-for-use of a spatial database by describing why, when and how the data are created, and how accurate the data are. The elements include an overview describing the purpose and usage, as well as specific quality elements reporting on the lineage, positional accuracy, attribute accuracy, logical consistency and completeness. This information is provided to users for all spatial data products disseminated for the census.

Lineage

Lineage describes the history of the spatial data, including descriptions of the source material from which the data were derived, and the methods of derivation. It also contains the dates of the source material, and all transformations involved in producing the final digital files.

The geographic area boundaries, names, codes, and the relationships among the various geographic levels are found on Statistics Canada's Spatial Data Infrastructure. The data for administrative areas are updated using information from provincial and territorial sources. The data for statistical areas are updated using the results of the previous census and input from users.

Creation of the 2006 Digital Boundary Files

The Spatial Data Infrastructure is the source for all 2006 Digital Boundary File products. Primary data manipulation of the product layers included preserving the geographic hierarchy of attributes inherent within a geography. This data manipulation included copying source data to a production environment and the joining of hierarchical geographic attributes. The final data treatment was an output of various file formats supported by Geographic Information System (GIS) software.

Creation of the 2006 Cartographic Boundary Files

The 2006 Cartographic Boundary Files were created using the 2006 Digital Boundary Files and a set of hydrographic features from the National Geographic Database. The hydrographic features included coastal features (e.g., oceans, bays), the Great Lakes and the St. Lawrence River. These data were used to remove from the digital boundary files that portion of the geographical area within these major coastal water features.

Additional formatting

The files were transformed from Lambert conformal conic projection into latitude/longitude coordinates. Finally, the files were verified, translated into French and English versions and appropriately labelled.

The files were converted into three output formats: ArcInfo^® (.shp), Geography Markup Language (.gml) and MapInfo^® (.tab).

Coastal features

The coastal features were created by selecting water features exterior to Canada's land mass from the National Geographic Database's hydrographic reference layers. These reference data were sourced from the National Topographic Data Base (1:50,000 and 1:250,000) and the Digital Chart of the World (1:1,000,000). This included polygon features forming the Pacific, Atlantic and Arctic oceans, as well as the Beaufort and Labrador seas and all related channels, straits, passages, inlets and bays including Hudson Bay and James Bay. In addition, features forming the Great Lakes, Lake of the Woods and the St. Lawrence River were included.

The coastal features were then generalized by removing all islands smaller than 100,000 square metres except when the islands accounted for the only land area for geographic areas or when they were intersected by road arcs found on the road network file.

Inland water

The inland water file was created by selecting water features from the National Geographic Database's hydrographic reference layers. These reference data were sourced from the National Topographic Data Base (1:50,000 and 1:250,000) and the Digital Chart of the World (1:1,000,000). Each feature was assigned a rank based on its size and/or cultural importance. The largest and most important features have lower rank values. These ranks can be used to select and format features for map display at different scales.

Positional accuracy

Positional accuracy refers to the absolute and relative accuracy of the positions of geographic features. Absolute accuracy is the closeness of the coordinate values in a dataset to values accepted as or being true. Relative accuracy is the closeness of the relative positions of features to their respective relative positions accepted as or being true. Descriptions of positional accuracy include the quality of the final file or product after all transformations.

The boundaries are derived from the Spatial Data Infrastructure. The data in the Spatial Data Infrastructure are stored in double precision. This precision allows features that are next to each other on the ground to be placed in the correct position on the map, relative to each other, without overlap. However, the absolute positional accuracy of the features in the database varies depending on the source of the features.

The Spatial Data Infrastructure is not Global Positioning Systems (GPS)-compliant. However, every possible attempt is made to ensure that the geographic area boundaries maintained in the Spatial Data Infrastructure respect the limits of the administrative entities that they represent (e.g., census division and census subdivision) or on which they are based (e.g., census metropolitan area or census agglomeration). The positional accuracy of these limits is dependent upon source materials used by Statistics Canada to identify the location of limits. In addition, due to the importance placed on relative positional accuracy, the positional accuracy of other geographic data (e.g., road network data and hydrographic data) that are stored within the Spatial Data Infrastructure is considered when positioning the limits of the geographic areas.

Attribute accuracy

Attribute accuracy refers to the accuracy of the quantitative and qualitative information attached to each feature (such as population for an urban area, street name, census subdivision name and code).

As noted under Lineage, the attributes (names, types and codes) for all geographic areas displayed on the maps are sourced from the Spatial Data Infrastructure. The names and types for administrative geographic areas have been updated from the 2001 Census using source materials from provincial and territorial authorities.

The attribute data associated with the polygons in the Boundary Files for the 2006 Census were independently verified against the data in the Spatial Data Infrastructure and found to be accurate.

Logical consistency

Logical consistency describes the fidelity of relationships encoded in the data structure of the digital spatial data.

In each boundary file, all geographic areas have been verified to have a unique identifier that is valid for the 2006 Census.

Boundaries found in this product are consistent with those found in other spatial products produced as part of the suite of 2006 Census products.

The hydrographic data files were specially created for the boundary files to enable thematic mapping at local and regional scales.

The land area for geographic areas present in GeoSuite may not be consistent with that computed from the cartographic boundary files. This is because the water features used in the creation of the cartographic boundary files are based on a set of hydrographic features that was created for thematic mapping.

Completeness

Completeness refers to the degree to which geographic features, their attributes and their relationships are included or omitted in a dataset. It also includes information on selection criteria, definitions used, and other relevant mapping rules.

Each boundary file contains the complete set of geographic areas for that level of the geographic hierarchy.

It is important to note that in both digital boundary files and cartographic boundary files, a geographic area may be depicted by more than one polygon. In the digital boundary files there are some geographic areas that have two or more parts. This is particularly the case for some census subdivisions. In cartographic boundary files, this is due to having removed the coastal water area from the digital boundary files, thus creating several polygons for one geographic area. In the cartographic boundary files this impacts only on geographic areas that are situated on the coastal areas.