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4. Data quality

Lineage
Positional accuracy
Attribute accuracy
Logical consistency
Consistency with other products
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

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.

Road layer

The data in the road layer were derived from Statistics Canada's Spatial Data Infrastructure based on a copy of the National Geographic Database. The National Geographic Database is a spatial database that contains the road network in Canada, as well as road attributes (name, type, direction, and address ranges). The National Geographic Database was originally built from four main data sources:

  • Statistics Canada Street Network Files
  • National Topographic Database 1:50,000 and 1:250,000 maps
  • Digital Chart of the World 1:1,000,000 maps
  • Elections Canada road data

Additional road information was incorporated from a variety of other sources, including municipal maps and road data from private companies. However, the timeliness of the National Geographic Database varies from region to region depending on the source data. Table 4.1 provides details on the distribution of road arcs by source.

Table 4.1 Road arc counts and summed length values by data source
Table 4.1
Road arc counts and summed length values by data source

Described below are the steps taken to incorporate data from various sources into the 2006 Road Network and Geographic Attribute File:

The 1996 Street Network Files

In census metropolitan areas and larger census agglomerations, the 1996 Street Network Files from Statistics Canada were the primary data source. These files were created from various source maps at different scales, and maintained by the Statistics Canada Geography Division over more than twenty-five years. They contained road names, address ranges and a rich set of road arcs. The maps used to build and maintain theses files had varying scales and different vintages. Therefore, the quality of its geometry varies from place to place in terms of absolute positional accuracy.

Information within the 2006 Street Network Files was updated, enhanced, and incorporated into the National Geographic Database (NTDB) during a build phase. Features that were not roads were removed. Streets were geometrically adjusted (i.e., rubber sheeted) to match the superior positional precision of the NTDB. The format of address ranges was not changed, except the type of the values was changed from character to numeric. In most cases, road names in all upper case letters were converted to names in upper and lower case. The length of the field that contains the road names was also increased to accept full names instead of abbreviations.

National Topographic Database

The National Topographic Database (NTDB), produced by Natural Resources Canada, has a stable and precise geometry and a standardized road classification scheme. In the more densely populated parts of Canada, its scale is 1:50,000, while in the more northern and sparsely populated areas the scale is 1:250,000. Unlike the Street Network Files, the NTDB contains no civic address range or road name information. The NTDB served as the source of the road network for most of southern Canada, outside of census metropolitan areas and large census agglomerations that were covered by the SNFs and Elections Canada data.

The National Topographic Database (NTDB) geometry is the adopted standard for the National Geographic Database (NGD). All spatial data used in the creation of the NGD were vertically adjusted (rubber sheeted) and edge matched to approach the largest scale NTDB geometry.

Digital Chart of the World

The Digital Chart of the World is a 1:1,000,000 scale digital map, built primarily for aeronautical charts. It was used in the database to add road geometry to the sparsely populated portion of Canada, mainly in the north. It does not contain road names or address ranges.

Elections Canada Geographic Database

In 1993, Elections Canada started to compile their Geographic Database, using data from the Street Network Files (SNFs), National Topographic Database and Digital Chart of the World. Paper maps were created for areas not covered by the SNFs and distributed to the Elections Canada returning officers who added the road names with information from the field. Elections Canada updated the road network with new roads and added the road names but not address ranges.

Due to the addition of new roads, the resulting geometry does not always match the initial National Topographic Database geometry. Wherever more recent Elections Canada data would improve the quality and quantity of road information, it was added to the Statistics Canada Street Network Files to form the National Geographic Database. The content derived from Elections Canada is primarily new roads and road names. These were left in the format used by Elections Canada, with upper and lower case letters, accents, road type and direction, but no civic address ranges.

Other sources

In addition to digital maps from other federal, provincial, municipal and licensed private sources, portions of the National Geographic Database contain information from Elections Canada Returning Officers (maps for the 38th General Election, held in the summer of 2004), Statistics Canada Regional Offices (1996 Enumeration Area Collection maps), data from the Statistic Canada's 2001 Census, and other materials prepared by private companies such as PLANET, which originates from the New Brunswick real property information system.

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.

Absolute positional accuracy

Absolute positional accuracy describes the degree to which the position of features in a geographic database reflects their true position on the ground (i.e., the closeness of reported coordinate values to values accepted as true).

The information present in the Spatial Data Infrastructure (SDI) road layer is provided for the purposes of statistical analysis and census operations only. The absolute position of roads on the SDI varies with the source files and documents used to build and maintain the database. Therefore, the SDI is not suitable for high precision measurement applications such as engineering, property transfers, or other uses that might require highly accurate measurements of the earth's surface.

During the build phase, the road layer was rubber sheeted to match the position of those on the National Topographic Database (NTDB) (which was used for reference purposes). After the build, the rubber sheeting process was applied to the materials used to maintain and improve the content of the road network. It is therefore expected that these geometrically matched arcs will have a positional accuracy similar to the corresponding reference data used during development of the database. It should be noted that the reference source selected for different geographic areas depended on a variety of factors such as population size, geographic location (urban or rural) and the availability of NTDB/ Digital Chart of the World (DCW) data. For example, in major urban centres 1:50,000 NTDB data were generally used as the reference data. As a result, in these areas, roads that were geometrically matched have a positional accuracy similar to roads on the NTDB data (i.e., approx. 10 metres). In areas that used 1:250,000 NTDB and DCW reference data, the positional accuracy of roads that were geometrically matched is approximately ±300 metres (NTDB) and between ±2,100 and ±4,300 metres (DCW), respectively.

The positional accuracy of arcs that could not be matched because they were not present in the reference data is not measured. These arcs were digitized on screen from paper maps annotated by Elections Canada's returning officers and Statistics Canada's regional officers. Although accurate in their attribute information and their relative position in relation to other features, the absolute positional accuracy of these roads is unknown.

Absolute positional accuracy is not a requirement for electoral and census processes.

Relative positional accuracy

Relative positional accuracy describes the degree to which the position of features in a geographic database reflects their true ground relationships.

For the National Geographic Database, relative positional accuracy is important. A road must appear in the proper position relative to other roads and physical features. During the build phase, the dataset was thoroughly tested for relative positional accuracy. The road network was overlaid onto the hydrographical, power line and railroad layers.

Attribute accuracy

Attribute accuracy refers to the accuracy of quantitative attributes and the correctness of non-quantitative attributes. Two road attributes were tested for accuracy: road name (name) and road address range ('ADDR_FM_LE', 'ADDR_TO_LE', 'ADDR_FM_RG', 'ADDR_TO_RG'). Road address range considers the completeness of addressing on individual arcs.

Road name

During the build phase, every effort was made to insure a proper transfer and association of a specific attribute (i.e., name, type, direction, and address range) to a specific geometric feature. This includes the association as well as its accuracy. The attribute data associated with the polygons in the boundary files were independently verified against the data in the Spatial Data Infrastructure and found to be accurate.

Measures on number of road name and address range attributes are presented in Table 4.2.

Road address range

Two tests were conducted to determine the attribute accuracy of address features on the base. First, the results from the current version of the Spatial Data Infrastructure were compared to the previous version of the Spatial Data Infrastructure within the National Geographic Database in order to identify any increases or decreases in the number of addressable roads. Secondly, a check was run on the 2001 addresses to determine which 2006 geographic area they fell into, then those same addresses were compared to the 2006 geographic area derived using a 2001/2006 correspondence file.

Logical consistency

Logical consistency refers to the fidelity of relationships between all variables in a dataset. For example, a road arc that does not have a road name should not have a road type.

During the build phase, the National Geographic Database dataset was thoroughly tested for logical consistency. Any violations of logical consistency were corrected, and 100% of the data are logically consistent.

Node-line-area relationships satisfy topological requirements as specified in the ArcInfo® data model.

Consistency with other products

The position of the arcs in the 2006 Road Network and Geographic Attribute File is generally consistent with the 2006 Road Network File, 2005 Road Network File and 2006 Cartographic and Digital Boundary Files, but is not necessarily consistent with those of the 2001 Cartographic Boundary Files or the 2001 Road Network and Skeletal Road Network Files.

Completeness

Completeness refers to the presence or absence of features, their attributes and relationships. Many new road features that were not previously found on earlier digital files at Elections Canada and Statistics Canada were added to the National Geographic Database (NGD) in order to create a more complete NGD road layer for all of Canada.

Roads

Many roads not present in the 2001 Skeletal Road Network File product were added to the 2006 Road Network and Geographic Attribute File in order to improve nation-wide coverage. Table 4.2 shows the number of road arcs on the 2006 Road Network and Geographic Attribute File.

Table 4.2 Number of road arcs in the 2006 Road Network and Geographic Attribute File
Table 4.2
Number of road arcs in the 2006 Road Network and Geographic Attribute File