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More information on Representative point

Censuses:

2006, 2001, 1996, 1991, 1986, 1981, 1976, 1971

Remarks:

Representative points are located by the following methods:

1. Block-face representative points

The block-face representative points are generated using the ArcInfo® GIS software (version 8.2) in conjunction with the Spatial Data Infrastructure, including selected water polygon layers. The points are initially calculated and stored based on the Lambert conformal conic projection; they are also transformed to latitude/longitude coordinates.

The block-face representative points are computed along addressable and non-addressable streets, midway (or approximately midway) between two consecutive features intersecting a street. The features can be other streets or boundaries of standard geographic areas.

The points are set back a perpendicular distance of 10, 5, 1 or 0.5 metres from the street centre line to ensure that all points have unique coordinates, and are located in the correct block and on the correct side of the street.

Some block-face representative points may fall in water bodies if the points are adjacent to bridges or causeways.

Figure 34 shows an example of block-face representative points.

2. Geographic area representative points

The representative points for dissemination blocks (DBs), dissemination areas (DAs), census subdivisions (CSDs), urban areas (UAs) and designated places (DPLs) are generated using ArcGIS® software (version 9) in conjunction with their respective cartographic boundary file (CBF). The most detailed dissemination hydrography in the CBF is used. The points are initially calculated and stored based on the Lambert conformal conic projection; they are also transformed to latitude/longitude coordinates.

A. Unweighted representative points

The representative points for DBs, CSDs, UAs and DPLs are unweighted. The points are generated using the ArcGIS® software. The software locates the point suitable for label or symbol placement in each polygon. If the geographic area is in multiple parts, the point is located in the portion having the largest area.

Topology checks are applied to ensure that the points fall within the appropriate geographic area. Since some dissemination blocks and designated places are located in water only, their representative points fall in water.

Figure 34 shows an example of dissemination block representative points.

B. Weighted representative points

The representative points for dissemination areas (DAs) are weighted using the centre of population (weighted mean centre). The first equation in Formula 1 employs the unweighted representative points for dissemination blocks and final population counts of each DB within the DAs, and then the weighted mean centre formula is applied. The second equation in Formula 1 is the simple mean (average) of the x,y coordinates, and is only applied when all dissemination blocks in a DA have no population.

Formula 1 Mean centre weighted by population

If any weighted representative points fall outside the dissemination area (e.g., for crescent-shaped polygons) or fall in water bodies, the points are generated using the minimum squared distance weighted by population (Formula 2). The first equation in Formula 2 selects an existing unweighted dissemination block representative point in the DA having the minimum population-weighted squared distance to all other dissemination blocks in the DA. The second equation in Formula 2 is not population-weighted and is only applied when all dissemination blocks in the DA have no population.

Formula 2 Minimum squared distance weighted by population

Topology checks are applied to ensure that the points fall within the dissemination area. Since some DAs are located in water only, their representative points fall in water.

Refer to related definitions of block-face; cartographic boundary files (CBFs); census subdivision (CSD); designated place (DPL); dissemination area (DA); dissemination block (DB); geocoding; postal code; urban area (UA); Spatial Data Infrastructure (SDI) and to the Postal Code Conversion File (PCCF), Reference Guide (Catalogue no. 92-153-GIE).

Changes prior to the current census:

Prior to 2001, enumeration area (EA) representative points were disseminated.

Prior to 1996, all representative points were called 'centroids'.

1. Block-face representative points

• For 2001, block-face representative points were set back a distance of 10, 5 or 1 metre(s) from the street centre line. As well, points were generated when streets crossed the limits of National Topographic Database (NTDB) map tiles.
• Prior to 2001, block-face representative points were not generated when streets crossed the limits of map tiles, since map tiles were not used.
• For 1996, block-face representative points were generated within street network file (SNF)coverage only, and the points were set back a distance of 22, 11, 5 or 1 metre(s) from the street centre line. The points were calculated in Universal Transverse Mercator (UTM) coordinates, but were disseminated in latitude/longitude coordinates.
• Prior to 1996, some block-face representative points did not have unique coordinate values, and all points were set back a perpendicular distance of 22 metres from the street centre line. The points were calculated and disseminated in UTM coordinates.
• Prior to 1991, block-faces were not created when EA boundary segments did not follow visible features.

2. Geographic area representative points

A. Blocks, dissemination areas, census subdivisions and designated places

• For 2001, the representative points for blocks, DAs, CSDs and DPLs could fall in water bodies. In addition, the DA points were not weighted.

B. Enumeration areas

• For 1996, EA representative points were disseminated in latitude/longitude coordinates and in x,y coordinates of the Lambert conformal conic projection. The points were generated as follows:
• EAs within street network file coverage: the points were created using the ArcInfo® GIS software, which located the point suitable for label or symbol placement in each polygon; steps were taken so that the points did not fall in water bodies. If the EA was in multiple parts, the point was located, when possible, in the portion with the largest number of occupied private dwellings (based on the 1991 block-face counts). In some cases, however, the representative point was located in the EA portion having the largest land area.
• EAs outside street network file coverage: the points were located by a manual procedure based on the visual inspection of building and/or street patterns on EA reference maps (some of which had topographic base map information). The representative point was located, when possible, within a predominant cluster of buildings and/or streets. If there was no predominant cluster, then the point was located between two or more clusters. In the absence of any cluster, the point was placed at the visual centre of the EA. If an EA was in multiple parts, the point was located in the portion with the largest number of dwellings. The representative point was located in the land-based portion of the EA.
• For 1991, the EA representative points within SNF coverage were created using the ArcInfo® GIS software, which locates the point suitable for label or symbol placement in each polygon; some points were located in water bodies. In addition, for EAs in multiple parts in SNF coverage, there was no rule for selecting the EA part to which the representative point was assigned. The EA representative points were disseminated in latitude/longitude coordinates, UTM coordinates, and in x,y coordinates of the Lambert conformal conic projection.
• Prior to 1991, EA representative points within SNF coverage were computed by a different method. An algorithm selected one of the existing block-face representative points (based on their number and concentration) within an EA as the overall EA representative point. The points were calculated and disseminated in UTM coordinates.