Urban greenness and normalized difference vegetation index by 2021 population centre (38100158)

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Ecosystem condition accounts organize biophysical data representing key abiotic and biotic characteristics of different ecosystem types. Urban greenness and normalized difference vegetation index (NDVI) are variables that represent urban vegetation condition.

The variables are derived from NDVI from moderate resolution imaging spectroradiometer (MODIS) satellite imagery and compiled for population centres, NDVI is an indicator of vegetation presence and quantity. It is an index with values ranging from -1 to +1, where high values (close to +1) correspond to healthier vegetation (dense green leaves) whereas low NDVI values (0.1 and below) indicate less or no vegetation (barren rock, sand, snow, water or impervious surfaces such as roads and buildings). Weekly NDVI images during peak summer conditions (Julian weeks 26 to 34, approximately late-June to late-August) were averaged to provide a measure of vegetation condition for each year for the same physical area using the 2021 population centre boundary to ensure consistency.

Population centres have a population of at least 1,000 and a population density of 400 people or more per square kilometre, based on population counts from the Census of Population. Population centres are classified in three groups: small (population from 1,000 to 29,999 inhabitants), medium (population from 30,000 to 99,999 inhabitants) and large urban (100,000 inhabitants or more).

For each year, the average NDVI of each population centre is the average of the NDVI values, over peak summer, of all pixels contained within the population centre boundary. For average greenness, each non-water pixel is classed as ‘green’ if its average NDVI value is greater than or equal to 0.5; otherwise, it is classed as ‘grey.’ The average greenness of each population centre is the ratio of the number of green pixels over the sum of the number of non-water (classified as either green or grey) pixels.

Urbanization processes such as densification and urban expansion can result in significant reductions in the quantity and quality of ‘green' areas and related increases in ‘grey’ areas that consist of buildings, impervious surfaces, bare soil and low density vegetation. Long-term and temporary changes in greenness can be linked to these urbanization processes, as well as the addition or maturing of urban vegetation and changes in vegetation condition related to natural factors such as drought, insects or disease.

This assessment of greenness has several limitations associated with the use of NDVI to represent greenness, including the coarse resolution of the MODIS data and the selection of the 0.5 NDVI cut-off as a threshold to classify green or grey pixels. The data used in this study has a spatial resolution of 230 m, which corresponds to an area of 0.05 km2 (i.e., a 52,900 m2 footprint) and the resolution of the pixel (230 m x 230 m) is a limitation, particularly for small population centres.

The urban green class defined in this analysis corresponds to areas with an NDVI greater than or equal to 0.5, representing areas that are predominantly vegetated. Areas with a value lower than 0.5 are considered ‘grey’ and are largely non-vegetated, though patches of grass, shrubs or crops, or other unhealthy/poor condition vegetation will be included. Higher resolution data is needed for the identification of detailed urban green spaces.

Fixed 2021 population centre boundaries are used for all years of the assessment, which may therefore bias results towards a higher proportion of green area in earlier years compared to later years as a result of development in peri-urban areas.

This table replaces table 38100149, which calculated greenness for population centres from the 2016 Census of Population. The new table uses the population centre boundaries from the 2021 Census of Population. In addition, different water boundaries were used. As of 2020, all the boundaries maintained by Statistics Canada were adjusted to the more current CanVec hydrographic features (Topographic Data of Canada – Natural Resources Canada), which comply with international geomatics standards seamlessly across Canada. For this analysis, the same CanVec hydrographic files were used to mask water from the NDVI data. This differs from the analysis done for table 38100149, which used the 2016 Coastal Waters and Lakes and Rivers polygon files to mask water bodies from the NDVI data.

The production process of urban greenness statistics was automated in 2023. Production was changed from the geographic information system ESRI ArcGIS Pro to the statistical computing environment R. Minor differences in the historical data values were observed resulting in a revision to the full time series to ensure comparability through time. Larger differences between the original and revised data values were more likely to occur for small population centres whose geographical extents cover a small number of pixels.