Urban greenness and normalized difference vegetation index by population centre (38100149)

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) were averaged to provide a measure of vegetation condition in summer for each year for the same physical area using the 2016 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).

Average greenness is the percentage of land area that is classed as green (NDVI pixel >= 0.5). Water areas were excluded from this analysis.

Average normalized difference vegetation index (NDVI) is the average of all pixel NDVI values within the population centre. Water areas were excluded from this analysis.

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 lower values 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 2016 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.

On November 17, 2022, this table was archived and replaced by table 38100158. Table 38100149 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.