Open spaces and green areas

The data referenced herein is calculated using urban boundaries defined using the Degree of Urbanization approach to defining cities and urban areas, which may be larger or smaller than the official municipality boundaries. Within each city/urban area, the green areas are extracted using satellite imagery analysis for four time periods 1990, 2000, 2010 and 2020 based on the Normalized Difference Vegetation Index (NDVI), which assesses the level of greenness from satellite imagery. In this analysis, green areas are defined as parts of the city that are green for most parts of the year, and include individual trees, forests or forested areas, shrubs, perennial grasses and such other types of long-term vegetation. Waterbodies are not considered as green areas in this assessment. Since the stability the input data can be affected by climatic conditions such as long-term drought or rain, a median image was created over a three-year period, reducing thousands of images available for each point(pixel) to the most stable pixel. The median was used as opposed to the mean, because the median is less influenced by extremes (in this case extreme rains or drought). Unlike majority of the existing datasets which use basic NDVI thresholds, the most common of which include values of -1 to =0.2 – 0.5 for low vegetation and >0.5 for high vegetation , the premise of this new analysis was that each city is unique, and using a constant NDVI threshold would work to the disadvantage of cities in drier regions. As such, NDVI thresholds were manually defined for each city per year by GIS experts at UN-Habitat. The analysis framework – including selection and mosaicking of images from the Landsat and Sentinel missions, pre-processing and NDVI analysis and thresholding was based on the Google Earth Engine platform. The results from this analysis were then used to calculate two indicators: a) the average share of green area in city/urban area (percentage) and b) the green are per capita (m2 per person). Population data used to calculate the green area per capita is sourced from GHS-Pop for 1990, 2000, 2010 and 2020. The calculation of global and regional averages for the indicator on share of green area in city/urban areas are based on simple averages, while those on green area per capita are based on population weighted averages per analysis year.

Read more at https://unhabitat.org/knowledge/data-and-analytics