Why does every city have a different threshold for NDVI?

Given that NDVI generally represents the total amount of live green vegetation (eg, street trees, green corridors, and general vegetation from public and private spaces), positive and higher NDVI values indicate more greenness. However, it is important to consider that the natural characteristics of each city (eg, latitude, type of landscape, weather, etc.) can influence the natural vegetation (by variations in type and density), and, consequently, the NDVI value that its vegetation presents.

In this sense, the authors have used mathematical models in a way to consider the possible geographical, climatic, and biological variations in vegetation when determining each city’s NDVI levels (eg, cities in Southern Europe having naturally different NDVI levels than cities in Northern Europe).