Implications for future research
We argue that NDVI and other vegetation indices are not just unable to
differentiate vegetation types but are also easily confounded by
urbanicity and thus air pollution. When investigating associations where
knowing the amount of greenspace or the distance to greenspace is not
enough, like with allergic outcomes, we suggest that more detailed
vegetation measures be considered. In our study, number of trees
appeared to be a good proxy for number of allergenic trees, but this
unlikely to always be the case across study areas. Digital tree registry
data are scarce but available for some cities, including New York,
Frankfurt am Main, Vienna, Hamburg and Melbourne. Similarly, rich
datasets are available for different areas, like the maps of 12 key
allergenic taxa in the UK for the year 2012 in 1 x 1 km resolution
produced by McInnes et al. 53