Abstract
Introduction: Exposure to green space, also referred to as greenness or natural areas, is associated with several diverse positive health outcomes. One of the proposed mechanisms of action for green space is psychological and based on human observation of green space elements. To date the majority of green space health studies have utilized satellite imagery (NDVI) or land use to estimate green space exposures. However, the correlation between satellite-based measures and amount of observable green space at ground level that individuals are actually exposed to (i.e. view) is largely unknown. Methods: We developed a methodology to extract green space measures from Google street view imagery. We compared monthly Landsat 7 NDVI (50m buffer) to greenspace estimates based on 700 randomly selected Google street view panoramas (360⁰ images) in Portland, Oregon. Street view panoramas were converted to hue and saturation vectors and filtered for green pixels with hue between 85 and 155 and saturation between 5 and 250. Summary statistics for each panorama image include mean and standard deviation hue and saturation and percent green pixel coverage. Results: NDVI was positively correlated with percent green pixel coverage (r=0.27) and mean saturation (r=0.26) and negatively correlated with mean hue (r=-0.21). Correlations between NDVI and green pixel coverage significantly varied by month (r=0.18 in August and 0.66 in November). Conclusions: Low to moderate correlation was observed between satellite and street-based green space measures. Monthly variations in correlation suggest NDVI and street view are capturing different components of the greenspace environment, potentially related to different stages of the greenspace annual cycle. We are performing crowd-source surveys to compare how well NDVI and novel street view summary statistics captures human greenspace attitudes and perceptions. These measures will be incorporated into future epidemiological analyses.
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