Seasonal NDVI by year and ZIP code

We used Landsat 7 (Collection 1 Tier 1 DN values, representing scaled, calibrated at sensor radiance) and 8 (Collection 1 Tier 1 Operational Land Imager DN values, representing scaled, calibrated at sensor radiance) images for the entire US. Landsat 7 and 8 images are generated every 16 days at 30m resolution. Landsat images were used from 2000 till winter 2013, Landsat 8 images were used from spring 2013. We created cloud-free Landsat composites for the winter (December-February), spring (March-May), summer (June-August) and fall (September-November) for the years 2000-2016 using GEE (function ee.Algorithms.Landsat.simpleComposite). We calculated spatially weighted average NDVI values for each zip code in the US, after setting negative NDVI values to zero.