The light pollution map has two base layers (road and hybrid Bing maps), VIIRS/World Atlas/Clouds overlays and a point feature overlay (SQM, SQC and Observatories).
Direct annual comparisons are to be avoided as there are too many factors involved. If you want an analysis of radiance (VIIRS/DMSP) changes over a period of time, then please use the Lighttrends application
For more information view the help
Send comments about the map to Jurij Stare