Darkness card, sky brightness with greenhouse lighting off

The map concerns the sky brightness in North Holland in 2011-2012 with greenhouse lighting off. If there is no direct enlightenment nearby, the night sky determines the degree of darkness. The sky is lit up by shining light from a city or greenhouse from an area with a radius of about 30 kilometers.
In 2011 and 2012, 225 locations in the province of North Holland measured the brightness of the sky in the point straight up (zenit), as a measure of the ‘darkness’.
Previously carried out measurements in Amsterdam and on the island of Texel have also been incorporated into this darkness map.