this is explained in the FAQ. They merge different sources, notably data from SQMs which "stands for Sky Quality Meter. It's a small device that can measure sky

this is explained in the FAQ. They merge different sources, notably data from SQMs which "stands for Sky Quality Meter. It's a small device that can measure sky quality ie. darkness. It's widely used by amateur astronomers because it's fairly cheap and gives you fast results which can be easily compared to other SQM users." https://www.lightpollutionmap.info/help.html https://www.lightpollutionmap.info/help.html