OK. I found this:
Quote:
Originally Posted by Robh
On the significance of the measure magnitudes per square arcsecond:
....Now, the skyglow can be measured as the surface brightness of the sky i.e. how bright each square arcsecond of the sky appears.
This is what a Sky Quality Meter measures.
Roughly speaking, the surface brightness of the sky is ...
17 centre of a major city
18 high density suburb around city centre
19 outer suburbs with lower density population
20 low density fringe suburbs, uninhabited areas adjacent
21 rural area with a town in the distance
22 isolated location. Darkest sky possible.
....
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but still interested in how those other site compared, and your own backyards?