Testing sky brightness models against radial dependency: A dense two dimensional survey around the city of Madrid, Spain: SQM data

We present a study of the night sky brightness around the extended
metropolitan area of Madrid using Sky Quality Meter (SQM) photometers. The map is the first to cover the spatial distribution of the sky brightness in the
center of the Iberian peninsula. These surveys are neccessary to test the light
pollution models that predict night sky brightness as a function of the
location and brightness of the sources of light pollution and the scattering of
light in the atmosphere. We describe the data-retrieval methodology, which
includes an automated procedure to measure from a moving vehicle in order to
speed up the data collection, providing a denser and wider survey than previous
works with similar time frames. We compare the night sky brightness map to the
nocturnal radiance measured from space by the DMSP satellite. We find that i) a
single source model is not enough to explain the radial evolution of the night
sky brightness, despite the predominance of Madrid in size and population, and
ii) that the orography of the region should be taken into account when deriving
geo-specific models from general first-principles models. We show the tight
relationship between these two luminance measures. This finding sets up an
alternative roadmap to extended studies over the globe that will not require
the local deployment of photometers or trained personnel.