Satellite images show NZ much brighter

Researchers at Waikato University came upon these images, taken in 1992 (L) and 2012 (R)

Night-time images captured by US defence satellites show a New Zealand much brighter than it was in the early 1990s.

Researchers at Waikato University came upon these images, taken in 1992 and 2012, while analysing a rich, 22-year digital archive of material from the United States Air Force defence Meteorological Satellite Program.

Since the 1970s, its satellites have been circling the planet 14 times per day, recording the intensity of Earth-based lights with their Operational Linescan System sensors.

The sensors are designed to collect low-light imaging data for the purpose of detecting moonlit clouds, but as a fascinating by-product have captured lights from human settlements.

Every night, at some point between 8.30pm and 10pm, the satellites observe every location on the planet at night, before US scientists at the National Oceanic and Atmospheric Administration's National Geophysical Data Center process and distribute the images.

During processing, they remove intense sources of natural light, leaving mostly man-made light, and observations where cloud cover obscures the globe's surface are also excluded.

Data from all orbits of a given satellite in a given year are eventually averaged over all valid nights to produce a five-year dataset.

Dr Lars Brabyn, a senior lecturer in geography, tourism and environmental planning at Waikato University's School of Social Sciences, said the 2012 sensor in these images was more sensitive than the 1992 sensor.

"This explains the brighter 2012 image, but development has also added to the brightness."

In the 2012 image, the Waikato Expressway between Hamilton and Auckland was clearly visible, as were gas flares off the coast of Taranaki, he said.

His colleagues were using the data to compare changes in forest cover around the world.