I spotted a function named “Minus” which told me I could subtract the values of one image from the values of another.

I opened both the 20 Earth at Night images in ArcGIS Pro and started investigating my options in the exceedingly fun and helpful Raster Functions toy box. I wondered if a simple change detection tool could show where lights had newly appeared or brightened, and where lights had dimmed or altogether vanished, within a single image. My eyes darted back and forth between excerpts of the 20 images, storing a bit of the lights in my working memory and trying to tease out in my mind where lights were coming on or where they were going out. Upon seeing the new Earth at Night imagery, and some beautiful side-by-side comparisons provided by NASA’s Joshua Stevens, I marveled at the extent of change some areas experienced in the intervening five years. NASA released an update to its Earth at Night imagery in 2017, sparking an opportunity to compare change against the prior imagery from 2012. This is one of the reasons geographic visualization is so exciting and surprising-maps are much more than colors and pixels arranged on a canvas. We spiral out from cities, perched along transportation lines like spools of holiday lights. We avoid rugged and barren areas such that our absence shouts the geographic form of mountain spines and deserts. We gather in lush river valleys, tracing the ancient desirability of agriculture along riparian floodplains. We concentrate in great numbers within brightly lit urban centers and perch along tendrils of connected settlements that link, via thin strands of humanity, cities to towns, towns to villages, villages to outposts.

In what other way could we so clearly see the echo of seven billion humans on the face of our shared planet? We are a moving people-creatures who continually strive for improvement, vie for resources, flee from danger, and generally spread out on the surface of the Earth. (Click on the image to see it full size.) NASA's Earth at Night image reveals a new understanding about humanity.

Earth at night nasa