NASA's satellites and astronauts take some great photographs. NASA's Astronomy Picture of the Day is often a striking picture of the cosmos, but NASA's satellites also point back towards mother Earth. NASA's Earth Observatory posts an image of the day, and those images can be just as breathtaking as NASA's astronomy photos. Wednesday's is a crimson thermal emission image of the Philippines, showing a dramatic change in vegetation over the past decade.
On Monday, the Earth Observatory posted an informative guide to better understanding its satellite photos. Or any satellite imagery, really. The basics are good tips: "Look for a scale, Look for patterns, shapes, and textures, Define the colors (including shadows), Find north, Consider your prior knowledge." It's easy to scroll around Google Earth and get the exact coordinates of any location, but when all you have is a single satellite image, it can take some intelligent analysis to identify some key information.
Color, for example, can tell you a ton about the geography in an image. Water absorbs light, so it usually appears dark blue or black. But "sediment reflects light and colors the water," Earth Observatory explains. "When suspended sand or mud is dense, the water looks brown. As the sediment disperses, the water’s color changes to green and then blue. Shallow waters with sandy bottoms can lead to a similar effect. Sunlight reflecting off the surface of the water makes the water look gray, silver, or white. This phenomenon, known as sun-glint, can highlight wave features or oil slicks, but it also masks the presence of sediment or phytoplankton."
Agricultural land shows up as a brighter shade of green than natural vegetation. Fall vegetation takes on red, brown and orange hues. Some high, thin clouds are only visible by the shadows they leave on the ground.
Patterns in land offer clues as to what an image contains. On a basic level, square and rectangular features and straight lines are almost certainly manmade. Identifying familiar features, or unique features, that can be compared to a map can help you identify the scale. ID a lake in a satellite image, and you can compare that lake to a map to extrapolate the satellite's scale. Identifying familiar features can also help establish which direction is north.
Earth Observatory's images already include scale, but it's still fun to dig through the archive of more than 10,000 photos.