Why a forest can look red in a satellite image

A forest that looks red in a satellite image can be full of green leaves. A scientist may have assigned red to a measurement the human eye cannot see. The display is called a false-colour image, and its colours can help pick out features that are harder to distinguish in ordinary photographs.

From separate bands to a single picture

A satellite sensor can record reflected light in several bands. A band covers a particular range of wavelengths. A natural-colour display can put visible red, green and blue measurements into the corresponding screen channels. A false-colour display assigns one or more channels to different bands.

NASA Earth Observatory shows a common example in which near-infrared measurements drive the red channel. Healthy leafy vegetation tends to reflect a lot of near-infrared light, so it can appear bright red in that image. The point is to make the vegetation easier to see and compare. A different band combination can give the same forest a different colour.

The U.S. Geological Survey’s Landsat guide identifies the specific bands used for a common colour-infrared combination. Look up the sensor before using band numbers. Different Landsat generations assign different numbers to some of the same spectral regions.

What a comparison can reveal

Imagine looking at a river floodplain twice, using images from before and after a flood. An analyst could choose a combination that separates water from vegetation and soil. Compare the same bend and low-lying fields in both scenes. Changes are easiest to interpret when the image dates, viewing area, band combination and legend are clear.

The display still leaves questions. Water, wet ground and shadow can all appear dark in some images, and clouds can hide the surface. More detail may come from another image date, a companion natural-colour view, a map or observations made on the ground. Treat a distinctive patch as a place to investigate.

Look for the bands, date and scale

First find the image caption or legend and look for the bands assigned to red, green and blue. Check whether a pair of images uses the same combination and display scale. Then check the date and the size of each pixel. Small features may blend together within one pixel, while a broad patch can stand out clearly.

Compare the date printed in the legend with the date an image was posted. They can be different, especially in a gallery that reuses an older scene to teach a method.

NASA’s examples place several versions of a scene side by side. Try picking a single river, field or patch of cloud and following it across the versions. That exercise makes the effect of the band assignment easier to see. The image is most informative when the colours stay attached to the measurements they represent.