What do colours mean in satellite imagery?

False colour image of phytoplankton blooming off the coast of Patagonia. Acquired 2nd Dec 2014. Image Courtesy of NASA/NASA's Earth Observatory

Phytoplankton blooming off the coast of Patagonia on 2nd Dec 2014.
Image Courtesy of NASA/NASA’s Earth Observatory

Satellite images are a kaleidoscope of colours, all vying for attention. It’s important to be clear what the colours are showing, and more importantly, what they may not be showing, to interpret the image correctly. For example, a patch of white on an image might indicate snow or ice, sunglint off the ocean, fog or it could just mean it was cloudy.

On the earth’s surface different colours represent different land types:

  • Vegetation appears as shades of green from pale for grasslands to dark for forests – although some forests will progress from green to orange to brown in autumn.
  • Ocean colour is significantly influenced by phytoplankton, which can produce a range of blue and green colours. A fantastic example of this can be seen in the image at the top of the blog showing phytoplankton blooming off the cost of Patagonia.
  • Snow and ice can appear white, grey, or slightly blue.

As noted in the opening, colours can also mislead with cloud cover being the natural nemesis of optical remote sensing. However, you also have to be careful with effects such as:

  • Smoke: ranges from brown to grey to black.
  • Haze: a pale grey or a dirty white.
  • Dust: can be brown, like bare ground, but also white, red and black.
  • Shadow: Clouds or mountain shadows can look like dark surface features.

There is a good article here from NASA’s Earth Observatory giving more details on the different colours of surface land types. So far, we’ve focussed on natural colour signatures; but man-made structures also appear on imagery. Generally, urban areas tend to be silver or grey in colour; although larger objects also show up in their own right such as the bright red roof of Ferrari World in the middle of the Abu Dhabi Grand Prix Circuit – as discussed in a previous blog.

Composite Google Earth image of the entrance to the Panama Canal: Data courtesy of DigitalGlobe

Composite Google Earth image of the entrance to the Panama Canal: Data courtesy of DigitalGlobe

We tried to repeat the identification of man-made objects for this blog using the coloured roofs of the Biomuseo building, located on the Amador Causeway – at the entrance to the Panama Canal in the Pacific Ocean. Sadly, Landsat 8 pixels are too coarse; and Google Earth has fallen prey to cloud cover preventing visibility, as shown in the image on the right. What you can see though is the buildings in Panama City and the yachts in the marinas and clustered around the four islands (Naos, Perico, Culebra and Flamenco) at the end of the Amador Causeway.

The final thing to remember when considering colours, is the format of the image itself. Some images use true-colours from the red, green and blue wavelengths, which produce colours as if you were looking at the scene directly, so trees are green, sea is blue, etc. However, other images incorporate infrared light to enhance the detection of features not easily distinguished on a true-colour image; this means colours aren’t what you would expect, for example, the ocean may appear red.

Colour is central to use of satellite imagery, but you need to know the properties of the rainbow you are looking at or you may never find the pot of satellite gold.