| First come the principal components
When the Earth is seen from on high, the natural landscapes
ranges of colours are surprisingly limited and there are very few
bright colours. Seen from space, the temperate continental areas
look green and the vast desert regions palettes are dominated
by grey, beige, and brown.
This explains why, when the pictures are converted into digital
images, the corresponding histograms of the Earths surface
show little contrast and the spectral components do not vary independently.