Natural Colour and False Colour Composites (below)
These Landsat images show the salt ponds at the southern end of San Francisco
Bay, which now form the San Francisco Bay National Wildlife Refuge. Salt making
in
these naturally shallow waters at the edge of the bay was begun by Native Americans.
The two images demonstrate the difference between 'natural colour' (left)
and 'false colour' (right) composites.
The image on the left is made from visible red, green and blue wavelengths (bands
3, 2, 1, depicted in red, green and blue respectively). The colours correspond
closely to those one would observe from an aircraft. The salt ponds appear green
or orange-red due to the colour of the sediments they contain. Urban ares, such
as Palo Alto and San Jose, appear grey and vegetation is either dark green (trees)
or light brown (dry grass).
The right-hand image is made up of near-infrared, visible red and visible green wavelengths
(bands 4, 3, 2, depicted in red, green and blue respectively). Since chlorophyll in healthy
vegetation strongly reflects near-infrared light, this is clearly visible as red in the image.
False-colour composite imagery is therefore very sensitive to the presence of healthy
vegetation. The right-hand image above thus provides better discrimination between the
'leafy' residential urban areas, such as Palo Alto (south-west of the Bay), from other
urban areas by the 'redness' of the trees. The high chlorophyll content of watered
urban grass areas shows as bright red, contrasting with the dark red of trees and
the brown of natural, dry grass.
