Main types of satellite images: Combining images
Since different objects absorb/emit energy at different wavelengths one of the most powerful ways to use satellite images is by combining the different channels.
This highlights the differences between various elements, such as clouds and snow or land and sea.
One way of doing this is to subtract one channel from another and the resulting image is used.
Small signals that are only in one channel then become much easier to see. This is particularly useful for the detection of fog and volcanic ash and the image above shows enhanced yellow-to-red colours which indicate volcanic ash from the Sangeang Api volcano.
Another common way of combining images is the creation of composites of RGB images (red, green, blue), in which different channels are displayed as different colour schemes.
For example, Natural Colour RGBs (below) are created by combining visible and near-infrared channels (1.6 µm for red, 0.8 µm for green and 0.6 µm for blue).
The 1.6 µm has a different sensitivity to snow and ice than the other two channels, but the same sensitivity to water. This means water (low) clouds appear white and ice (high) clouds and snow appear cyan.
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