In our recent blog we described the five simple steps to select, download and view LandsatLook Natural Colour Images. However, did you know that the Natural Colour Image isnâ€™t actually a single image? Instead, itâ€™s a combination of three separate images!
This is because remote sensing works by exploiting the fact that the Earthâ€™s surfaces, and the substances on it, reflect electromagnetic energy in different ways. Using different parts of the electromagnetic spectrum makes it possible to see details, features and information that arenâ€™t obvious to the naked eye. Some remote sensing satellites carry instruments that can measure more than part of the electromagnetic spectrum, with each different measurement known as a spectral band.
Landsat 8 currently has two instruments, measuring eleven different spectral bands:
- Three visible light bands that approximate red, green and blue
- One near infrared band
- Two shortwave infrared bands
- Two thermal bands used for sensing temperature
- Panchromatic band with a higher spatial resolution
- The two final bands focus on coastal aerosols and cirrus clouds.
Combing the red, green and blue bands produces a single image that is very similar to what your eye would see; and this composite is the Natural Colour Image product that Landsat offers. However, you can also create your own colour composites using Image Processing Software, as Landsat offers the possibility of downloading an image for each of the individual spectral bands, known as the Level 1 GeoTIFF files.
Once imported into an image processing package, itâ€™s straightforward to create different composites by combining different variations of the spectral bands. For example, combing the red, green and blue bands creates an image like the one at top of the blog showing the eastern edge of Paris, with the Bois de Vincennes, the largest public park in Paris, on the left hand side.
This image has colours your eyes expect to see, for example, trees are green, water is blue, etc, known as a true colour or RGB composite. Combining other spectral bands produces images where the colours are different to what you would expect, these are known as false colour composites. As they use different parts of the electromagnetic spectrum, the surface of the earth reacts differently to the light and allows features hidden when showing true colour to become far more prominent.
An example of a false composite can be seen on the right, it uses the near infrared, red and green bands. Like in the RGB image, the park is easily distinguishable from the surrounding Paris; but in the false colour image, the parkâ€™s water features of the Lac Daumesnil and the Lac des Minimes have become visible as black swirls.
A second example of a false colour composite is shown on the right, which this time combines the near infrared, shortwave infrared 2 and the coastal aerosol band. In this case, the vegetation of Paris appears orange and jumps out of the image when compared to urbanisation shown in blue.
Using different combinations of spectral bands is just one remote sensing technique to create valuable information and knowledge from an image. However, every satellite measures different spectral bands and you need to be aware of what you are looking at. For example, weâ€™ve described Landsat 8 in this blog, previous Landsat missions have measured similar, but slightly different spectral bands; full details of all Landsat missions and their spectral bands can be found here.
Using the individual spectral bands, rather than relying on the set Landsat products, means you may gain new insights into the area you are looking at and you can great some fantastic images. You can literally make things appear before your eyes!