Congo River Basin - Landsat data - Continued
|  | Congo River Basin, 18 February 2001 | |
Multispectral Image Combination
It is difficult for the human eye to select different features of the Earth's surface in a greyscale image. It is therefore useful to combine 3 greyscale Landsat images into one RGB colour image.
RGB means red (channel 3), green (channel 2) and blue (channel 1) and uses the physical features of the additive colour system.
The colours of the different features depend on the bands selected for the combination, because every object has its own radiation characteristics. Different combinations allow different views of the same feature. We will try out some of these in the next exercise.
True-Colour Combination
The true-colour combination requires the Red (band 3 - 0.63 - 0.69 µm), the Green (band 2 - 0.52 - 0.60 µm), and the Blue (band 1 - 0.45 - 0.52 µm) Landsat channels. This combination leads to an image which closely resembles a colour photograph.
Open the LEOWorks programme. If you have not downloaded the images of the Congo River Basin yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Congo and the images Congo_Landsat_Band_1.tif, Congo_Landsat_Band_2.tif, and Congo_Landsat_Band_3.tif.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select images Congo_Landsat_Band_1.tif for Red, Congo_Landsat_Band_2.tif for Green, and Congo_Landsat_Band_3.tif for Blue, and click OK.
The new image is the true-colour image produced from the 3 visible light channels. But it is not in true colours yet, because of the unenhanced data. Select (activate) the first image Congo_Landsat_Band_3.tif and choose Enhance>Interactive Stretching. A histogram will appear. Shift the left blue bar in the Input Histogram to the left initial point of the Input Histogram. Then shift the right red bar to the right initial point of the Input Histogram and click Apply. Note the changes in the combined image. Convert the other two images in the same way.
Describe the image and try to divide the image features into 5 classes: dense forest, shrubs, clear water, muddy water, and clouds.
False-Colour Combination
To increase the interpretability of satellite images, false-colour images are used.
In most cases, a false-colour image uses at least one infrared channel. The infrared range is very useful for interpreting the Earth's surface, because it consists of reflected and emitted energy.
Infrared is not visible to the human eye but transmits a lot of information. Plants in particular reflect much more energy in the near infrared than in the visible range of the electromagnetic spectrum. Even the health status of a plant can be ascertained from the intensity. Sometimes false-colour images even uncover invisible features.
Which are the unseen features in the true-colour image? Are there hidden parts of vegetation or water surfaces?
Open the LEOWorks programme. If you have not downloaded the images of the Congo River Basin yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Congo and select the first image Congo_Landsat_Band_2.tif. Open Congo_Landsat_Band_4.tif and Congo_Landsat_Band_7.tif, too.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select image Congo_Landsat_Band_7.tif for Red, Congo_Landsat_Band_4.tif for Green, and Congo_Landsat_Band_2.tif for Blue, and click OK. Improve the raw data in the same way it was done in the True-Colour Combination exercise.
How did the colours of the classes change between the true- and the false-colour image?
Can the classes be identified in both images?
Which image would you prefer in order to differentiate the satellite image's features?
Try another combination. Choose File>Open. A windows dialogue box will pop up. Choose the folder Congo and select the images Congo_Landsat_Band_1.tif, Congo_Landsat_Band_2.tif and Congo_Landsat_Band_4.tif.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select image Congo_Landsat_Band_4.tif for Red, Congo_Landsat_Band_2 for Green, and Congo_Landsat_Band_1.tif for Blue, and click OK. Improve the raw data the same way it was done in the True-Colour Combination exercise.
This new image is a false-colour combination of three greyscale pictures. Describe the image and try to divide the image features into 5 classes: dense forest, shrubs, clear water, muddy water, and clouds.
What conclusions do the green coloured parts of the image allow you to draw? Explain your thoughts.
Compare the two false-colour images. Explain the differences and the similarities of the images.
Combine a false-colour image by using channel 7 for Red, channel 1 for Green, and channel 2 for Blue. What can you say about the usefulness of the image?
Now try a combination of your choice and note the variations.
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