Lagos - Exercises using Landsat data - Continued
|  | True-colour combination of Lagos using bands 3,2,1 | |
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 renderings of one and 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 - 0.69µm), the Green (band 2 - 0.52 - 0.60µm), and the Blue (band 1 - 0.45 - 0.52µm) Landsat channels. This combination leads to an image closely resembling a colour photograph.
Open the LEOWorks programme. If you have not downloaded the images of Lagos yet, do so now.
Choose File>Open. A dialog box will pop up. Choose the folder Lagos and select the first image Lagos_Landsat_Band_1.tif. Open Lagos_Landsat_Band_2.tif and Lagos_Landsat_Band_3.tif, too.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select image Lagos_Landsat_Band_3.tif for Red, Lagos_Landsat_Band_2.tif for Green, and Lagos_Landsat_Band_1.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 unimproved raw data.
Select (activate) the first image Lagos_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 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 7 classes - dense forest, densely built-up areas, moderately built-up areas, residential areas with gardens and industrial areas, natural cover (green space), beach, and water.
Why is the true-colour image so blurred?
Are the 7 classes easy to identify in the image?
Which options do we have to improve the image information?
| | | False-colour combination of Lagos using bands 4,5,3 |
False-Colour Combination
To increase the interpretability of satellite images, false-colour images are often 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 of the energy it reflects.
Open the LEOWorks programme. If you have not downloaded the images of Lagos yet, do so now.
Choose File>Open. A dialog box will pop up. Choose the folder Lagos and select the first image Lagos_Landsat_Band_3.tif. Open Lagos_Landsat_Band_4.tif and Lagos_Landsat_Band_5.tif, too.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select image Lagos_Landsat_Band_4.tif for Red,
Lagos_Landsat_Band_5.tif for Green, and
Lagos_Landsat_Band_3.tif for Blue, and click OK.
Improve the raw data in the same way you did for the True-Colour Combination exercise. Save the image as lagos_landsat_453 (TIF) into your folder Lagos.
This new image is a false-colour combination of three greyscale pictures.
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 differentiates the various features better?
Are more features identifiable in that image than in the true-colour image?
If so, which and why?
Interpret the surface features of Lagos and allocate the following classes: dense forest, densely built-up areas, moderately built up areas, residential areas with gardens and industrial areas, natural cover (green space), beach and water.
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