|  | Landsat 7 ETM | |
Cairo - Exercises using Landsat data
Description of Landsat data
The US satellite Landsat 7 ETM is NASA's 6th operational Earth observation satellite of a series (the launch of Landsat 6 failed in 1993) dating back to 1972. The Landsat system therefore constitutes the longest continuous record of the Earth's surface. The main tasks of all Landsat satellites are environmental monitoring, disaster assessment, land use and regional planning, cartography, range management, and oil and mineral exploration.
Landsat's mode of operation has been continuously improved. Today, the Landsat ETM features 8 channels, from visible light (channels 1,2,3) to near and middle infrared (channels 4,5,7) and thermal radiation (channel 6). The ground resolution is 15 metres for the panchromatic channel (8) and 30 metres for channels 1 to 5 and 7. The thermal channel 6 provides a resolution of 80 metres. The Landsat 7 satellite operates in a circular, sun-synchronous, near polar orbit.
See the
Technical data of Landsat bands table.
Description of the Cairo Images
In order to do the exercises you need to download the required images.
Download the file cairo.zip and save the Cairo images to your hard disk in a folder named Cairo.
In this exercise we will use Landsat 7 ETM images from the year 2000.
Open the LEOWorks programme.
To open the first image, choose File>Open. A dialogue box will pop up. Choose the folder Cairo and select the first image Cairo_Landsat_2000_Band_1.tif. Open all other single band images from the year 2000.
All the images need to be enhanced in order to get a clearer view of the area in the image.
Select (activate) the first image Cairo_Landsat_2000_Band_1.tif and choose Enhance>Histogram Equalization. The image will change. Convert all other images in the same way.
There are now 9 images on your screen. Every single image reflects a part of the electromagnetic spectrum between 0.45 µm and 12.5 µm (the part of the visible light portion ranges from 0.45 µm to 0.7 µm).
There are many different bands because different features of the Earth's surface can be seen in different parts of the electromagnetic spectrum and the same features appear unequal in different parts of the spectrum. That means that all the imaged features/objects appear in different colours when illuminated by light (including infrared). In order to distinguish the objects by their colour we need an instrument capable of measuring in many bands, covering all the visible colours and also the infrared sector.
Now have a look at the images and group them into two categories of similar appearance. Note the water and the northwestern area of vegetation in particular. Compare the two groups with the table containing the different Landsat data bands. Are these two groups identifiable here, too?
What is the significant difference with respect to image details of Cairo_Landsat_2000_Band_61.tif/Cairo_Landsat_2000_Band_62.tif (Landsat ETM band 6) and Cairo_Landsat_2000_Band_8.tif (Landsat ETM band 8) in comparison with the others bands?
| | | Cairo as seen from Landsat 7 ETM on 23 August 2000 |
Multispectral Image Combination
It is difficult for the human eye to select different features of the Earth's surface in a greyscale image. Therefore it is 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 of Cairo using bands 3, 2, 1 | |
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 3 - 0.45 - 0.52 µm) Landsat channels. This combination leads to images closely resembling a colour photograph.
Open the LEOWorks programme. If you have not downloaded the images of Cairo yet, do so now.
Choose File>Open. A dialogue box will pop up. Choose the folder Cairo and select the first image Cairo_Landsat_2000_Band_1.tif. Open Cairo_Landsat_2000_Band_2.tif and Cairo_Landsat_2000_Band_3.tif, too.
Choose Image>Combine from...>Red Green Blue. A pop-up menu will open. Select image Cairo_Landsat_2000_Band_3.tif for Red,
Cairo_Landsat_2000_Band_2.tif for Green, and
Cairo_Landsat_2000_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 unenhanced data.
Select (activate) the first image Cairo_Landsat_2000_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 - parks and farming areas, water, desert and plain soil, densely built-up areas, loosely built-up areas.
What other features can you identify?
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