Exercise 1 - Landsat ETM
|  | True-colour image of the Niger Inland Delta | |
The image on the left is a true-colour combination of the three visible channels, red, green, and blue (RGB), of a Landsat scene. The resolution is 120 metres per pixel by 512 pixels in each direction. The image therefore covers an area of about 61.4 x 61.4 km². Which part of the 10,000 - 30,000 km² Niger Inland Delta does the image show?
Open the LEOWorks programme. If you have not downloaded the images of the Niger Inland Delta yet, do so now (see right of page).
Open the image Nigerdelta_Landsat_Band_321_120m.tif. Choose File>Open. A dialogue box will pop up. Choose the folder Niger and select the image Nigerdelta_Landsat_Band_321_120m.tif.
Now choose View>Cursor Position/Value. A pop-up window opens. Place the crosshair at the junction of the western river, and write down the latitude and the longitude. That is only possible because of the GeoTIFF format used for the data. GeoTIFF embeds geographical data as tags.
Take a map of Central Africa, identify the names of the geographical features, and locate the area of the Niger Inland Delta shown by using the written information.
What can you say about the location of the area shown by the Landsat image?
Which rivers and which lake are covered by this image?
| | | Landsat ETM |
Description of Landsat data
a) Landsat data
The US satellite Landsat 7 ETM is NASA's 6th operational Earth observation satellite of this series (the launch of Landsat 6 failed in 1993). It is the latest in this series of Earth observation satellites dating back to 1972. The Landsat system constitutes the longest continuous record of the Earth's surface. The main tasks of all Landsat satellites include environmental monitoring, disaster assessment, land use and regional planning, cartography, range management, and oil and mineral exploration. Over the years, its operation mode 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.
| | Niger Inland Delta, Mali, 27 September 2000 | |
b) Description of the Niger Inland Delta images
Open the LEOWorks programme. If you have not downloaded the images of the Niger Inland Delta yet, do so now.
Open the images. Choose File>Open. A dialogue box will pop up. Choose the folder Niger and select the first image Nigerdelta_Landsat_Band_1.tif. Open all the other single band images, too.
All the images need to be improved in brightness and contrast.
Select (activate) the first image Nigerdelta_Landsat_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 (visible light ranges from 0.45 µm to 0.7 µm).
There are so 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 or objects appear to us in different colours when illuminated with light (including infrared). In order to distinguish objects by their colour, we need an instrument capable of covering many bands, as well as all visible colours, including the infrared sector.
Now have a look at the images and group them into two categories of similar appearance. Note the water and the north-western 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 of Nigerdelta_Landsat_Band_61.tif and Nigerdelta_Landsat_Band_62.tif (Landsat ETM band 6) if you compare them with the other images?
What is the special feature of the Nigerdelta_Landsat_Band_8.tif image, taken by Landsat ETM band 8?
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