Exercise 1: Exploring the volcano
Landsat 7 image taken on 26 September 1999
Open the images in LEOworks
Click on File > Open. Choose the file named etna_1999-09-26_all.tif. A window appears showing the image preview. Click on > OK. The different files in this folder are the different bands on the Landsat image. Save the image with the band number and the suffix _stretched (for example etna_1999-09-26_band1.tif) by choosing Image > Save As. Repeat the same process for all bands (band 1, 2, 3, 4, 5, 6, 7, 8). You can also see the option to Crop (cut) images.
The images have previously been cropped to an extent of 58 x 60km (for comparison, look at the etna_landsat_large_1999-09-26.jpg image in the Etna_Landsat_GIS.zip folder, which has a size of approx. 185 x 185km). The image is already orthorectified. The reason the image is shifted is because the sensor that was recording the image is not pointing north.
You can see that the volcano is a very large natural phenomenon. This is one reason why satellite data is used for monitoring volcanoes. As it covers large areas, it can monitor large scale phenomena which cannot be observed from the ground.
Explore the image
Look at the different bands and try to zoom in and look for features connected to volcanic activity.
1. Why do you think they all look much darker than in the preview and have a low contrast? We will explore this later.
2. Which volcanic features can you detect? Can you find old lava flows, hot lava, craters, cones, a volcanic plume?
3.Can you find clear, visible differences between the bands? Pick an area (of what you believe is e.g. vegetation, lava…) and look at the grey values (from black to white) in different bands. Consider the principles of remote sensing on the Eduspace website and the Landsat bands (worksheet introduction) .
Click on the Histogram icon: The histogram shows the distribution of the grey values. As the image is recorded in an 8 bit format, each band consists of 256 grey values. On the horizontal axis the 256 grey values are displayed, and on the vertical axis, the number of pixels having this grey value. Only some parts of the available colour space from 0 - 255 are used. Using the cursor you can see the number of pixels and the percentage of a certain grey value (see tutorial p.16 onwards).
Click on the Image Information icon, and look at the image size (width and height in pixel = image coordinates), pixel size in metres, and projection (WGS 84, UTM 33N).
4. Do all bands have the same extension and pixel size?
5. Calculate the extension and area in metres for the image.
This is easier to do if you use the measurement tool. Go to Image > Measurement Tool and draw four lines at the borders of the image, until you arrive at the starting point. You can get an idea of how large your area is. You can switch between displaying the values in pixels or in metres. To get information about a single pixel you can use the cursor. Go to View > Curser Position/Value. The window displays the image coordinates, grey value, UTM coordinates, and geographic coordinates.
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