| | Exercise 1: Exploring the volcano - continued
Contrast enhancement
As mentioned before, the colour space is not completely used, and the images seem to have low contrast. We will therefore stretch the image. Stretching means to stretch the histogram and spread the values over a larger range of grey values (max. 0-255), which improves the contrast.
Pick one band. Go to Enhance > Interactive Stretching, and drag the blue and pink vertical lines to the borders of the histogram. If you open the histogram of the image again, the appearance will be different. The values have been stretched over the possible space of 256 grey values but now have gaps in between. Save the image with the band number and the suffix _stretched (for example etna_1999-09-26_band1_stretched.tif) by choosing Image > Save As.
LEOWorks is able to display the pixel values on a line along the X or Y axis of the images in a profile view. Go to Image > Profile X (or Profile Y, or both). Open the same band again without applying stretching (don't close the stretched one) and compare the profile to the one of the stretched image (see tutorial p.16).
Create a stretched image for each band and save it with the band number and the suffix _stretched. Leave the 7 new stretched images open and close the rest.
Band combinations and visual interpretation
Using combinations of three different bands which are assigned to be displayed in the colours red, green, and blue, we can create an RGB composite. A ‘Natural Colour’ or ’True Colour’ image shows objects in the same colours human eyes
normally see them. In a ’False Colour Composite’ (FCC) the colours have been assigned to any three different bands with different wavelengths that human eyes might not see.
Image > Combine from > RGB > 3-2-1 (= Natural colour composite)
Image > Combine from > RGB > 4-3-2 (= False colour composite)
You can save the images choosing Image > Save As.
 | | | FCC 3-2-1 of the Landsat image of 1999 |
In the natural colour composite (band 3-2-1) healthy vegetation is green, unhealthy vegetation is brown/yellow, urban areas grey/reddish/brown, roads are grey, water is blue, and clouds are white.
| | FCC 4-3-2 of the Landsat image of 1999 | |
In the false colour image (band 4-3-2), vegetation appears in shades of red (the near-IT band is assigned to red), urban areas are cyan blue, soils and bare land are browns or bluish, water is dark blue, and clouds are white.
6. Inspect the two images.
7. How does volcanic lava appear in the two images? Do you think that you can distinguish different types of lava very well?
8. Notice the cloud and its shadow, west of the summit.
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