Exercise 5 - Thorung La Region - Continued
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|  | Thorong La at 5,416 metres | |
Differentiation between snow/ice and clouds or pale rocks using a near-natural colour image is sometimes not so easy. Snow and clouds may have the same or a similar reflection coefficient in the visible bands, so that they appear white.
However, as you noted in the previous exercise, snow and ice produce almost no reflection of irradiated light in the middle infrared (band 5), whereas clouds reflect strongly here as well. A false-colour combination using at least one middle infrared should therefore be a good way of assessing the area covered by snow and ice. This is what we will look at now.
To prepare the false colour image, we will use Image>Combine From>[Red Green Blue] and select bands 4, 5 and 2.
Have a look at the false-colour image.
What colour represents the areas that are covered by ice/snow?
What about water surfaces? Compare this with the 3-2-1 combination you opened before.
In order to optimise the appearance of the image, we must enhance the colours. Choose Enhance>Interactive Stretching and drag the colour bars in the histogram towards the centre until the overall impression of the image is good.
Once your result satisfies you, save it to your folder 'Annapurna' as anapurna_2000_452.tif.
Ice and snow can be clearly identified by their colour, which is somewhere between magenta and violet. Bare rock shows up in green, and vegetation in a red-green tone.
What percentage of the image is covered by snow or ice?
Where in the image can we find vegetation?
Is there a difference between north-facing and south-facing slopes?
Moraines consist of rocks and gravel, transported from the mountains for kilometres by the glaciers. They are also what is left when glaciers retreat. The lateral moraine indicates the maximum lateral extent of the glacier, the end moraine the maximum length of the glacier, and the area formerly covered by the glacier is filled with the ground moraine. When the valley is illuminated from the side, the moraine can be identified easily by the shadows within its structure.
Where in the image can we find lateral moraine?
Where in the image can we find ground moraine?
Describe the shape (cross section) of a typical valley formerly filled by a glacier.
Let's now concentrate on a glacier visible north of the image centre, which flows from the mountain massif to the west/northwest.
What is the length of the glacier?
What is the length of the ground moraine below the glacier?
What is the length of the side moraine below the glacier?
Now that we know what moraine looks like, we are able to draw a geomorphological map of the region. This is prepared by printing the image anapurna_2000_452.tif you saved before.
We now lay some tracing paper over the printed satellite image. On this paper we draw the following features:
- the river courses in blue,
- the mountain ridges in black,
- the lateral moraine in brown, and
- the end moraine in magenta.
In a final step we delineate the glaciers, paint them in a light blue and indicate the flow direction for the glaciers in blue.
Look at the map that has been prepared this way. What is the path taken by water falling as snow on top of the mountains?
How is the lateral moraine distributed over the image?
What percentage of the image area has been formed by glaciers?
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