|
|
Exercise 3: NDVI dynamics by biogeographical zones (continued)  Part 2: Comparison of biogeographical zones over time Use the image histogram to obtain useful information about NDVI behaviour. Select an image, click on the historgram icon, and the histogram will appear (or click on View/Histogram). The histogram for the month of June shows that most of the data accumulated ranges from 100 to 240. For the rest of the values (0-99) there are almost no pixels. Other interesting information include the minimum, maximum, mean, standard deviation, median, etc.
mean_NDVI/255 = real_mean_NDVI
This must be done for all mean values collected. You can then create the graph. In the abscissa (‘x’) put the time lag (months), and in the ordinate (‘y’) put the NDVI values. Repeat the process for any type of vegetation cover you want.
1. Describe the variation of NDVI over the year for the selected bio-zones. 2. Compare the results with the description of the bio-zones in the background articles. Are the results of your work as expected? 3. Can you explain the seasonal variations? Part 3: Analysis of horizontal and vertical profiles Another way to analyse data is to study the horizontal and vertical profiles. After selecting the image, go to Image/Profile X (horizontal) or Image/Profile Y (vertical). Two new windows will appear with the values of the NDVI in the row or column selected. What is shown in the figure is how the NDVI value drops when there is a channel of the river, for instance.
1. Analyse selected image files in the same way. Try to give explanations for the variation of the NDVI along the profiles.
2. Find out how latitude plays an important role by analysing areas relatively less vegetated nearer the Equator, where the area is more affected by the effects of the Intertropical Convergence Zone (ITCZ) which moves its position to the northern hemisphere. Last update: 16 April 2013
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
