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7.0.0.0.0.0.1 Analysis of vegetation index and rainfall estimate images
Learning objectives and requirements
Objectives
  • Accessing vegetation index and rainfall estimate time series
  • Producing a difference image
  • Analytical interpretation of NDVI (Normalised Difference Vegetation Index) and RFE (Rainfall Estimate) images
Climatic conditions of Burkina Faso Image interpretation



Why difference images?
3. Creation of difference images

3.1 What are the uses of difference images ?

A difference image displays the difference between the values of two images. It is a newly created image, showing the result of a subtraction of one image from another. There are several applications:
  • Computing the difference between an NDVI image of a certain dekad and a reference image (e.g. the historical average) of the same dekad allows to assess positive or negative deviations of the growing season. Instead of using dekad images you can also use monthly images.
  • Calculation of an increase in the NDVI for two consecutive dekadal images means a vegetation growth in the first dekad. This can be caused by a rainfall event in the same or, most likely, in the previous dekad.
  • The difference between representative and comparable images can be calculated to detect changes in vegetation cover over years.

A difference image can be created with the Diagnostic functions build into the IDV as introduced at the beginning of this exercise. Remember that for drawing conclusions on structural changes such as degradation, desertification, etc. the rainfall conditions at both dates have to correspond.
Difference image = Image A - Image B

Where A = recent image
B = reference image
To create a formula select the Edit->Formulas->Create Formula menu item. This brings up the Formula Editor dialog box used to define a formula as discussed in Formulas and Functions.

Once you have defined an IDV formula, it is saved and will appear in future runs of your IDV.

The difference image can be displayed in the same way as described above. However since the image values are the result of the subtraction they are not the same as the original NDVI values. Consequently, the color table should be adjusted. A "NDVI difference" color table could be used here.

Never confuse the input images!!


Colors of output image
When creating difference images always keep in mind the following rules:

  • If the NDVI value for a pixel in an image for a specific dekad is higher than the average situation for a pixel, than you should have a positive resulting value for that pixel in your difference image. If you would confuse the image A and B, the result would be negative!.
  • A positive difference, which means increase should always be displayed in green, as green is analog to something positive. A negative difference, which means a decrease, should always be displayed in red (perceived as signal for danger or something negative as with traffic lights).
An example difference image Difference of the 1996 season with regard to the 1982-1995 average

NDVI Difference Burkina Faso
Question 8
Suppose you wish to calculate the increment (increase) of the NDVI for the second half of July 1997, from one dekad to the next. Give the one-line mathematical expression that should be used in the formula editor to perform this calculation. In other words: construct a simple formula using variables A and B to calculate a difference in NDVI values. Let A represent the latest dekad and B the previous dekad.

Formula   

 


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U09-NRM-127: The role of Distributed Data Access Technologies in NRM - for ITC-IDV version 2.7 > Thematic Expert Models > Food security > Remotely Sensed Indices > NDVI > Exercises > Indexed approaches to monitoring of Crops, Rangeland and Food Security at National Level