5. Implementation Using Image Data (4/4)

How do you do this in ILWIS? Import the bindslev_crops, bindslev_rvi and bindslev_refl data files by using Import Using GDAL in ILWIS. You will see them displayed in the directory when Navigator is active. Check that they have been imported correctly by displaying them. The bindslev_crops image will display in three colours; green has value 1 and represents fields of Spring Barley, whilst red has value 2 and represents fields of Winter Wheat. The bindslev_refl image must be created as a colour composite if this has not happened, and it will display with band 2 in red and band 1 in both blue and green. The bindslev_rvi should be displayed using the Finegrey Table rather than the Pseudo colour table.

Now to use these to create an image of GLAI, highlight the Operations List and activate Map Calculation. A window opens. In the Expression Text Box, type in the following expression

IFF((bindslev_crop = 1), 0.254203·bindslev_rvi - 0.4335,
IFF((bindslev_crop = 2), 0.162576·bindslev_rvi + 0.2882, -0.1))

IFF(a,b,c) is a command in which b is acted upon if a is true, otherwise c is acted upon. In this case, we have used a nest of two IFF(a,b,c) commands, and any pixel in bindslev_crops that is neither 1 or 2 will be given the value -0.1.

Once the Expression has been completed, enter the output name of bindslev-glai and display the result. The output is automatically saved under the name that you have given it. You can compare your result with the attached file, bindslev_glai that you can also import into ILWIS.

Exercises

  1. Use the Map Calculator to derive RVI values and compare them with the bindslev_rvi image.


  2. Strictly, neither RVI nor GLAI can be negative, yet the regression equation for Spring Barley could give you negative GLAI values when RVI is very low. Modify the expression so that negative values are set to zero. (Clue - Replace the Spring Barley regression equation with a third embedded IFF(a,b,c) command in which a and b or a and c include this regression equation, depending on the test, a).


  3. There are other Spring Barley and Winter Wheat fields in this image. Use ILWIS to classify all of the fields that are similar to Winter Wheat and Spring Barley, and then repeat the estimation of GLAI.