I have two raster classifications. They have the same number of classes which use the same ID (each class has a unique ID). I would like to get information on the changes between classes i.e., class 2 has changed to class 4 for polygon X. I would also like to calculate area for these changes (to be able to see which class has experienced the most change).

How would you do it in QGIS?

(The equivalent in ENVI is the 'Thematic change' tool)

1 Answer 1


The general name for the operation that will allow you to compare two classified images is cross tabulation or what is sometimes called a contingency table. This will allow you to calculate change in class values. In the SAGA toolbox of QGIS there is a tool called Cross-classification and tabulation that will perform this operation.

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  • That's brilliant, thank you! Just a quick question, could you possibly explain the Grid and Table produced in a bit more detail? I'm not sure I understand what the image is showing. Also, with the grid, how do you know which image stats are across and which are down? Sorry if that's obvious... Many thanks!
    – AMK
    Jan 29, 2015 at 9:33

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