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Using the Swipe tool to compare thematic imagery

by sbent on ‎02-26-2019 02:16 PM (193 Views)

Question

No observable differences can be found when visually comparing two change detection images using the Swipe tool. The images are Highlight Change files created with the Change Detection Using Image Difference tool.

ERDAS IMAGINE > Raster tab > Change Detection Tools menu > Image Difference

 

If the images are displayed in separate 2D Views they each appear as expected, but when the images overlay one another in the same 2D View no differences can be observed when using the Swipe tool if the image with less change is displayed on top.

 

compare1.png

 

compare2.png

 

Answer

The phenomenon witnessed here is due to the opacity setting for the other classes in the change images (pixel values 2, 3, 4). Only classes 1 (Decreased change) and 5 (Increased change) have their opacity values set to 1. The other classes (2=Some Decrease, 3=Unchanged, 4=Some Increase) have opacity values set to 0 which makes them transparent. If you have the image with “less” change displayed below the image with “more” change you will not discern any differences when using the Swipe tool. But if you have the image with “less” change displayed on top then you should see the differences when comparing them with the Swipe tool.

 

If you change the opacity values to 1 for all of the classes of change images you will always notice a change when using the Swipe tool. You can also change the colors for these classes to something other than black if you wish to see the areas that represent some decrease change (pixel value 2) or unchanged (pixel value 3) or some increased change (pixel value 4). 

 

compare3.png

Open the image’s raster attribute table to change the opacity values from 0 to 1.
ERDAS IMAGINE > Table tab > View group > Show Attributes 

 

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