Change detection is a powerful tool to compare two images.  Typically this will be a before/after pair. 

Change detection

TABLE OF CONTENTS

• Step 1: Understanding the general approach in Earth Blox
• Step 2: How to create the before and after images
• Step 3: Using the change detection block

Step 1: Understanding the general approach in Earth Blox

• For change detection in Earth Blox, we first create two images from two separate workflows.
• Each of these two new images is given an appropriate name using the block called Save image as:
• We then create a third workflow using the Change detection block to identify the differences between the two images.

Step 2: Create the before and after images

• Build a workflow to create the image output for the two images you want to compare. 
• (Refer back to the Building a workflow article if you need to be reminded how to build a workflow.) 
• Usually, these will be a before/after pair, but not always. 
• Add in a Save image block so that each workflow output has a name. 

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• Remember that you can use a right-hand mouse click on a workflow and choose Duplicate to copy an entire workflow. You can then simply edit the dates (or whatever parameter you want to change) on the duplicate. 
• Ensure you have given each output a different name. 

Step 3: Using the change detection block

• The Change detection block is found in the ANALYSE Toolbox and looks like this:

• The drop-down menu for Image 1 and Image 2 will update automatically as you label images in your previous workflows. 
• Choose the two images from the drop-down that you want to compare. 
• The default threshold is that more than a 5% difference is classified as "change".  
• You can apply a custom threshold by checking the box.  Note that by using a less-than (<) rather than a more-than (>) you can identify areas of no change. 
• RUN WORKFLOW will run the two workflows to create the two images and then run the change detection.  
• Both images and the change detection result will appear on the map.
• The Table and Histogram blocks are optional. These will appear in the DASHBOARD tab.

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