Change detection is a powerful tool to compare two images.  Typically this will be comparing images (or data) over time.

A good place to start is to check there is change over time. The quickest way to do this is to use the Add Multitemporal Map Layer Block. This will highlight if and where there has been any change in the data over time. 

Change detection

TABLE OF CONTENTS

• Step 1: Use two different workflows
• Step 2: Add the Calculator Block

Step 1: Use two different workflows

• 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 Data for Re-use.
• For example, here are two Sentinel 1 workflows that calculate the mean value over June and July, but for different years:

Step 2: Add the Calculator Block

• Now add a calculator block to one of the workflows, and select the band you want to difference from the drop down menu. 
• Enter the calculation you want to apply (for a simple difference map, it is just subtracting one band from the other). 
• Your calculator block will look something like this:

• Save the new band to use later. 
• Note: when you are doing a difference, expect to get both positive and negative values, so adjust the contrast settings in the visualisation block accordingly. 

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