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Detecting change between two datasets
Detecting change between two datasets

How to detect and measure changes between the greenness index, DSM, and DTM from to different dates / times.

Karen Joyce avatar
Written by Karen Joyce
Updated over a week ago

Who can use this feature

Editors of a project within a Professional or Pro + workspace.

If you're capturing data over the same location over time, chances are you are keen to assess the amount of change on the ground. The best way to measure changes between two dates is to use the detect change tool.

Applying the detect change tool

  1. Select your two datasets in the table of contents. These need to have an area of overlap.

  2. Click the Detect change icon on the top menu bar.

  3. Select the layer/s that you would like to use in the calculation. You can choose from the greenness index, DSM, DTM, or height.

  4. Click run.

A new dataset group will appear in the table of contents and be visible on your map.

You can also access the Detect change tool the following ways:

  1. Right click on one of the selected datasets

  2. Go to Time series

  3. Select Detect change

OR

  1. Open the toolbox on the top menu bar

  2. Go to Time series

  3. Select Detect change

How is the change dataset calculated?

By default, the most recent dataset will be listed as 'dataset 1', and the older dataset will be 'dataset 2'.

The algorithm then simply subtracts the value in every pixel in dataset 2 from the corresponding one in dataset 1.

Example 1

The value of a pixel in my DSM from data captured today is 10m.

The value of a pixel in my DSM from data captured six months ago is 11.

Today's dataset would be considered dataset 1, and the one from six months ago would default as dataset 2.

The 'change' would therefore be 10-11 = -1. As a negative value, this indicates a decrease in elevation. This could be an indication of recent erosion (of course, dataset dependent).

This calculation is applied to every pixel in the image to show the change across the entire scene.

Example 2

The value of a pixel in my greenness index from data captured today is 0.3.

The value of a pixel in my DSM from data captured six months ago is 0.25.

Today's dataset would be considered dataset 1, and the one from six months ago would default as dataset 2.

The 'change' would therefore be 0.3 - 0.25 = +0.05. As a positive value, this indicates an increase greenness. This could be an indication of increased vegetation vitality or biomass (of course, dataset dependent).

Alternatively, you can use the following visualization and exploration tools that are available for all uses on the GeoNadir platform regardless of subscription level.

Other options

Who can use this feature

Viewers, Commenters and Editors of a project within an Essential, Professional, or Pro + workspace.

Toggle visibility or change layer transparency

As you hover over a layer in the table of contents you will see a slider and an eyeball to the right. Move the slider to change transparency (all the way to the left is completely transparent). Toggle the eyeball open or closed to change its visibility.

These simple tools allow you to quickly view the difference between one layer and the one below it.

Compare layers tool

  1. Select a layer in the table of contents.

  2. Click the Compare icon on the top menu bar.

  3. Swipe from left to right to reveal the differences between the top layer and the one below.

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