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As part of my thesis, I'm looking at the atmospheric and landscape effects of the 2018 wildfire in the Algarve. As part of the process, I want to estimate total biomass loss so I have classified two Landsat 8 images (pre and post-fire, 5 and 6 classes respectively) and performed a land cover change analysis using SCP plugin. I'm still fairly new to QGIS/remote sensing so am struggling to understand the output from the change detection and how I can produce a layer showing classes which have changed/calculating total biomass loss from the vegetation classes.

Dialog box Change Detection Change detection output

closed as too broad by Vince, nmtoken, whyzar, Fran Raga, HDunn Jul 17 at 9:00

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  • The key to understanding the 'change raster' is that you have a 'before raster' with 6 classes and a 'after raster' with 7. Now 6 * 7 = 42, which is the number of classes in your 'change raster'. As such, each of the classes in your 'change raster' corresponds to a combination of classes between 'before' and 'after'. Now you just need to spend time identifying which number in the 'change raster' corresponds to which pair of input classes. – Mikkel Lydholm Rasmussen Jul 18 at 10:43
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How to calculate change in total biomass

Use a spreadsheet to make the following calculations.

  1. Assign a biomass per square meter value to each landcover category. These are the fundamental assumptions on which your analysis is based, so they need to be the best estimates you can make. If your biomass per square meter values aren't reasonable, then your final calculated change in total biomass results will be equally unreasonable. Since you're doing a thesis on this topic, I assume you have enough information and knowledge to make this estimate. If not, that should probably be an entire separate question.
  2. For the pre-fire raster, multiply biomass per square meter by the area (in square meters) of that landcover type. This gives you total biomass in landcover type 1, total biomass in landcover type 2 etc. Repeat for the post-fire raster.
  3. Add up the total pre-fire biomass in landcover type x values calculated in step two. Repeat for post-fire values. This gives you total biomass pre-fire and total biomass post-fire.
  4. Subtract total biomass pre-fire from total biomass post-fire. This gives you change in total biomass.
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have a look at this:

https://docs.qgis.org/2.8/de/docs/training_manual/rasters/changing_symbology.html

choose singleband pseudocolor for your percentage band.

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