Calculating volume difference

A volume difference calculation is used in situations where you want to know the excavation or fill of areas. This is useful in mining for example. It requires you to have georeferenced, scaled DEM data of the same area at different points in time.

In QGIS, you can use the "raster calculator" tool. You should have 2 DEM Tiffs that are georeferenced. Then in the raster calculator, you just subtract one DEM from the other. In this subtraction, you'd typically apply a formula where the old is subtracted from the new: "new@1" - "old@1"

This gives you a new DEM which demonstrates the "fills" as positive numbers and the "excavations" as negative numbers. It probably looks mostly black at the start, because the styling hasn't been set. So you need to do a couple of things to be able to use this as a 2D map:

• In the "transparency" tab, you can set ranges for pixels that should appear as transparent. This is useful, because no DEM is 100% correctly registered. Here you can thus deal with slight discrepancies and only show values > x cm of elevation difference. You can also set a different "no data" number there, like 0, which is another way to set transparency on specific values.
• Reload the min/max values for the dataset. This way, you can use a pseudo-color visualization style and classify ranges of elevation difference with different colors. This is a very powerful way to show where material was removed and added on a 2D map and the intensity of that removal.