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QGIS — Differential Change Detection

TOOL
📖4 min read

Introduction​

Differential change detection compares a before image and an after image of the same place. Only what changed between the two dates survives the subtraction: destroyed structures, rubble piles, cleared vegetation, new construction. The output is a change layer — a map of what is different, and where.

In the QGIS workflow this sits beside the gather → prepare → export pipeline. It answers a different question: not "what does the site look like" but "what is different, and where". For a reconstruction, the answer decides what you model. The data is free (Sentinel-2 archive, 10 m, 5-day revisit) and the method is fully repeatable in open-source software.

Usage​

Run the workflow in this order.

Source the pair· Copernicus Browser

Download Sentinel-2 tiles of the same area from before and after the event, the same band set (RGB plus NIR for vegetation). Note the capture date, sensor, and cloud cover for each tile.

Load and align· same CRS

Add both tiles to one project. Both are already georeferenced, so no manual registration is needed. Confirm they sit in the same CRS.

Subtract· Raster Calculator

Raster → Raster Calculator. Subtract one band from its counterpart in the other tile. Unchanged ground cancels to near zero. The change layer is the remainder.

Threshold· isolate the signal

Most pixels are noise near zero. Threshold the result so only significant differences remain. The threshold is a judgment call — record the value you used.

Read the change layer· build the work list

What remains is your work list of what to model: footprints of lost structures, rubble extent, cleared areas. Digitize the features you care about into a vector layer.

Record the run· reproducibility

Save the QGIS project file and export the change raster and the vector layer. Log the tile dates, band set, threshold, and QGIS version. A third party must be able to re-run the same subtraction.

In an Investigation​

The change layer is evidence of what changed, not of who or how. Keep it honest in the record:

  • Method section: both tile capture dates, sensor, band set, CRS, threshold value, and the QGIS version used for the subtraction.
  • Decision log: why you picked these two dates (closest clear-sky pair around the event), and why you picked the threshold.
  • Verification: the saved QGIS project file plus the exported rasters let anyone re-run the exact subtraction. If a report leans on the change layer, the project file is the artifact that makes it checkable.

Into Blender​

The change layer does not become geometry by itself. It tells you what to build:

  • Import the digitized vector layer with BlenderGIS to place footprints of destroyed structures and rubble extent in the scene.
  • The change map drives scope: model what changed, reference what did not.
  • 10 m pixels mean a "building" in the change layer may cover several footprints. Treat the polygons as areas to investigate, not as walls to extrude.

Limitations​

  • 10 m resolution: small structures and single vehicles do not register.
  • Cloud cover limits usable before/after pairs. A pair from different seasons adds vegetation noise that is not damage.
  • The subtraction shows change, not cause. A cleared area may be demolition, construction, or damage.
  • For cloud-free, all-weather comparison, SAR coherence is the sibling method (see To be expanded).

To be expanded: NDVI-based comparison for vegetation, SAR coherence as a sibling page, one worked example with a real before/after pair.

Resources​

  • QGIS — the hub: installation, CRS, the gather → prepare → export pipeline
  • QGIS Baseline Setup — the stage before this one: basemap, site-centred orthographic CRS, origin point, capture regions
  • QGIS Data Acquisition & Export — basemaps, elevation, OpenStreetMap data, export as GeoTIFF or shapefile
  • Global Building Atlas — LoD1 building footprints over WFS, extruded in 3D, imported into Blender
  • Geolocation — establishing where the site is before you compare dates
  • BlenderGIS — the add-on that brings the exported QGIS data into Blender

Sources​

  1. Copernicus Browser — free Sentinel-2 tile browser
  2. QGIS Raster Calculator documentation — the subtraction step