Error matrices based on the building object (polygon) were produced for the two scenarios of the difference DEMs. Comparing with the pixel-based assessment method that classifies the difference DEM pixels into collapsed and not collapsed pixels by examining the height change of the building that a pixel represents, the advantage of the object-based assessment method is that the percentage of difference DEM pixels that fall inside a building polygon is used to determine whether a building has collapsed, which might provide a more realistic accuracy assessment of the detection of collapsed buildings.