Using the matched image coordinates of the homologous points
The DEMs generated from the pre- and post-seismic IKONOS
stereo images are then differenced. The differenced DEM, which
provides the height difference for the buildings between pre- and
post-seismic times, is used to determine the region of collapsed
buildings. At the same time, in digital topographical maps for the
time before the earthquake, there are plenty of ground control
points and benchmark points with 3D coordinates located in the
streets and building areas. Furthermore, the elevations of the
building roofs are calculated based on the elevations of the ground
points and the heights of the buildings. With the 3D coordinates of
ground points and building roofs obtained from the digital topographical
map, the pre-seismic DEM based on the topological
map is generated accordingly. Therefore, we have two kinds of difference
DEMs. The first one is the difference between the DEMs for
the pre- and post-seismic IKONOS stereo images, and the second is
the difference between the DEMs for the pre-seismic topographical
map and the post-seismic IKONOS stereo images. A detailed analysis
of the result of determining the region of the collapsed buildings
by differencing the DEMs is presented in the next section.