You can clearly read the altitude of the buildings. However, also in the streets there is a difference, which should not be there, but is due to inaccuracies in both DEMs.
We have calculated that the average difference is 3 meters. We will use this value to reduce the map Difference. Naturally this is a major simplification, as the difference is not always the same. We can also use the information from the urban landuse types related to the mapping units, as we can assume that in the vacant areas, river and parks there are no buildings. Also we can mask out the areas occupied by the streets.