Comparing with
the two independent sets of results, we found that the area
differences were less than 10% with the same mapping
standard. The resultant area from TM/ETM+ is bigger than
that from Google Earth. There are four possible reasons.
First, mixture effects cause some inaccuracies and make
the boundaries a little bigger in the coarser resolution
TM/ETM+ data. Second, bare land was confused with urban
areas in TM/ETM+ data and was interpreted as urban areas.
Third, in the rural-urban fringes, some rural areas were interpreted
as urban areas due to low spatial resolution in the
TM/ETM+ data. Fourth, in the Yangtze delta, it is difficult
to determine the boundary of urban areas due to high river
density and connected build-up areas along the water network
(Figure 2(d)).