the NDWI also produces a positive value for built-up land just
as for water. Table 3 shows that the built-up land class in
both Quanzhou and Fuzhou images has positive mean
values. To remedy this problem, Xu (2005) modified the
NDWI by using a middle infrared (MIR) band such as TM5 to
substitute the NIR band in the NDWI. The modified NDWI
(MNDWI) is expressed as follows:
.
(3)
Figure 3 shows that the mean of TM5 is much greater
than that of TM2. Therefore, if a MIR band is used instead of a
NIR band, the built-up land should have negative values while
keeping the water values positive. Accordingly, the enhanced
water features will no longer have built-up land noise in a
MNDWI image. This substitution has no impact on vegetation,
as vegetation still has negative value when calculated using
Equation 3 (Figure 3). Therefore, this test employed MNDWI
instead of NDWI to enhance water features in the built-up
land-dominated urban area (Figure 1c and 1g).