for pixel vector (130, 100, 70, 76) the code will be 222220.
Because the reflectance of clear water is high in green band, low
in red and near infrared and almost absorbed in short wave
infrared band therefore it is always b,>b;, b1>b4, b,>b3, b;»b;,
so the Cy13, C4, C23, C24 are always 2. For turbid water it can
happen that reflectance in red band due to sedimentation is
higher than reflectance in green band. Based on experiments the
author found that water body is involved mainly in pattern
222222, 022222, and 002222. Modulation of spectral
reflectance curve itself is not enough for water extraction
because there are many spectral mixtures that have the same
spectral reflectance as water. To separate water from possible
mixtures the ratio of green / SWIR, red /SWIR or reflectance
value of the SWIR could be used depending on type of water:
clear or turbid (organic matter contaminated).
2.2 Data Used for Experiments
160
140 :
Niadulation
wie OTIS
Figure 1. Three main modulations of spectral reflectance curve
for water body
X&
West Lake
Clear water
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
To demonstrate automatic water body extraction the Landsat
TM scene 127-45 observed on 27/12/1993 and Spot 5 scene
269-307 observed on 24/06/2003 have been used as input data.
Image windows of Hanoi area have been cut from full scenes
for study. The calibration coefficients gain and bias for
conversion from DN to reflectance are given in the metadata
files along with the image data. Only 4 bands of Landsat 5 TM
image have been used. The bands 2, 3, 4 and 5 of Landsat 5 TM
and bands 1, 2, 3 and 4 of Spot 5 HRG have almost the same
spectral sensitivity as showed in Table 1.
Landsat 5 TM sensor SPOT 5 HRG sensor
Band number Sensitivity Band number Sensitivity
(pm) (pm)
1 0.45-052 N/A N/A
2 0.52-0.6 1 0,50 - 0,59
3 0.63-0.69 2 0,61 - 0,68
4 0.76-0.9 3 0,79 - 0,89
5 1.55-1.75 4 1,58 - 1,75
6 10.4-12.5 N/A N/A
7 2.08-2.35 N/A N/A
Table 1. Spectral characteristics of Landsat 5 TM and SPOT 5
HRG
Figure 2 displays windows of Landsat 5 TM and Spot 5 HRG
respectively for study area. Colour composition is red=SWIR,
green=NIR, blue=red. The nominal spectral sensitivities of both
sensors TM and HRG are almost the same but in practice we
can observe differences in image data.
Red River
Turbid
water
Figure 2. Study area by Landsat 5 TM (left) and Spot 5 (right)
Pixels which have spectral reflectance curve with modulation
222222 are showed on Figure 3. In this modulation while the
Spot 5 image shows almost only clear water bodies like lakes
and ponds in the study area, the Landsat 5 TM image display
beside clear water also many other ground objects which are
mostly infrastructure and construction. The largest water body
in these images is the West Lake which is natural lake in Hanoi.
On the TM image the mixture of urban and construction appears
in dark purple colour. We can see few pixels along the Red
River in blue colour. The Spot 5 image in this spectral pattern
displays more correctly clear water bodies. We do not see
almost any mixture at all.
The pixels with modulation 022222 for Spot 5 image show
some ponds and lakes together with mixtures of construction
objects. However the Landsat 5 TM image tends higher mixture
rate when comparing to Spot 5 data. Modulation 022222 of
Landsat 5 TM shows turbid water, sandy surface, irrigated
paddy field and also construction.
Though the spectral sensitivity of both sensors of Landsat 5 TM
and Spot 5 HRG is almost the same for 4 bands from green to
SWIR but image data created by them are quite different in
Inte
implemet
different
of the sei
in Lands
Spot 5
022222,
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Figure
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