Kagoshima University Research Center for the Pacific Islands Occasional Papers No.39, 65-71, 2003
Section 2, Report 6. The Progress Report of the 2000 and 2001 Survey of the Research Project“Social Homeostasis of Small Islands in an Island-zone”

65

PROBABILITY OF MONITORING AND ANALYZING SEA SHOREENVIRONMENT IN YAP USING SATELLITE DATAISHIGURO Etsuji1), KASHIWAGI Sumitaka1) , KIKUKAWA Hiroyuki2) ,HIGASHI Masataka2) , YOSHINAGA Keisuke2) , FUKUDA Ryuji2), andMORIYAMA Masao3)

Abstract
Remote sensing technique is the effective technology for monitoring and analyzing environmentalchanges on the Earth surface. We focused to clarify the environmental changes in the Yap islands andUlithi atoll using satellite data, especially the distribution of water depth around the Yap islands andin the Ulithi atoll. One of these results was compared with the environment of near shore of Japan. To
serve our purpose, the sea-truth data were collected using portable spectrometer.
Keywords: remote sensing, estimation of sea depth, turbidity, sea-shore

Introduction
Remote sensing is the effective procedure to analyze and to monitor the environmental
changes because of its potentials of monitoring the large area at the same time and of the timeseries analyzing. Many researchers and we have been clarifying several studies using satellite
2-6, 9-11, 13)
data. Especially, it was clarified that the water depth, shallower than 20 m, can be esti1, 7, 8, 12)mated by satellite data
. We reported that water depth shallower than 30 meters could be es7)
timated by the satellite data . If the distribution of water depth is determined, it will help native
people to easily find the waterway, and will help in evaluating the rising sea level in relation tothe greenhouse effect.
This study was carried out as a project of the Research Center for the Pacific Islands in
Kagoshima University. This project concerned clarify the self-reliance of islands, based on theYap Islands in Micronesia.

Principle of the Evaluating of Water Depth
It is assumed that the extinction coefficient of the sea water, α λ, is homogeneous verticallyat each point. At the sea surface, radiance intensity, I0, decreases in the sea water and becomes
I1 at the bottom. I 1 is expressed as I1 =I 0exp(-α λh), where h is the depth. As the radiance I1 is reflected by the sea-bed, I 1 becomes I 2, I21= γ λ I1, where γ λ denotes bottom reflection coefficient.
When the radiance reaches the sea surfaces, the intensity becomes I3, I3 =I 2exp(-α λh). Then I3 isexpressed as I 3= γ λ I0 exp(-2 α λh). I3/I 0 is the reflectance at the sea surface. If the extinction coef1)

Faculty of Agriculture, Kagoshima University. Kagoshima 890-0065, Japan.
Faculty of Fisheries, Kagoshima University. Kagoshima 890-0056, Japan.
3)
Faculty of Technology, Nagasaki University. Nagasaki 852-8521, Japan2)

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ISHIGURO Etsuji, KASHIWAGI Sumitaka, KIKUKAWA Hiroyuki, HIGASHI Masataka, YOSHINAGA Keisuke, FUKUDA Ryuji, and MORIYAMA Masao

ficient of the water collected at the Yap and reflectance is measured, the depth can be evaluated.

Methodology
Sea Truth
Actual sea depths were measured by throwing the rope from the boat at several points.
Spectral reflectances were measured by the handheld spectroradiometer (Model: 2703, Abesekkei Co. Japan), ranging from 400 nm to 1050 nm with 25 nm intervals. Sea sediments werecollected on the same points and spectral reflectances were measured by the handheld spectrometer. Seawater, in the middle depth of each sampling point, was collected at the same time,and extinction coefficients of water, from 400 nm to 1100 nm, were measured by the spectrometer (Model: 121-0001, Hitachi Co.). These points were recognized and recorded by the handheld GPS instruments (Model: GPS-315, Magellan Co.). These points were shown in Figs.1 and
2 respectively.

Fig. 1. Sea truth point at Yap islands

Fig. 2. Sea truth points at Ulithi atoll

Image Analysis
To date, many satellites have been observing the Earth surface. These satellite data havebeen received only bases, Hatoyama, Japan, Beijing, China etc. These data, observing Yapislands, have been limited in use. Moreover, reflected and emitted radiation from ground objects
cannot penetrate the clouds. From these reasons, we determined to use the satellite data ofLandsat-2/Multi Spectral Scanner (MSS), Landsat-5/Thematic Mapper (TM) and MOS1/MESSR.
Landsat-2/MSS data consists of 4 bands, i.e., Band-4 (0.5 to 0.6 µm), Band-5 (0.6 to 0.7µm), Band-6 (0.7 to 0.8 µm), and Band-7 (0.8 to 1.1 µm). Landsat-5/TM data consists of 7bands, Band-1 (0.45 to 0.52 µm), Band-2 (0.52 to 0.60 µm), Band-3 (0.63 to 0.69 µm), Band-4(0.76 to 0.90 µm), Band-5 (1.55 to 1.75 µm), Band-6 (10.40 to 12.50 µm), and Band-7 (2.08 to2.35 µm). On the other hand, MESSR is consists of 4 bands, Band-1 (0.51 to 0.59 µm), Band-2(0.61 to 0.69 µm), Band-3 (0.72 to 0.80 µm), and Band-4 (0.8 to 1.1 µm). It has to be noted that

PROBABILITY OF MONITORING AND ANALYZING SEA SHORE ENVIRONMENT IN YAP USING SATELLITE DATA

67

these observing regions slightly differed from satellite. These resolutions are 80 m, 30 m (exceptBand-6; Band-6 is 120 m) and 50 m for L-2/MSS L-5/TM and MESSR, respectively.

Results and Discussion
Characteristic of spectral reflectance of sea- sediment
The spectral reflectance of several sea-sediments, collected at many positions, is demonstrated in Fig. 3. As the collected sands differed in diameter according to sampling places, thespectral reflectance differed from collected place to places in the visible region. However, in theinfrared region, these differences were observed to be decreasing. The tendency, that is increasing the wavelength increasing the reflectance, is the same tendency of the soils on the earth
ground. The characteristics of spectral reflectance of coral differed according collection point.
That is, the coral collected at point-1 was active with green color and the coral collected at point43 was a white skeleton. The active coral had a small peak at 575 nm and showed high reflectance on the infrared region as is the same tendency with plants grown on the earth. On the otherhand, skeletoned coral showed low reflectance among observed regions and was decreasing withincreasing wavelength. The distribution of the sea sediments divided into five parts and these
spectral reflectances were determined with these measuring results.

Fig. 3. Spectral characteristic of sea sediments

Changes of spectral reflectance by the sea depth
Spectral reflectance was measured at each point. One of these examples is demonstrated inFig 4. In this point, sea depth was 35.3 m and Halimeda spp. were grown. Three measurements
were done and the variations were not large. The spectral reflectances increased with increasingwavelength.
The relationships between the depth and mean reflectance corresponding with Landsat/MSS
band was shown in Fig. 5. These relations are shown with the following equations.
Y4= -0.2066x+13.215, R2 =0.4074
2
Y5= -0.2018x+ 9.057, R =0.4476

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ISHIGURO Etsuji, KASHIWAGI Sumitaka, KIKUKAWA Hiroyuki, HIGASHI Masataka, YOSHINAGA Keisuke, FUKUDA Ryuji, and MORIYAMA Masao
2

Y6= -0.0456x+ 2.970, R =0.0726
Y7= -0.0318x+ 3.501, R2 =0.0297
Where Y is the mean reflectance of each MSS band and x is depth. In these cases on Ulithi
atoll, as the sea sediments were not divided, the correlation coefficients were very low. On theother hand, our previous work at Yap islands could be represented with the following equations.

Fig. 4. Changes of Spectral Reflectance at Poin
(35.3 m depth, Halimeda spp.)

Fig. 5. Relationships between sea depth and meanreflectance corresponded with Landsat/MSS band

PROBABILITY OF MONITORING AND ANALYZING SEA SHORE ENVIRONMENT IN YAP USING SATELLITE DATA69

On sandY4= -0.3718x+16.444, R2 =0.9743
2
Y5= -0.5308x+20.270, R =0.9643
2
Y6= -0.2804x+10.863, R =0.9480
Y7= -0.1358x+ 7.8323, R2=0.6736
On mud2
Y4= -0.0715x+6.4792, R =0.5901
Y5= -0.0993x+6.8819, R2 =0.7596
2
Y6= -0.0672x+4.7687, R =0.8609
2
Y7= -0.0095x+5.6078, R =0.0306
The decreasing rate in the case of sand is larger than the case of mud. These results corresponded to the principle of evaluating the water depth.
To compare the results of Yap and Ulithi, Ulithi’s data were not divided by type of sea sediment. To analyze the satellite data observed on Ulithi atoll, we would have to correlate these datawith sea sediments.
Extinction coefficient of the seawater
The relationships of the extinction coefficient of sea water collected at several points inUlithi atoll and wavelength were shown in Fig. 6. The extinction coefficients of sea water werecompared with distinguished water. As these data were very changeable values at each wavelength, the extinction coefficient was observed decreasing with increasing wavelength.
These coefficients were calculated with corresponding satellite bands in Table 2. In Yap(1999) data, mean distinction coefficients differed from satellite bands as the observing region
differed from satellite. These mean extinction coefficients decreased with increasing region. Thistendency agreed with the theory of Rayleigh scattering. Comparing Ulithi (2001) and Yap (1999)
in Landsat/MSS, the values of Ulithi were slightly smaller than the values of Yap. This showsthat the sea water in Ulithi atoll is more transparent than the Yap near shore. Izumi has the worst
transparency among the sites studied.

Fig. 6. Changes of extinction coefficient on several sea depth

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ISHIGURO Etsuji, KASHIWAGI Sumitaka, KIKUKAWA Hiroyuki, HIGASHI Masataka, YOSHINAGA Keisuke, FUKUDA Ryuji, and MORIYAMA Masao

Table 1. Spectral reflectance corresponded with Landsat/MSS on ea ach sediment
Bandregion (nm)Sand

Cactus(P42)

Cactus(P15)

Coral(P1)

Coral(P43)MSS4

400-500

39.78

5.70

41.15

13.00

5.70

MSS5
MSS6

500-600
600-700

46.21
55.51

2.14
0.68

54.33
59.73

17.39
24.42

2.19
0.68

MSS7

800-1100

70.33

1.06

79.83

82.27

1.06

Table 2. Comparing the mean extinction coefficients at each studying area
Band
MSS4
MSS5
MSS6
MSS7Region (nm)400〜500
500〜600
600〜700
800〜1100

Ulithi (2001)
0.00100.00140.00130.0006

Yap (1999)
0.00220.00160.00140.0009

TM1TM2
TM3
TM4

450〜520520〜600
630〜690760〜900Izumi (1997)
0.00800.00820.00700.0064

Yap (1999)
0.00270.00200.00150.0014

Conclusion
We focused on clarifying the environmental changes in Yap islands and Ulithi atoll using
satellite data, especially about the distribution of water depth around the Yap islands and in theUlithi atoll. One of these results was compared with the environment of near shore of Japan. This
study showed the following results.
1. The distribution of the sea sediments in Ulithi atoll divided into five parts and these spectral reflectancse were determined with these measuring results.
2. Relationships between the depth and mean reflectance correspondent with Landsat/MSS
band were determined. This shows the possibility of estimating the sea depth using satellite data.
3. The extinction coefficients were measured and compared with Yap, Ulithi and Izumi. It
was clarified that Izumi has the worst environmental conditions among them.
We are now in the process of collecting satellite data from NASA in USA and NASDA inJapan. We hope to have success in making visual maps. And these visual maps will provide theenvironmental changes during 30 years.

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