Kagoshima University Research Center for the Pacific Islands, Occasional Papers No.34, 77-85, 2001
Part 1, Section 2, Report 2. The Progress Report of the 1999 Survey of the Research Project“Social Homeostasis of Small Islands in an Island-zone”

77

STUDIES ON THE EVALUATION OF WATER DEPTH AROUNDSEASHORE AND THE LAND CLASSIFICATION IN YAP ISLANDSUSING SATELLITE DATA
Etsuji ISHIGURO, Katsumi TATSUNO, Motoi KAWAKATSU, Shinsaku HIRAYAMA,Din Ara WAHID, Sentaro KONGO, Kiyoshi SHIMADA, Masataka HIGASHI,Akimasa HABANO, Takafumi AZUMA, Hiroyuki KIKUKAWA, Masao MORIYAMA,Andy TAIFILEICHIG, Patrick P. PECKALIBE , Tamdad SULOG, Francis LIYEG,
and Edward Tukasa KANEMASU

Abstract
Remote sensing technique is an effective technology for monitoring and analyzing environmental
changes on the Earth surface. We have tried to clarify environmental changes in Yap islands using satellite data, especially, on the distribution of water depth around the Yap islands and land classification.
To serve our purpose, a sea-truth and ground truth data were collected, using a portable spectrometerand a digital camera.
Key words: remote sensing, water depth, land classification, monitoring

Introduction
This project is a joint program with the Research Center for the Pacific Islands in
Kagoshima University. The project seeks to enhanced the self-reliance of Yap Islands inMicronesia. Remote sensing is an effective procedure simultaneously to analyze and to monitor
environmental changes because of its potentials to cover a large area and generate time series
2-6, 9-11, 13)analysis. Many researchers have been undertaking studies using satellite data. Especially,
it was clarified that the water depth, shallower than 20m can be estimated by satellite
data1, 7, 8, 12) .
If the distribution of water depth map can readily indicate a waterway easily for a native
person and also evaluate the rising sea surface for purpose of understanding the greenhouseeffect. And if a method of classification the land can be achieved, it will be possible to evaluate
and analyze biomass and surface temperatures on the ground and thus the potential of yield of
several crops. Our primary results highlights these methods to be effective for the Yap Islands.

Principle of the Evaluating of Water DepthIt is assumed that the extinction coefficient of the sea water αλ, is homogeneous verticallyat a point. At the sea surface, radiance intensity, I0 , become I 1 at the bottom. I1 are expressed asI 1=I 0exp(- αλh), where h is the depth. As the radiance I1 is reflected by the sea-bed, I1 becomes
I 2, I 2=γλI 1, where γλ denotes bottom reflection coefficient. When the radiance reaches at the seasurfaces, the intensity becomes I 3, I3 =I 2exp(- αλh). Then I 3 is expressed as I 3= γλI 0 exp(-2 αλh).
I 3/I0 is the reflectance at the sea surface. If the extinction coefficient of the water collected at the78

Etsuji I SHIGURO et al.

Yap and reflectance are measured, the depth will be evaluated.

Methodology
Sea Truth
Actual sea depths were measured by throwing a rope from our boat at several points.
Spectral reflectances were measured by a handheld spectroradiometer (Model: 2703, Abe-sekkei
Co. Japan), ranging from 400nm to 1050 nm at 25nm intervals. Sea sediments were collected at
the same points and spectral reflectance was measured by the handheld spectroradiometer.
Seawater, in the middle depths, was collected at the same time, and extinction coefficients ofwater at several wavelengths, from 400nm to 1100nm, were measured by the spectrometer(Model: 121-0001, Hitachi Co.). These points were recognized and recorded by handheld GPSinstruments (Model: GPS-315, Magellan Co.).
Ground Truth
Spectral reflectances of the leaves of several crops and trees, grown in the Yap islands, weremeasured by handheld spectroradiometer. Making a land classification map for Yap islands,
several points were surveyed. The data were recorded by digital camera and video camera. Thepoints were recognized and recorded by handheld GPS instruments.
Image Analysis
In recent times, many satellites have been observing the Earth surface. Satellite data for theWest Pacific have been received mainly at bases in Hatoyama, Japan, Beijing, China etc. Few
data based on observing Yap islands have been available. Moreover, reflected and emitted radiation from ground objects cannot penetrate clouds. For these reasons, we determined to use theFig. 1. Measured points in Yap.

Studies on the Evaluation of Water Depth Around Seashore and the Land Classification in Yap Islands Using Satellite Date79

satellite data of Landsat-2/MSS (L-2/MSS) and MOS-1/MESSR.
L-2/MSS data consist of 4 bands; Band-4 is from 0.5 m to 0.6 m, Band-5 is from 0.6 mto 0.7 m, Band-6 is from 0.7 m to 0.8 m, and Band-7 is from 0.8 m to 1.1 m. MESSR consistof 4 bands, too, but it regions are slightly different, Band-1 is from 0.51 m to 0.59 m, Band-2
is from 0.61 m to 0.69 m, Band-3 is from 0.72 m to 0.80 m, and Band-4 is from 0.8 m to 1.1
m. The resolutions are 80m and 50m for L-2/MSS and MESSR, respectively.

Results and Discussion
Spectral reflectance of sea-sediment
Spectral reflectance of several sea-sediments, collected at many positions, is shown in Fig.2.
In the case of sea-grass, the spectral reflectance curves have a low peak at 575 nm, a red-edge
between 600nm to 700nm and a higher reflectance in the infrared region. This is the same tendency as for plants grown on land. For mud and sand, spectral reflectance increased with increased wavelength. These results are the same as on bare land.
Coral and sea-grass are randomly distributed on the seabed around these islands.
Moreover, we found sludge in some parts of the area. If the seabed was classified into several categories, it become possible to estimate sea-depth by adapting the reflectances of eachcategory.
These measured reflectances were divided into corresponding bands of satellite calculated.
The result is shown in Table 1.

Fig. 2. Reflectance on several measured points.

Etsuji I SHIGURO et al.

80Table 1. Characteristics of measured pointsPointDepth (m)Sediment

St-5

1.1

Sand + Sea grass

St-12

1.75

SandSt-14

8.0

MudSt-18

2.5

Sand + Mud

St-19

9.0

White sandSt-21

2.4

ClaySt-22

21.0

Coral + Sand

St-26

6.5

CoralSt-30

13.5

Mud + ClayFig. 3. Relationships between reflectance and depth at each MSS Band on sand.

Changes of spectral reflectance with sea depth
Relationships between the spectral reflectance of sand and mud, which correspond toLasndsat-2/MSS and water-depth, are shown in Figs.3 and 4, respectively. These relationships
are indicated by the following equations.
On sand2
B4: y=-0.3718x+16.444, R =0.9743
2
B5: y=-0.5308x+20.270, R =0.9643
B6: y=-0.2804x+10.863, R2=0.9480
2
B7: y=-0.1358x+7.8323, R =0.6736

Studies on the Evaluation of Water Depth Around Seashore and the Land Classification in Yap Islands Using Satellite Date81

Fig. 4. Relationships between reflectance and depth at each MSS band on mud.

On mud2
B4: y=-0.0715x+6.4792, R =0.5901
B5: y=-0.0993x+6.8819, R2=0.7596
2B6: y=-0.0672x+4.7687, R =0.8609
2
B7: y=-0.0095x+5.6078, R =0.0306
Where y is the mean reflectance of each MSS band and x is depth. The decreasing rate in
the case of sand is larger than in the case of mud. These results correspond to the principle ofevaluating the water depth.
Extinction coefficient of the seawater
The absorbencies of the collected water were changed at wavelength and collection points
as shown in Fig.5. As wavelength increase absorbance decreases. At 900 nm, it shows the bottom for all curves as the typical absorbance of the water. From this curve, the extinction coefficients of seawater corresponding to the L-2/MSS band was calculated as shown in Table 2.
Table 2 shows the extinction coefficient collected on Yap and at Izumi, Kagoshima, Japan,
for corresponding bands with L-2/MSS and L-5/TM, respectively. It is evident from Table 2 thatthe values of extinction coefficients decrease with increasing wavelength, which accords with theRayleigh Scattering Theory.
Distribution of the water-depth
The distribution of sea-water-depth at Izumi, Kagoshima Japan using L-5/TM data is shownin Fig.6. White and black color indicate shallow and deeper waters, respectively, from 0 to 20 m.

Etsuji I SHIGURO et al.

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Fig. 5. Changes of absorbance at each wavelength on collected sea water.
Table 2. Extinction coefficients of seawater
On Yap (’99 Oct.) for L-2/MSSBandWavelength

Extinction
coefficientBand-4500〜600nm

0.0022Band-5600〜700nm

0.0016

Band-6700〜800nm

0.0014Band-7800〜1100nm

0.0009On Izumi (’98 Oct.) for L-5/TM
Band-1450〜520nm

0.0080Band-2520〜600nm

0.0082Band-3630〜690nm0.0077

Band-4760〜900nm

0.0064The sea-truth points were recorded by GPS and actual depths were also measured. The linear
regression coefficient between depth and indices (color) was 0.8562. This procedure shows thepotential for measuring the sea-depth around Yap islands.
Spectral reflectance of crops
Spectral reflectances of the leaves of crops and trees grown in Yap islands are shown inStudies on the Evaluation of Water Depth Around Seashore and the Land Classification in Yap Islands Using Satellite DateFig. 6. Distribution of the water depth using by L-5/TM (1997 Oct).

Fig. 7. Spectral reflectance on several crops.

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Fig.7. In the visible region, differences of spectral reflectances are not recognized and lower reflectance for all crops. However, peaks occurred at about 575 nm. On the other hand, higher
reflectances in the near-infrared region were shown with different crops. These results show thepossibility of land-classification using satellite data.

Conclusion
Sea truth and ground truth show the possibility of estimating the sea-depth around the Yap
islands and also land-classification using satellite data.
We are now in the process of collecting satellite data from NASA in USA and NASDA in
Japan. We hope to succeed in making visual maps which will indicate environmental changes
over the passed during 30 years.

References
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