九州大学大学院総合理工学府報告第 35 巻 第 2 号 7-13 頁 平成 26 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 35, No. 2, pp. 7-13, FEB, 2014

Compilation and validation of bathymetric data for theSouth China Sea with an emphasis on shallow region
Katsuto UEHARA*1,†
E-mail of corresponding author: uehara@riam.kyushu-u.ac.jp†(Received January 20, 2014, accepted January 22, 2014)
Bathymetric data with a resolution of 5 min called gbscs5 have been compiled for a shelf region
shallower than 200 m in the South China Sea by referring to paper charts, electronic charts, andtrackline sounding data. Through a comparison between the new dataset and existing datasets (etopo5,
etopo1, gebco08), characteristics of each dataset, including the existence of artificial features derived
from erroneous sounding data or a conversion error of depth units, were documented for each dataset.
It was found that small-scale bathymetric features observed along the shelf edge in gebco08
bathymetry are bounded by survey lines and were likely to be a spurious figure generated when
applying a high-order interpolation scheme. Tidal simulations conducted with changing bathymetries
have shown that the newly compiled dataset will produce smaller deviation from observed values thanthe existing bathymetries. While all four datasets analyzed in this study were found to be suitable for
estimating overall tidal features of the South China Sea, a care must be taken when applying the tidal
model results to a particular coastal region.
Key words: South China Sea, continental shelf, bathymetry, tidal model, internal tide generation

1.

Introduction

Ocean bathymetry is one of the major factors
which control dynamical processes such as
oceanic currents, coastal upwelling, tides,
tsunamis, and internal waves. Accurate water
depth is therefore essential when studying
ocean dynamics.
During the last 25 years, a number of digitalterrain models (DTMs) covering global lands
and oceans have been developed. While thequality and resolution of such datasets have
improved greatly for open oceans as satellitealtimetry data become available in the late
1990s1), large uncertainties still remain forbathymetries in shallow waters where thespatial scale of bottom features are small.
Further refinement of bathymetric dataset for
shallow regions is necessary to improve theoverall performance of ocean models.
In this study, bathymetry data of the South
China Sea are compiled for depths shallower
than 200 m and compared with existing
bathymetry data.
*1 Center for East Asian Ocean-Atmosphere Research,
Research Institute for Applied Mechanics

2.

Materials and methods

2.1 Existing bathymetry datasets
Three existing DTMs, etopo5, etopo1, andgebco08, which contain land altitudes andocean depths on a global longitude-latitude gridwere used for analyses. The study area ranges
from 99°E to 125°E and from 0° to 23°N (Fig. 1).

Fig.1

Map of the study region. Circles denote locations oftide-gauge stations used for analyses.

8Compilation of South China Sea bathymetric data

Etopo5 (Fig.2a) is a 5 min resolution dataset
developed in 1988 which is distributed by U.S.
National Geophysical Data Center (NGDC) in
National Oceanic and Atmospheric Administration (NOAA). Water depths of etopo5 arebased on ship sounding data. This dataset wassuperseded by a 1 min resolution etopo1 in2009 (Fig.2b)2). Ocean depths in etopo1 arederived mostly by satellite altimetry technique.
Gebco08 dataset was released in 2010 by
British Oceanographic Data Centre (BODC)
and has a spatial resolution of 0.5 min (Fig.2c).
It was generated by combining ship depth
sounding data with interpolation of sounding
points guided by satellite derived gravity data.
2.2 Gbscs5 bathymetry
The newly compiled 5 min resolution dataset(gbscs5; Fig.2d) was based on gebco08. Firstly,
the 0.5 min resolution gebco08 was remapped
onto a 5 min resolution grid compatible to thatof etopo5 (cell-registered configuration, i.e., celledges are along lines of latitude and longitude)
by taking average of the second and third
quadrants of 100 depth values allocated within
the enlarged grid cell (Fig. 2e).
Grid cell values were modified over an area
shown in Fig.2f, which includes a wide shelfwest of 110°E and an area along southernChinese coasts. The water depth of the revised
region was mostly shallower than 200 m, as ouraim was to improve the data quality in shallow
waters.
The revision was made by overlaying (a)scanned paper charts issued by U.S., Russia,
U.K., Vietnam, and Thailand, (b) electronic
charts (ENCs) supplied as a part of U.K.
Hydrographic Office (UKHO) Admiralty Vector
Chart System (AVCS) and South China Sea
Electronic Navigational Charts (SCS ENC) by
East Asia Hydrographic Commission (EAHC),and (c) trackline bathymetry data distributedat NGDC website which contain serialsounding records along survey sections, and (d)depth contours generated from gebco08
bathymetry for every 10 m from 0 m to 200 m
and for 500 m depth (displayed for reference
purpose only and not used for depth estimation)
on a single screen of a GIS software. A
representative depth at a certain grid cell was
estimated by referring to depth sounding
values appear inside the cell, normally bytaking an average of maximum and minimum
depths found in the cell.
Comparison between the depth sounding
values derived from charts and trackline datawith depth contours of gebco08 indicated thatthe number of depth points used to compile
gbscs5 was larger than that for gebco08, which
was probably because we employed large scalechartscontaining
detailedbathymetryespecially at nearshore regions.
Horizontal datum of scanned paper charts
were converted from local datum to WGS84
coordinates if necessary. Tidal correction was
not applied here because we could not obtainappropriate offset values to make a correction
at the current stage.
We did not employ an automated method
such as an interpolation scheme to obtain agridded dataset mainly for two reasons. Firstly,
it was found that navigational charts,
especially small-scale ones, tend to indicate
depth values at shallow shoals more frequently
than those in deep troughs or those at amoderate depth, so that a simple interpolationof depth values without considering such bias
often result in an underestimation of arepresentative depth.
Another reason was that the distribution ofavailable sounding points in the South China
Sea are generally not dense enough forautomated interpolation scheme to resolve
subgrid scale anisotropic features such as
narrow troughs and elongated ridges.
The low spatial resolution of the new dataset(5 min) compared to the existing ones (up to 0.5min) was also due to the overall sparseness ofthe depth sounding data, which is evident inregions south of Vietnam (especially in offshore
area between 4°N and 8°N), central Gulf ofThailand, Gulf of Tonkin west of 108°E or southof 18°N, and southern Taiwan Strait 118°E119.5°E and 22°N-23°N (Fig. 3).
2.3 Tidal experiment
A two dimensional finite difference model
was introduced to verify the performance of the
new bathymetry data. The model was forced by
specifying harmonic constants (M2, S2, N2, K2,
K1, O1, P1, and Q1 constituents) of a global tidal
model TPXO7.23) along open boundaries andalso by applying astronomical forcing (M2, S2,
K1, and O1). A series of numerical experiments
were conducted with different bathymetries(etopo5, etopo1, gebco08, and gbscs05; alldatasets were converted into 5 min resolution)but other conditions were kept unchanged.
Model results were compared against tidegauge data for M2 and K1 constituents at all
stations listed in Zu et al.4-5) (except for 5
stations situated north of 23°N) (see Fig. 1 forlocations). Each numerical run was conducted
for 222.625 days with a time step of 12 sec and平成 25 年度

Fig.2

九州大学大学院総合理工学報告第 35 巻第2号

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Depth contour of bathymetric datasets used in this study: (a) etopo5, (b) etopo1, (c) gebco08, (d)gbscs5, (e) gebco08 (converted to 5min resolution), and (f) difference between Figs. 2d and 2e. Contour
lines are drawn for (a-e) depth 0, 20, 50, 75, 100, 500, 1000, 2000 and 4000m or (f) 0 and 200m.

the last 182.625 days were used for analyses.
Quadratic bottom friction coefficient was set as0.0025 and horizontal eddy viscosity coefficientwas 100m/s2. Details of the model formulation
are found in other article6).

10Compilation of South China Sea bathymetric data

locations, such as those north of Gulf ofThailand, in southwestern Sumatra Island, in
the Mekong River delta, and along thenorthwestern coast of Gulf of Tonkin, grid pointvalues were padded with 0 m and coastlines
were not represented correctly. Furthermore,
depths at some coastal regions, such as thosealong the east coast of Laizhou Peninsula(110.3°E, 21°N), off the Pearl River mouth,
northeast and southwest of Palawan Island(120.5°E, 11°N and 117°E, 7.5°N), were paddedwith an erroneous value of 10 m.
In offshore regions, there were three majordifferences between etopo5 and gbscs5: (1)depth of etopo5 in the central part of Gulf ofThailand was deeper than that of gbscs5, (2) an
isolated shoal situated along the shelf edge inthe southwestern South China Sea (109.8°E,
5.6°N) was existent only in etopo5, and (3) the
depth of a shoal in the southeastern Taiwan
Strait (118°E, 22.7°N) was much shallower in
etopo5. We could not collect enough materials
to make further discussion on this discrepancy
because depth information was sparse in thesethree areas.
3.1.2
Fig.3 Screenshots of overlaid depth information derived
from paper charts, encs (dots and blue lines), andtrackline sounding data (thick brown lines) innorthwestern (upper panel, 105°E-121°E and 15°N23.5°N) and southwestern (lower panel, 98.5°E-115°E
and 0°-14°N) South China Sea. Red lattices denotelatitudes and longitudes in 0.5 degree interval, which
are six times coarser than the resolution of gbscs5.
Thin brown lines denote depth contour (every 10 m up
to 200 m and 500 m) derived from gebco08 bathymetry.

3.

Results

3.1

Comparison between depth datasets

Figure 2 shows the water depth of three
existing datasets as well as that of gbscs5
developed in this study. Though the overall
features were similar among the four datasets,
discrepancies were also evident. Here, we
confine our view to areas shallower than 200 m.
3.1.1

Etopo5The depth contour of etopo5 show patterns
similar to those of gbscs5 at depths between 20
m and 100 m (Figs. 2a and 2d) except that thecontour of etopo5 was generally smoother. The
resemblance is probably because both datasets
are based on ship sounding data. The contourof etopo5 is smooth because a smoothing filter
was applied when producing the grid data.
On the other hand, etopo5 seems to haveproblems with the coastline position and depth
shallower than 20 m. At lowlands in several

Etopo1The position of coastline in etopo1 was
improved significantly from that in etopo5 as
the former incorporates Global Self-consistent,
Hierarchical, High-resolution Shoreline data
(GSHHS) published by NGDC in 2003 (Fig.2b).
However, etopo1 still have small thoughsystematic errors at around shorelines adjacent
to lowlands such as Mekong and Pearl Riverdeltas and the northern coast of Gulf of Tonkin,so that a care must be taken when the dataset
is used with its original resolution.
It was also found that bottom features incoastal areas shallower than 50 m weresmoothed excessively and 20 m contour was
located much farther from the coast thanetopo5 and gbscs5. Anomalous shallow depths
were also found at Paracel (114.5°E, 16°N) andSpratly (116.5°E, 11.5°N) islands located in thecentral South China Sea and at a region
northwest of Palawan Island (120°E, 11°N).
While undulating features observed in the
Malacca Strait (102°E, 2°N) are obviously
artificial, we could not acquire enough material
to confirm the existence of two shoals observedin Gulf of Thailand (103.7°E, 6.5°N) and in Gulfof Tonkin (107.7°E, 19°), which were not foundin etopo5 and gbscs5.
3.1.3

Gebco08

Depth contours shown in Fig.2c indicate thatgebco08 bathymetry, which uses both sounding
and altimetry information, has more realistic

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九州大学大学院総合理工学報告第 35 巻第2号

11

Fig.4 (a) Location of Figs.4(b-f). (b) Overlay of land area derived from gebco08 (dark green) and from anautical chart (light green). (c) Anomalous depth (exceeding 2000m) observed in sounding data. Color
contours indicating the depth of gebco08 in 10 m intserval from 0 m to 200 m as well as 500 m, and aseries of anomalous sounding data exceeding 2000 m. (c) a circular 20 m contour of gebco08 crossing
two points showing 17 fathom (31 m) and 25.6 m depths. Bold figures (e.g., 25.6 and 29.2) indicate
depth in meters, obtained from different chart. (e) Depth contour of gebco08 (color contours) andsounding survey lines (thick brown lines) at a shelf edge south of Taiwan Strait (118°E-119.5°E, 22°N23°N). (f) Same as (e) except for a shelf edge south of Vietnam in the southwestern South China Sea.

coastline and higher resolution than etopo5(Fig.2a) and has more accurate bathymetry atshallow regions than etopo1 (Fig.2b). Depth
difference between gebco08 and gbscs5 was
found mainly in areas where the depth
sounding information was sparse (Fig.2f).
Irrespective of overall improvements in thedata quality, some erroneous values still
remains in the depth data of gebco08 (Fig.4).
For example, inaccurate coastline position wereobserved at several lowlands such as Mekong(Fig.4b) and Pearl River deltas, which seems tohave inherited from etopo1. It was also foundthat gebco08 incorporates erroneous sounding
data at a shallow shelf north of Brunei, which
could be recognized by a series of isolated
hollows deeper than 500 m (Fig.4c). Several
small shoals appear in Gulf of Thailand in the
gebco08 bathymetry seems to have made by aconversion error of depth units, i.e., depths
indicated in fathoms (1.83 m) were regarded asmeters and were underestimated (Fig.4d). It is12Compilation of South China Sea bathymetric data

to be noted that many sounding data obtainedin 1930s-1960s are still in use in the South
China Sea and several paper charts represent
depths in fathoms. Similar depth conversion
failure was also observed in SCS ENC.
In etopo1 and gebco08, spurious deep depths
were often found in nearshore areas such as an
inlet surrounded by high mountains and anintertidal zone where no depth information is
specified. The erroneous values seem to haveassociated with an overshoot caused by theapplication of high-order interpolation scheme.
Small-scale bathymetric features observed at
the shelf edge (Figs.4e and 4f) might also be
spurious features caused by the overshooting
effect, as ridges and troughs seem to bebounded by the survey lines and the depth
sounding points, and the features were more
apparent at areas where the overall depthchanges abruptly from 2000 m to 200 m. In thesouthern Taiwan Strait, the magnitude of
depth variation along the shelf edge wassmaller at region west of 119°E, where thedepth point is distributed more densely than atareas east of 119°E, which is consistent withthe overshooting hypothesis. We cannot
present clear conclusion on whether thefeatures are artificial or not, because the area
where such feature is evident generally
coincides with an area where depth information
is extremely sparse. More detailed survey on
the bathymetry of shelf-edge region in theSouth China Sea is anticipated, as the shelfedge is considered as major generation site ofinternal waves.
3.2

Tidal experiment

The rms deviation between observed andsimulated results indicates that the model
outputs derived by using gebco08 and gbscs5
bathymetries show better performance thanthose derived by using etopo5 and etopo1
datasets (Table 1). Larger rms deviation in
etopo5 and etopo1 cases were caused mainly bylarge discrepancies observed at several specificstations, which indicate that all four datasets
analyzed in this study would be suitable toobtain overall tidal features in the South China
Sea while a care must be taken when focusing
on a specific coastal location.
For example, etopo1 is not suitable to be used
in a numerical model predicting tides in theupper Gulf of Thailand (station 26 in Fig.1) as
the bathymetry is much shallower thanobserved and tides around the area are largely
underestimated. Erroneous configuration of
the Pearl River Estuary in etopo1 and gebco08

Table 1 Root-mean-squared (rms) deviation of tidal
harmonic constants (ampliutde and phase) between
model run output using different bathymetry andobserved values at 49 tide-gauge stations listed in Fig.1.

M2
Datasetetopo5etopo1
gebco_08
gbscs5

K1

amp

phase

amplitude

phase

(cm)

(deg)

(cm)

(deg)

13.4
11.0
9.0
8.9

29.2
40.2
24.1
19.1

9.0
10.6
7.0
6.4

22.4
37.9
15.4
6.0

influenced tidal estimates along the coast of
Guanxi Province in China (station 5).
Though tidal simulation using gbscs5
produces optimal results in overall sense, themodel tends to overestimate tidal amplitudes
along peninsula Malaysia (stations 28-31) andat southwest Borneo (station 38).
These large errors might be due partly toinsufficient accuracy of model bathymetry inthe central part of Gulf of Thailand and thearea south of Vietnam, as the number ofsounding data used for revision was small
compared to other regions. In addition, usage ofold sounding data may also have influenced thetidal output, because a recent study for theBohai Sea in northern China indicates that a
decadal shoreline change may alter M2
amplitude for more than 10 cm7).

4.

Summary and Conclusion

New bathymetric dataset of the South China
Sea shelf was compiled and compared withexisting datasets. It was found that the newdataset, which refers to larger number ofsounding data than in previous cases, seems toshow better performance than the three
existing datasets.
Additional sounding data are necessary tocompile more accurate bathymetry dataset of
the South China Sea shelf, especially at regions
such as the central Gulf of Thailand, offshore
area south of Vietnam from 4°N to 8°N, andshelf edge in the southwestern South China
Sea, southern Gulf of Tonkin, and southern
Taiwan Strait (118°E -119°E ).
Acknowledgments
This work was supported by Japan Society
for the Promotion of Science (JSPS) KAKENHI(23510009), JSPS G8 Research Councils
Initiative, and Research Institute for East Asia
Environments, Kyushu University.

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九州大学大学院総合理工学報告第 35 巻13

Appendix

References
1) W.H.F. Smith and D.T. Sandwell, Science 277 (1997)
1957-1962.
2) C. Amante and B.W. Eakins, NOAA Technical
Memorandum NESDIS NGDC-24 (200) 19pp.
3) G.D. Egbert and S.Y. Erofeeva, J. Atmos Ocean. Tech.
19 (2002) 183-204.
4) T. Zu et al., Deep-Sea Res. I 55 (2008) 137-154.
5) G. Fang et al., Cont. Shelf Res. 19 (1999) 845-869.
6) K. Uehara, Rep. Res. Inst. Appl. Mech, Kyushu Univ.
No.143 (2012) 69-73.
7) H.E. Pelling et al., J. Geophys. Res. 118 (2013) doi:
10.1002/jgrc.20258, 11pp.

第2号

A1.

URL of the data source

Etopo5
http://www.ngdc.noaa.gov/mgg/global/etopo5.HTML

Etopo1
http://www.ngdc.noaa.gov/mgg/global/global.html

Gebco08
http://www.bodc.ac.uk/data/online_delivery/gebco/

Trackline sounding data
http://maps.ngdc.noaa.gov/viewers/bathymetry/

SCS ENC
http://scsenc.eahc.asia/main.php