九州大学大学院総合理工学府報告第 37 巻 第 2 号 12-18 頁 平成 28 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 37, No. 2, pp. 12-18, FEB, 2016

Compilation of bathymetric data for the South China Sea
2: High resolution dataset based on multiple sources
Katsuto UEHARA*1,††
E-mail of corresponding author: uehara@riam.kyushu-u.ac.jp(Received January 18, 2016, accepted January 22, 2016)
New grid bathymetry dataset covering the whole South China Sea with a spatial resolution of 1
min (1/60 degrees) has been developed by compiling sounding data derived from various sources
including recent multi-beam sounding records. This dataset was found to resolve more realistic bottomfeatures than existing global DTMs (etopo1, gebco08, gebco2014) especially at shallow regions such as
Spratly Islands and the Java Sea.
Key words: bathymetry, South China Sea, tidal model, Spratly and Paracel Islands, DTM

1.

Introduction

The performance of ocean models, especiallythose predicting tides, rely greatly on thequality of bathymetric data used. For example,
root-mean-square (RMS) differences between
observed and predicted M2 tides in the East
China Sea were 17 cm and 15 degrees for
amplitudes and phases, respectively1), whereas
corresponding values were 5.6 cm and 6.5
degrees in the case of the Northwest European
Shelf seas2). As both studies used a same
numerical model and the target seas weresimilar in size, the smaller RMS values foundin the latter case may be ascribed to thedifference in the availability of well-managed
bathymetric data.
As for the case of the South China Sea (SCS),
Uehara3) verified three existing digital terrain
models (DTMs), i.e., etopo5, etopo1 and gebco08,
and found errors especially in coastal regions
shallower than 200 m, which were caused bymisinterpretation of units, inclusion oferroneous records and interpolation made oversparsely-distributed data.
Uehara3) revised the shallow along-shore
portion of gebco08 dataset by using depth
information obtained from nautical charts andsingle-beam sounding data and compiled a new
bathymetric dataset with a resolution of 1/12
degrees. Usage of the new dataset reduced the
RMS difference of M2 amplitudes from 13.4 cm(etopo5 case) to 8.9 cm.
In late 2014, gebco08 dataset was superseded
by a new version, gebco2014. As presented laterin this article, this new dataset has eliminated*1 Center for East Asian Ocean-Atmosphere Research,
Research Institute for Applied Mechanics

many errors existed in the former version.
While spatial smoothing applied to the dataset
assures overall quality over a wide area andmakes the data suitable for large-scale studies,
the procedure also seems to have suppressedsmall-scale features important for coastal
studies. In addition, the dataset still does not
represent gigantic coral reefs specific to SCS
such as Spratly and Paracel Islands correctly.
As an extension of Uehara3), this study
develops a gridded bathymetry covering thewhole SCS in a best-effort basis, i.e., compilingan unfiltered dataset which may resolvefeatures having 5-10 km scale at regions wheresounding data are fully available. The
compilation of the 1/60-degree dataset was
accomplished by collecting as many sounding
data as possible from various sources including
recent multi-beam survey products.
The next section describes the procedure tocompile the dataset which is followed by theverification of the data.

2.

Materials and methods

2.1 Existing bathymetry datasets
The final product of this study is comparedwith three existing global DTMs, i.e., etopo1,
gebco08 and gebco2014, which all represent
land altitudes or water depths on a longitudelatitude grid.
Etopo1 is 1 min resolution data developed in2009 by U.S. National Geophysical Data Center(NGDC; now National Centers for Environmental Information, NCEI). The oceanic part ofetopo1 is based on satellite altimetry
technology which has greatly improved the平成 27 年度

九州大学大学院総合理工学報告 第 37 巻

accuracy of water depths at deep oceans.
Gebco08 (General Bathymetric Chart of the
Oceans) gridded bathymetric dataset has been
published in 2010 by British Oceanographic
Data Centre (BODC), which was superseded bygebco2014 dataset in late 2014. Both gebco08
and gebco2014 have a spatial resolution of 1/2
min. Though they are based on the satellite
altimetry products, as in the case of etopo1,
depths in shallow areas were modified byreferring to in-situ sounding data. Differences
between gebco08 and gebco2014 will be
discussed later.
2.2 Tscs bathymetry
The regional bathymetry dataset compiled inthis study, tscs, is a successor of gbscs5 dataset
developed by Uehara3). Tscs covers the whole
SCS as well as adjacent waters such as Java,
Flores, Celebes and Sulu seas.
While gbscs5 used depth of gebco08 at region
deeper than 200 m, all depths in tscs were
newly compiled from sounding data, except at
an area south of the Sumatra Island west of
103°E where depths were obtained from
gebco2014 because the sounding data were
extremely sparse.
Tscs has higher spatial resolution (1 min) andlarger latitudinal extent (10°S-27°N) thangbscs5 (5min, 0°-23°N). On the other hand,both tscs and gbscs5 cover same longitudinal
extent (99°E--125°E) and uses land altitudes
derived from gebco products: either gebco08(gbscs5) or gebco2014 (tscs).
2.3 Data source
2.3.1 Navigational charts
The primary source of the sounding data wasnavigational charts: paper and electronic
charts issued by local countries/regions otherthan Brunei, Cambodia, Macao and Taiwan (forHong Kong, Indonesia and Singapore,electronic charts only), paper charts by U.S.
and Russia, and electronic charts by U.K.
2.3.2 Bathymetric survey records
Raw sounding records derived from singlebeam and multi-beam surveys were also used
for the compilation, which provide information
along ship tracks in high spatial density.
Single-beam surveys have been taken placemainly in the eastern part of the central SCS.
Data have been obtained from NCEI and Japan
Oceanographic Data Center (JODC).
In the last decade, increasing number of

第2号

13

multi-beam surveys have been conducted atregions such as the Manila Strait and the Sulu
Sea. The current study used multi-beamsounding data measured at 1995-2013 (mostly
at 2005-2013), which were available from NCEIand Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC).
2.3.3 Reference data
In this study, electronic charts by East Asia
Hydrographic Commission (EAHC-SCSenc)
and Navionics were used only for reference
because they contain errors which seem to havegenerated on data processing phase.
Gridded bathymetry data called JEGG500
issued by JODC were also used as reference forregion around Japanese waters. Though theiroverall quality seems to be high, they arespatially filtered and were not conformablewith other raw sounding data used.
2.4 Data processing
2.4.1 Navigational charts
Paper charts were scanned, tagged withgeographic position and converted into WGS84
coordinates if necessary.
Depths indicated in rasterized paper chartsand electronic charts were displayed altogether
on a computer display by using a GIS software
and were digitized into xyz (longitude, latitude,
depth) format. Depth contours were notdigitized unless contours (1) define the edge ofthe large coral reefs where water depth may
change for more than 2,000 m or (2) represent
the extent of elongated shoals or trenches
where depths vary in anisotropic manner.
It should be noticed that using all the spotsounding data contained in electronic charts
may often cause underestimation of depth in
the final product, especially when reading fromsmall-scale charts. It is because navigational
charts tend to indicate depths over a shoal oran obstacle rather than those representing theregion in concern.
Quality checks have been conducted before
and after the digitizing process. Pre-digitizing
check verifies the quality of the depth indicatedon charts by overlaying different charts andcompare each other on the screen, which may
help excluding erroneous values and extreme
values which may not represent the depth
around the sounding spot. Post-digitizing checksorts out duplications and input errors.
In areas where depths are expected to change
largely with time, we tried to use data
originated from a single source to retain the14Compilation of South China Sea bathymetric data 2

consistency of the depth field. In cases of thenorthern part of the upper Gulf of Thailand andthe area south of the Mekong River Delta, we
adopted data measured at comprehensive
surveys conducted in 1960s. For the Tonkin
Gulf west of 108°E, we followed depths found inrecent Vietnamese charts.
2.4.2 Bathymetric survey records
Sounding information obtained by singleand multi-beam surveys were displayed andcompared with other data for verification.
As for the single-beam data, depths obtainedby some cruises, mainly those conducted before
1980s, were excluded because water depths
were inconsistent with others and also because
old sounding records at offshore location tend
to contain positioning errors.
The multi-beam data derived from several
cruises conducted at a same area were merged
and converted to a grid data having a spacingof 0.2 min, because the original data resolution
was too high for the current purpose. While
multi-beam soundings provide detailed twodimensional information, the obtained productrequires careful verification because it often
include unrealistic values at shallow waters
and at along the edge of swath bands.
2.4.3 Final sounding data
The sounding data created from various
sources have been merged to a single file. It isrelatively easy to reflect the modification or
addition of the source data to this final filebecause individual files to be merged could be
replaced by changing the file name listed in a
fortran code used for the merging process.
Figure 1 denotes the distribution of thesounding data contained in the final file. The
color represents the number of the data that
falls into a 5 min bin. It is found that thesounding data is sparse in the Celebes Sea, theGulf of Thailand, the Java Sea and an area off
central Vietnam (ca. 11.5°N, 110°E), showingthat the final product of this study, which rely
solely on the sounding data, should be treatedwith care for these areas.
In addition to the seafloor depths based on
the sounding data, land altitudes derived from
1/2 min resolution gebco2014 dataset wereincorporated into the final data to obtainplausible water depth at around the coast andto avoid unnecessary overshooting at areas
devoid of data when the interpolation is made.
The maximum height of the altitude data usedin the interpolation was limited to 10 m, toFig.1 Histogram showing the number of sounding data
used to compile the bathymetry dataset in gridbins having the size of 5 min. White shadings
denote area without any depth data. Note that theactual number ranges up to ca.10,000.

obtain realistic nearshore water depth evenwhen sea is surrounded by high mountains.
2.4.4 Grid data generation
The sounding data were converted into agridded format with a spacing of 1min by using
Natural Neighbor Interpolation scheme ofSurfer 13 application (Golden Software) which
does not generate values beyond the range ofthe actual dataset. In this study, we havechosen this conservative interpolation scheme
to avoid generating spurious depth values, withthe cost of losing some information contained in
the original sounding data.

3.

Results

3.1 Data comparison
3.1.1 Overall features
Fig. 2 illustrates the depth pattern of etopo1,
gebco08, gebco2014 and tscs datasets. To makethe datasets comparable, gebco bathymetries
were converted into 1 min resolution andetopo1 was transformed from grid-registered(data point is defined on the cell boundary) tocell-registered format (defined on cell center).

平成 27 年度

九州大学大学院総合理工学報告 第 37 巻(b) gebco08(c) gebco2014(d) tscs

15

Full range

Shallow Area(a) etopo1

第2号

Fig.2 Depth distribution of four bathymetric datasets: (a) etopo1, (b) gebco08, (c) gebco2014
and (d) newly formed tscs. All figures use two color scales shown in the right: (upperrow) depth scale applicable to seas shallower than 100 m and (lower row) scale forother depth/altitude ranges.

16Compilation of South China Sea bathymetric data 2

(a)

Fig.3

(b)

(c)

Relative difference between gebco2014 and (a) etopo1, (b) gebco08, (c) tscs bathymetries. Areas
where the absolute difference is less than 0.05 are drawn in white. Note a positive value stands
for the situation that the dataset in concern was deeper than gebco2014.

The most notable features found in Fig. 2 isextensive shallow regions observed in etopo1(Fig. 2a), which seems to have caused because
depths shallower than 50 m in etopo1 wereobtained by interpolation. Shallow areas
around Paracel and Philippine islands werepadded with 1 m depth and the whole depth
within the upper Gulf of Thailand was less than
10 m, less than one half of the observed depth.
The relative difference between etopo1 (Fig.
2a) and gebco2014 (Fig. 2c) are shown in Fig.3a.
Judging from the distribution of the area where
the difference is small, it is speculated thatgebco2014 used satellite altimetry products at
depth deeper than ca. 1,000 m and that some ofthem were modified by referring to thebathymetric survey records. On the other hand,
depths shallower than 1000 m in gebco2014
seem to have based mainly on the in-situ
sounding data.
As mentioned in Uehara3), gebco08 dataset
tends to show spurious bumps and hollows inshallow regions (Fig. 2b), which seems to havesuppressed in newly introduced gebco2014 (Fig.
2c). Map showing relative difference between
gebco08 and gebco2014 (Fig. 3b) indicates thatthe modification was made mostly at coastal
region shallower than 200 m at longitudes
105°E-120°E. Fig. 3b suggests that thebathymetry of the gigantic reefs was notrevised in gebco2014.

3.1.2 Southern SCS
To verify the difference among the datasets
more in detail, enlarged bathymetric maps oftwo specific areas, the Southern SCS and theJava Sea, are shown in Fig. 4. Here, we willcompare gebco08, gebco2014 and tscs datasets
because basic features of etopo1 in deep seas
were similar to those of gebco topographies
while shallow features in etopo1 were notrealistic.
In the southern SCS, the bathymetric map ofgebco08 (Fig. 4a) again shows spurious featuresin the shallow shelf area (left and bottom edgeof the figure) whereas they were less apparent
in gebco2014 (Fig. 4b). Though the spatialsmoothing applied to gebco2014 has improved
the quality of the data in general, there weresome cases when the smoothing seems to havegiven rise to spurious features. In the easternside of the southern SCS, for example, thespatial pattern of shallow features such as
offshore coral reefs near the shelf edge ingebco08 (Fig. 4a) resemble more with those intscs (Fig. 4c) than in gebco2014 (Fig. 4b), while
their width in gebco2014 (Fig. 4b) were muchlarger than those found in nautical charts.
As found in the northeastern edge of Fig. 4c(tscs case), Spratly Islands is a large shoal
having a size of ca. 100 nautical miles (ca. 180km). Though the interior of this shoal istypically about 50 m deep, majority of the area

平成 27 年度

九州大学大学院総合理工学報告 第 37 巻

Southern SCS

第2号

17

Java Sea(d)(a)
Gebco08(e)(b)
Gebco2014

(f)

(c)
Tscs
Fig.4

Enlarged bathymetric image of Fig. 2 for (a & d) gebco08, (b & e)
gebco2014 and (c & f) tscs datasets at (a, b & c) southern SCS and (e, f& g) Java Sea, the locations of which are indicated as two red frames
in the right figure. All six figures are shown in a same scale. Note zonal
and meridional extent of the Spratly Islands (a large shoal near the NE
corner indicated in light green, most apparent in Fig. 4c) areapproximately 160 km and 190 km, respectively.

was shown as area deeper than 200 m in gebco
bathymetries (Figs. 4a and 4b).
These results suggest that the highresolution dataset does not necessarily resolve
small-scale depth feature.
3.1.2 Java Sea
Another close-up area illustrated in Fig. 4 isthe Java Sea, a marginal sea surrounded by
Java, Sumatra and Kalimantan Islands.
Difference between gebco08 (Fig. 4d) andgebco2014 (Fig. 4e) bathymetry is found only inthe northwestern Java Sea north of 2°S (Fig.
3b), where gebco2014 represents more smoothbottom features and resembles more to tscs
(Fig. 4f) than to gebco08. In the eastern Java
Sea, both gebco08 and gebco2014 show many
bumpy features in contrast to relatively smoothbathymetry found in the tscs case. A linear
elongated trench feature observed at around
2°S coincides with the route of a sounding
survey track which was not adopted in tscs as
depths obtained by the cruise did not match

18

Compilation of South China Sea bathymetric data 2

with those shown in recently publishedIndonesian electronic charts. These results
suggest higher performance of tscs compared to
the gebco bathymetry in the Java Sea, eventhough the available sounding data is not
sufficient.
On the other hand, comparison between
gebco and tscs datasets suggest two potential
issues reside in tscs. One is that thebathymetry along the northern Java coast
seems to be too smooth. Taking into accountthat many rivers in the Java Island discharges
a large amount of freshwater and sediments to
the Java Sea, the actual bottom features might
have been more complex than presented in tscs.
The smooth nearshore bathymetry is due torelatively smooth features found in electronic
charts, in which depth points are densely
allocated. Spatial smoothing might haveapplied to the charts on their production.
Similar smooth feature was observed at along
Guangxi coast in southern China.
Another issue is that tscs, which rely solely
on sounding data, does not represent detailed
features observed in gebco datasets at regions
south of the Java Island where the sounding
data is sparse. Large discrepancy off Sulawesi
Island (0° 122°E in Fig. 3c) may also be
ascribed to the lack of sounding data.
In summary, tscs dataset seems to represent
realistic small scale features in the shallow
region in SCS, while a care must be taken whenanalyzing the bathymetry at regions where thesounding data was sparse.
3.2 Preliminary tidal-modeling test
A series of tidal simulations have been
conducted to infer the performance of thebathymetric data by following the setting of
Uehara3) except that the spatial resolution was
improved to 2 min.
A preliminary result show that the RMS

differences between the model and tide-gauge
data were (12.2 cm, 8.0 cm, 7.8 cm, 6.3 cm) forM2 amplitudes in (etopo1, gebco08, gebco2014,tscs), respectively. Though tscs give rise to abest result among all cases including thosefound in Uehara3), further validation isnecessary to confirm this output.

4.

Summary and Conclusion

Bathymetry data covering the whole SCS,
aimed to resolve features having 5-10 km scale,
was developed by compiling sounding data
derived from various sources. The procedure tocompile the bathymetry considers future
replacement or addition of the source data. The
data was found to represent more realisticfeatures than existing global DTMs at regions
such as Spratly Islands, the Java Sea, andnorthwest
Borneocoasts,whichwasaccomplished by multiple-step quality checkand introduction of latest sounding data andusage of modest interpolation scheme. On theother hand, the source sounding data were lessavailable in regions such as the Gulf ofThailand, the Java Sea and off central Vietnam,
where the dataset may potentially containsome uncertainties.
Acknowledgments
Author would like to thank National Centers
for Environmental Information, British
Oceanographic
Data
Centre,Japan
Oceanographic Data Center and Japan Agency
for Marine-Earth Science and Technology forkindly providing the data.
References
1) K. Uehara et al., Mar. Geol. 183 (2002) 179.
2) K. Uehara et al., J. Geophys. Res. 111 (2006) C9025.
3) K. Uehara, Eng. Sci. Rep. Kyushu U. 35 (2014) 7.