# Improvement of the geoid model over Japan using integral formulae and combination of GGMs

- Patroba Achola Odera
^{1}Email author and - Yoichi Fukuda
^{2}

**66**:22

https://doi.org/10.1186/1880-5981-66-22

© Odera and Fukuda; licensee Springer. 2014

**Received: **2 August 2013

**Accepted: **6 November 2013

**Published: **28 April 2014

## Abstract

An improved high-resolution gravimetric geoid model covering the four main islands of Japan (Hokkaido, Honshu, Shikoku, and Kyushu) was developed on a 1 × 1.5 arc-minute grid from EGM2008, GOCO02S/EGM2008, and terrestrial gravity data. A modified form of the Stokes-Helmert scheme was applied for the determination of the geoid using an empirically determined optimal spherical cap size. Handling of the topographical effects on gravity was accomplished using the integral formulae of Martinec and Vaníček, which were found to be more suitable for geoid modeling over Japan than the classical formulae. EGM2008 was used in Hokkaido, Honshu, and Kyushu Islands, whereas a combination of GOCO02S and EGM2008 was used in Shikoku Island and its immediate surroundings. The global geopotential models (GGMs) used in this study were chosen based on our earlier evaluation of the performance of EGM2008- and GOCE-related GGMs in Japan. In comparison with the previous geoid model for Japan, our new model shows an improvement in the standard deviation from ±8.3 to ±7.5 cm.

## Keywords

## Background

Geoid determination over Japan has been the focus of numerous studies over the past few decades (e.g., Ganeko 1976; Kuroishi 1995, 2001a, b,2009; Fukuda et al. 1997; Kuroda et al. 1997; Kuroishi et al. 2002; Kuroishi and Denker 2001; Kuroishi and Keller 2005; Odera et al. 2012; Odera and Fukuda 2013), although precise geoid modeling remains a challenge. In this study, an improved high-resolution gravimetric geoid model is developed from EGM2008 (Pavlis et al. 2012), GOCO02S (Goiginger et al. 2011) combined with EGM2008 (GOCO02S/EGM2008), and local terrestrial gravity data. EGM2008 is complete to spherical harmonic degree and order 2,159 and contains additional coefficients extending to degree 2,190 and order 2,159.

The evaluated GOCE and related GGMs included GOCE-DIR1, 2, 3 (Pail et al. 2011; Bruinsma et al. 2010), GOCE-TIM1, 2, 3 (Pail et al. 2011; Pail et al. 2010b), GOCE-SPW1, 2 (Migliaccio et al. 2011), and GOCO01S, 02S (Goiginger et al. 2011; Pail et al. 2010a). The performance of EGM2008 and GOCE-related GGMs over Japan was shown by Odera and Fukuda (2013) to be almost identical at a harmonic expansion to degree 150, although EGM2008 performs better at the 180-, 210-, and 240-degree spectral bands. Furthermore, comparisons over the four main islands reveal that EGM2008 performs better than the GOCE and related GGMs in Hokkaido, Honshu, and Kyushu at 180, 210, and 240 degrees. However, the GOCE and related GGMs perform better than EGM2008 in Shikoku.

Odera and Fukuda (2013) developed a gravimetric geoid model for Shikoku and the surrounding area from GOCE-TIM3/EGM2008 and terrestrial gravity data, with the aim of determining the magnitude of improvement in the model by using GOCE-only data. However, GOCO02S/EGM2008 performs slightly better than GOCE-TIM3/EGM2008 in this region. Therefore, in this work, a combination of GOCO02S (up to 180 degrees) and EGM2008 (from 181 to 2,190 degrees) is used to improve the geoid model over the Shikoku area, and EGM2008 is used in Hokkaido, Honshu, and Kyushu. Ship-track gravity data and satellite altimetry-derived marine gravity anomalies in the coastal areas were not considered because of the use of a high-resolution gravitational model (EGM2008). Furthermore, the interest of this study is limited to the land areas.

where *C* is the DTE, *N*_{
ind
} is the PITE, *G* is the Newtonian gravitational constant, *ρ*_{0} is the constant topographic density, *H*_{
P
} is the orthometric height of the computation point, *H* is the height of the running point, *σ* is the surface integration element, and *l*_{
o
} is the horizontal distance between the computation point and the running point.

The current geoid model for Japan has been developed using the integral formulae of Martinec and Vaníček (1994a, b) for the computation of DTE and PITE. Details concerning the procedure and data sets used to develop the previous geoid model can be found in Odera et al. (2012), these being generally the same as for the current computations, with the exception of topographical reductions.

This study further assesses the appropriate topographical gravity reduction procedure for precise geoid modeling in Japan. Gridding of the residual gravity anomalies is accomplished by kriging (Krige 1951) on a 1 × 1.5 arc-minute grid, Stokes's integral formula (Stokes 1849; Heiskanen and Moritz 1967) is used for geoid determination, and the modified Stokes's kernel proposed by Meissl (1971) is applied to minimize truncation errors.

## Methods

### Gravity data reductions and gridding

The method chosen for downward continuation of observed gravity on the topographical surface to the geoid plays a central role in precise geoid determination. Stokes's formula for the gravimetric geoid determination requires that no masses occur outside of the geoid and that the gravity anomaly should be referred to the geoid. This reduction was performed following the approach applied by Odera et al. (2012) to the previous geoid model for Japan. Although the classical Moritz formula (Moritz 1980) was used to estimate DTE in the previous geoid model for Japan, here, we evaluated DTE using the integral formula of Martinec and Vaníček (1994b). While both formulae use average topographic density, it should be noted that the classical Moritz formula is also based on the assumption that the distance between the computation and the running point is much greater than the height of the computation point (*l*_{
o
} ≫ *H*_{
P
}), which implies that it can only be used effectively for the far-zone integration area (Martinec et al. 1996). As a result, the effect of the near zone and the Bouguer shell, which cannot be derived in the planar model, is completely missing (e.g., Martinec and Vaníček 1994a; Nahavandchi 2000).

*σ*) into a near zone (

*σ*

_{1}) and a far zone (

*σ*

_{2}) based on a spherical approximation of the geoid. The resulting expression for DTE is given as (Martinec and Vaníček 1994b)

*R*is the mean radius of the Earth,

*H*

_{ P }is the height of the computation point,

*H*is the height of the running point,

*σ*is the surface integration element,

*ρ*

_{0}is the average topographic density, and

*G*is the Newtonian gravitational constant. The spatial distances

*l*and

*l*

_{ o }between the computation and the running points are given respectively as

where *r* is approximated as *R* + *H*_{
P
}.

*ψ*

_{0}) given by Martinec and Vaníček (1994b) as

The other applied reductions include a secondary indirect topographical effect (Vaníček et al. 1999), a second-order free-air reduction (Heiskanen and Moritz 1967), and an atmospheric correction (Wichiencharoen 1982a). The residual gravity anomalies are obtained from the reduced gravity anomalies on the geoid and gravity anomalies obtained from EGM2008 (degree 2,190 and order 2,159) in the three islands (Hokkaido, Honshu, and Kyushu), and GOCO02S/EGM2008 (degree 2,190 and order 2,159) in Shikoku. Kriging (Krige 1951) on a 1 × 1.5 arc-minute grid was performed for gridding of the residual gravity anomalies.

### Geoid modeling over Japan

where *N* is the gravimetric geoid undulation, *N*_{
GGM
} is the geoid undulation obtained from EGM2008 or GOCO02S/EGM2008 after applying the zero-degree term (with respect to GRS80), *Δg*^{
r
} is the residual gravity anomaly, *N*_{
ind
} is the indirect topographical effect, and *S*^{
ME
}(*ψ*) is Meissl's modified kernel.

The optimal spherical distance was evaluated empirically by comparing gravimetric and GPS/leveling geoid undulations at various spherical distances. These comparisons were performed at all 816 GPS/leveling points, and a standard deviation (S.D.) was computed for each spherical distance. As a result, a spherical cap size of 40 km was adopted for computations in this study because it gives the smallest S.D. when compared to the other spherical distances tested.

## Results and discussion

**Statistics of the topographical effects over Japan**

Min | Max | Mean | S.D. | |
---|---|---|---|---|

DTE by classical Moritz formula (mGal) | 0.00 | 75.98 | 2.75 | 3.75 |

DTE by integral formula of Martinec and Vaníček (mGal) | -40.48 | 49.46 | 2.78 | 4.74 |

PITE by planar formula (cm) | -40.73 | 37.54 | -1.03 | 2.50 |

PITE by integral formula of Martinec and Vaníček (cm) | -88.22 | 0.05 | -1.82 | 4.71 |

**Statistics of the differences between gravimetric and GPS/leveling geoid undulations over Japan and the six sub-regions**

Area | Points | Min (cm) | Max (cm) | Mean (cm) | S.D. (cm) |
---|---|---|---|---|---|

Hokkaido | 163 | -7.88 (-12.28) | 27.54 (25.93) | 6.82 (2.52) | 6.22 (6.71) |

North Honshu | 171 | -14.19 (-14.97) | 22.18 (19.67) | 6.07 (3.17) | 6.90 (6.41) |

Central Honshu | 163 | -7.81 (-13.99) | 36.67 (34.92) | 7.70 (1.44) | 7.14 (7.34) |

West Honshu | 158 | -8.98 (-17.32) | 21.16 (15.22) | -1.04 (-6.93) | 4.93 (5.16) |

Shikoku | 56 | -14.27 (-23.37) | 16.98 (21.49) | -0.34 (-8.38) | 6.55 (8.69) |

Kyushu | 105 | -21.58 (-31.50) | 9.37 (3.07) | -2.77 (-9.32) | 5.18 (5.58) |

Whole | 816 | -21.58 (-31.50) | 36.67 (34.92) | 3.59 (-1.66) | 7.48 (8.29) |

Table 2 shows that the West Honshu area has the smallest S.D. (±4.9 cm) between the gravimetric and GPS/leveling geoid undulations, while Central Honshu (a mountainous area) has the largest (±7.1 cm). When comparing these 816 GPS/leveling data points with the previous geoid model for Japan (Odera et al. 2012), there is a marked improvement in the overall S.D. from ±8.3 to ±7.5 cm.

It is evident that much of the improvement of the geoid is due to this study's treatment of the topography, given that the same data sets have been used for both models. For example, if EGM2008 is used over the four main islands (Hokkaido, Honshu, Shikoku, and Kyushu) and integral formulae are applied for the computation of topographic effects, the S.D. between gravimetric and GPS/leveling geoid undulations over the whole of Japan is ±7.6 cm, whereas it is ±8.7 cm over Shikoku only. Hence, the integral formulae of Martinec and Vaníček (1994a, b) seem to be more suitable for computing the DTE and PITE over Japan than the classical formulae. We also note the interesting effect of a deterioration of the S.D. over North Honshu as opportunity for further investigation. We suggest therefore that a significant improvement in the geoid model over Shikoku, from ±8.7 cm previously to ±6.6 cm currently, is mainly due to the contribution of GOCE data.

## Conclusions

We described a procedure for the computation of an improved high-resolution gravimetric geoid model over Japan that exploits the currently available data sets. The integral formulae of Martinec and Vaníček (1994a, b) were used to compute the DTE and PITE over Japan for the first time and were found to perform better than the classical formulae over the studied area. A combination of GOCO02S and EGM2008 (GOCO02S/EGM2008) was used for geoid determination in the Shikoku area, while EGM2008 was used for geoid determination in the remaining islands (Hokkaido, Honshu, and Kyushu).

When comparing these 816 GPS/leveling data points with the previous gravimetric geoid model for Japan, a marked improvement in the overall S.D. from ±8.3 to ±7.5 cm was observed. It was noted that much of the improvement in the accuracy of the geoid over the study area was due to the use of the integral formulae for handling the topographical effects on gravity.

With the recent improvements in geoid determination techniques, the handling of the DTE and PITE needs further attention. However, the need for denser coverage and more accurate gravity data over Japan cannot be overemphasized. Despite this, the gravimetric geoid model obtained in this study performs significantly better than the previous versions and makes an important contribution towards the establishment of a geoid-consistent vertical datum over Japan.

## Declarations

### Acknowledgements

We would like to thank the Geospatial Information Authority of Japan for providing GPS/leveling and other additional data sets covering the study area. We appreciate the efforts of Nagoya University and other organizations for developing and providing a detailed gravity database covering the southwestern parts of Japan. We are grateful to Prof. Zdeněk Martinec and an anonymous reviewer for their constructive comments and suggestions that have helped to improve the paper.

## Authors’ Affiliations

## References

- Bruinsma SL, Marty JC, Balmino G, Biancale R, Förste C, Abrikosov O, Neumayer H:
*GOCE gravity field recovery by means of the direct numerical method*. Paper presented at the ESA living planet symposium, Bergen; 2010.Google Scholar - Fukuda Y, Kuroda J, Takabatake Y, Itoh J, Murakami M: Improvement of JGEOID93 by the geoidal heights derived from GPS/levelling survey. In
*Gravity, geoid and marine geodesy, IAG symposia 117*. Edited by: Segawa J, Fujimoto H, Okubo S. Tokyo; 1997:589–596. 30 Sept–5 Oct 1996 30 Sept–5 Oct 1996View ArticleGoogle Scholar - Ganeko Y:
*Astrogeodetic geoid of Japan. Special report 372. Smithsonian Astrophysical Observatory*. 1976.Google Scholar - Goiginger H, Höck E, Rieser D, Mayer-Gürr T, Maier A, Krauss S, Pail R, Fecher T, Gruber T, Brockmann JM, Krasbutter I, Schuh WD, Jäggi A, Prange L, Hausleitner W, Baur O, Kusche J:
*The combined satellite-only global gravity field model GOCO02S*. Presented at the 2011 General Assembly of the European Geosciences Union, Vienna; 2011.Google Scholar - Heiskanen W, Moritz H:
*Physical geodesy*. Freeman WH, San Francisco; 1967.Google Scholar - Krige DG: A statistical approach to some basic mine valuation problems on the Witwatersrand.
*J Chem Metall Min Soc S Afr*1951, 52(6):119–139.Google Scholar - Kuroda J, Takabatake J, Matsushima M, Fukuda Y: Integration of gravimetric geoid and GPS/levelling survey by least squares collocation (in Japanese).
*J Geogr Surv Inst*1997, 87: 1–3.Google Scholar - Kuroishi Y: Precise gravimetric determination of geoid in the vicinity of Japan.
*Bull Geogr Surv Inst*1995, 41: 1–93.Google Scholar - Kuroishi Y: An improved gravimetric geoid for Japan, JGEOID98, and relationships to marine gravity data.
*J Geod*2001, 74(11–12):745–755.View ArticleGoogle Scholar - Kuroishi Y: A new geoid model for Japan, JGEOID2000. In
*Gravity, geoid and geodynamics 2000, IAG symposia 123, Banff, 31 July–4 Aug 2000*Edited by: Sideris MG. 2001b, 329–333.Google Scholar - Kuroishi Y: Improved geoid determination for Japan from GRACE and a regional gravity field model.
*Earth Planets Space*2009, 61: 807–813.View ArticleGoogle Scholar - Kuroishi Y, Denker H: Development of improved gravity field models around Japan. In
*Gravity, geoid and geodynamics 2000, IAG symposia 123, Banff, 31 July–4 Aug 2000*Edited by: Sideris MG. 2001, 317–322.Google Scholar - Kuroishi Y, Keller W: Wavelet approach to improvement of gravity field–geoid modelling for Japan.
*J Geophys Res*2005, 110: B03402.Google Scholar - Kuroishi Y, Ando H, Fukuda Y: A new hybrid geoid model for Japan, GSIGEO 2000.
*J Geod*2002, 76: 428–436. 10.1007/s00190-002-0266-5View ArticleGoogle Scholar - Martinec Z, Vaníček P: The indirect effect of topography in the Stokes-Helmert technique for a spherical approximation of the geoid.
*Manuscr Geodaet*1994a, 19: 213–219.Google Scholar - Martinec Z, Vaníček P: Direct topographical effect of Helmert's condensation for a spherical approximation of the geoid.
*Manuscr Geodaet*1994b, 19: 257–268.Google Scholar - Martinec Z, Vaníček P, Mainville A, Véronneau M: Evaluation of topographical effects in precise geoid computation from densely sampled heights.
*J Geod*1996, 70: 746–754. 10.1007/BF00867153View ArticleGoogle Scholar - Meissl P:
*Preparations for the numerical evaluation of second-order Molodensky-type formulas*. Report 163, Department of Geodetic Science & Surveying, Ohio State University, Columbus; 1971.Google Scholar - Migliaccio F, Reguzzoni M, Gatti A, Sansò F, Herceg M: A GOCE-only global field model by the space-wise approach.
*Proceedings of the 4th international GOCE user workshop, Munich, 31 Mar–1 Apr 2011*2011.Google Scholar - Moritz H:
*Advanced physical geodesy*. Wichmann Verlag, Karlsruhe; 1980.Google Scholar - Nahavandchi H: The direct topographical correction in gravimetric geoid determination by the Stokes-Helmert method.
*J Geod*2000, 74(6):488–496. 10.1007/s001900000110View ArticleGoogle Scholar - Odera PA, Fukuda Y: Towards an improvement of the geoid model in Japan by GOCE data: a case study of the Shikoku area.
*Earth Planets Space*2013, 65(4):361–366. 10.5047/eps.2012.07.005View ArticleGoogle Scholar - Odera PA, Fukuda Y, Kuroishi Y: A high-resolution gravimetric geoid model for Japan from EGM2008 and local gravity data.
*Earth Planets Space*2012, 64(5):361–368. 10.5047/eps.2011.11.004View ArticleGoogle Scholar - Pail R, Goiginger H, Mayrhofer R, Schuh WD, Brockmann JM, Krasbutter I, Höck E, Fecher T Presented at the ESA living planet symposium, Bergen, 27 June–2 July 2010.
*Global gravity field model derived from orbit and gradiometry data applying the time-wise method*2010a.Google Scholar - Pail R, Goiginger H, Schuh WD, Höck E, Brockmann JM, Fecher T, Gruber T, Mayer-Gürr T, Kusche J, Jäggi A, Rieser D: Combined satellite gravity field model GOCO01S derived from GOCE and GRACE.
*Geophys Res Lett*2010b, 37: L20314.View ArticleGoogle Scholar - Pail R, Bruinsma S, Migliaccio F, Förste C, Goiginger H, Schuh WD, Höck E, Reguzzoni M, Brockmann JM, Abrikosov O, Veicherts M, Fecher T, Mayrhofer R, Krasbutter I, Sansò F, Tscherning CC: First GOCE gravity field models derived by three different approaches.
*J Geod*2011, 85(11):819–843. 10.1007/s00190-011-0467-xView ArticleGoogle Scholar - Pavlis NK, Holmes SA, Kenyon SC, Factor JK: The development and evaluation of the Earth gravitational model 2008 (EGM2008).
*J Geophys Res*2012, 117: B04406.Google Scholar - Shichi R, Yamamoto A Special report 9. In
*List of gravity data measured by Nagoya University*. Part I, Bulletin of the Nagoya University Museum; 2001a.Google Scholar - Shichi R, Yamamoto A Special report 9. In
*List of gravity data measured by organizations other than Nagoya University*. Part II, Bulletin of the Nagoya University Museum; 2001b.Google Scholar - Stokes GG: On the variation of gravity on the surface of the Earth.
*Trans Camb Philos Soc*1849, 8: 672–695.Google Scholar - Vaníček P, Huang J, Novák P, Pagiatakis S, Véronneau M, Martinec Z, Featherstone WE: Determination of the boundary values for the Stokes-Helmert problem.
*J Geod*1999, 73(4):180–192. 10.1007/s001900050235View ArticleGoogle Scholar - Wichiencharoen C Internal report. In
*FORTRAN programs for computing geoid undulations from potential coefficients and gravity anomalies*. Department of Geodetic Science and Surveying, Ohio State University, Columbus; 1982a.Google Scholar - Wichiencharoen C Report 336. In
*The indirect effects on the computation of geoid undulations*. Department of Geodetic Science and Surveying, Ohio State University, Columbus; 1982b.Google Scholar

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