• Journal of Korean Society for Geospatial Information Science
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• v.16 no.1
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• pp.83-93
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• 2008

The topographic effect is one of the most important component in the solution of the geodetic boundary value problem (geodetic BVP). Therefore, topographic effect should be considered properly for developing the precise geoid model, especially for the area where contains many mountains like Korea. The selection of gravity reduction method in the context of the precise geoid determination depends on the magnitude of its indirect effect, the smoothness and magnitude of the reduced gravity anomalies, and their related geophysical interpretation. In this study, Korean digital elevation model with 100m resolution was constructed and topographic effect was calculated by three reduction methods as like Helmert condensation method and RTM method and Airy-isostatic reduction method. Through the analysis of computation results, we can find that RTM reduction method is the best optimal method and the results shows that gravity anomaly and indirect effect of geoidal height are $0.660{\pm}13.009mGal$, $-0.004{\pm}0.131m$ respectively and it is the most gentle slow of the three methods. Through this study, it was found that the RTM method is better suitable for calculating topographic effect precisely in context of precise geoid determination in Korea than other reduction methods.

• Journal of Korean Society for Geospatial Information Science
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• v.16 no.3
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• pp.117-126
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• 2008

Earth geopotential models were used to determine the reference surface in geoid modelling and many global geopotential models were developed after 1980's. Nowadays, EGM96 and EIGEN-GL04C geopotential models have been most widely used in the world, but what so called EGM2008 earth geopotential model were developed in 2008 by NGA. In this paper, we intended to compare the results of spherical harmonic analyses using the three geopotential model, EGM96, EIGEN-GL04C and EGM2008. So, the spherical harmonic analyses were performed up to degree and order 360(in case of EGM2008, up to degree and order 720, 1440, 2190 in addition), on each $1'{\times}1'$grid point in and around Korean peninsula. Geometric geoid were calculated at 464 GPS/leveling points for accuracy evaluation and then the results of three geopotential models were compared to geometric geoid. The results show that the accuracy of EGM2008 is improved considerablely compared to EGM96 and EIGEN-GL04C and it is possible to calculate geoidal heights within 14cm standard deviation and 5.5cm standard deviation after LSC fitting in and around Korean peninsula using EGM2008 geopotential model.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.27 no.2
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• pp.139-147
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• 2009

The gravity data collected and reserved in Korea is seriously biased in its distribution. That is, only the west-southern part of the peninsula including Chungcheong and Jeonla area has dense distribution while only a part is covered in Gyoungsang area. Especially, the low density of the gravity data in mountainous area basically limits the accuracy of the gravimetric geoid in Korea. As one of the solution to overcome the problem, an airborne gravity survey were conducted from Dec. 2008 $\sim$ Jan. 2009. In this study, free-air gravity anomaly derived from the airborne gravity data which has consistent quality are presented. The data processing for the airborne gravity is composed of several corrections of errors such as errors from gravity measurement, errors from flight dynamics, errors from GPS, and errors from time synchronization. We presented detailed explanations on the data processing with the final cross-over results. The free-air anomaly from airborne gravity finally shows the cross-over accuracy of 2.21mGal which reflects the precision of each track is 1.56mGal. It is expected that the result from this study will play a role as input data in precision geoid determination with ground and ship-borne gravity data after appropriate fusion process.

• Journal of the Korean Association of Geographic Information Studies
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• v.24 no.2
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• pp.78-90
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• 2021

In order to construct a precision geoid, it has been diversified into land, sea, aviation, and satellite gravity measurement methods, and measurement technology has developed, making it possible to secure high-resolution, high-precision gravity data. The construction of precision geoids can be fast and conveniently decided through GNSS surveys without separate leveling, and since 2014, the National Geographic Information Institute has been developing a hybrid geoid model to improve the accuracy of height surveying based on GNSS. In this study, the results of the GNSS height measurement were compared and analyzed choosing existing public reference points to verify the GNSS height measurement of public surveys. Experiments are conducted with GNSS height measurements and analyzed precision for public reference points on coastal, border, and mountainous terrain presented as low-precision areas or expected-to-be low-precision in research reports. To verify the GNSS height measurement, the GNSS ellipsoid height of the surrounding integrated datum to be used as a base point for the GNSS height measurement at the public datum. Based on the checked integrated datum, the GNSS ellipsoid of the public datum was calculated, and the elevation was calculated using the KNGeoid18 model and compared with the results of the direct level measurement elevation. The analysis showed that the results of GNSS height measurement at public reference points in the coastal, border, and mountainous areas were satisfied with the accuracy of public level measurement in grades 3 and 4. Through this study, GNSS level measurement can be used more efficiently than existing direct level measurements depending on the height accuracy required by users, and KNGeoids 18 can also be used in various fields such as autonomous vehicles and unmanned aerial vehicles.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.22 no.1
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• pp.45-52
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• 2004

It can be classified in various methods to get the geoidal heights. It can be achieved geometric geoidal heights if we do GPS surveying in leveling point. The aims of this paper are calculation of geometric geoidal heights using GPS/leveling data in study area and evaluation of the global and local geoid models in and around Korean peninsula. For this study, study area was selected in the leveling line from Kunsan to Chonju city and GPS surveying was accomplished in the leveling line. And, also spherical harmonic analysis was made on the three global geopotential models, OSU91A, EGM96, EGM96m under same condition. Then the differences were calculated between geometric geoidal heights and geoidal heights of 3 geopotential models, KOGD2002 which was Korean gravimetric geoid model. The results shows that EGM96m is the best model because the differences between geoidal heights of E6M96m and geometric geoidal heights of GPS/Leveling data appear the smallest value among them.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.27 no.1
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• pp.713-722
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• 2009

Gravity measurement can determine the earth gravitational field, also is the fundamental to the research of earth gravitational field, geodesy and geodynamic, vertical movement of the crust, geoid surface, sea level and climate etc. Recently, National Geographic Information Institute (NGII) introduced FG-5 absolute gravity meter in order to lay a foundation for establishment of Absolute Gravity Network, and furthermore NGII plan to construct about 1,200 multi dimensional and function Unified Control Points(UCP) in nationwide. It will play an important role in development of high accuracy geoid model in South Korea. This paper explains the fundamental theory and method of relative gravity measurement, surveys the relative gravity of 21 stations using latest Scintrex CG-5 relative gravimeter. In addition, it calculates gravity values, compare and analysis gravity survey results using datum-free adjustment and weighted constraint adjustment. The results indicate show that datum-free and weighted constraint adjustment methods are available to determine high accuracy gravity achievement, datum-free method is more advantage than weighted constraint adjustment.

• Journal of Korean Society for Geospatial Information Science
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• v.10 no.2 s.20
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• pp.107-114
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• 2002

This study aimed to estimate accuracy of GPS/Leveling and to present availability of GPS/Leveling in public surveying. For this purpose, we carried out GPS survey for bench marks and control points of Hongsung area. Orthometric heights calculated by two GPS/Leveling methods were compared to reference height. The one is calculated by base of geoid models such as EGM96, OSU91A, KGEOID99, and the other is calculated by network adjustment using fixed point. The results of GPS/Leveling by geoid models show that RMSE of EGM96 is ${\pm}0.061m,\;OSU91A\;{\pm}0.725m,\;KGEOID99\;{\pm}0.598m$. The results of GPS/Leveling by network adjustment show that the best RMSE is ${\pm}0.043m$ in case of using three fixed bench mark, and this method can be used for leveling effectively. GPS/Leveling would be able to apply in forth order public leveling and height determination of public control points.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.261-266
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• 2005

According to development of satellite geodesy, gravity potential models which have high accuracy and resolution were released. Using the EIGEN-CG01C model based on low orbit satellite data such as CHAMP and GRACE and the EGM96 model, geoid and gravity anomaly were calculated and compared. The study area is located at $123^{\circ}{\sim}132^{\circ}$ E, $33^{\circ}{\sim}43^{\circ}$ including Korea. Comparing two models, very high correlation more than 0.90 in geoid and gravity anomaly was observed, but in amplitude analysis the EIGEN-CG01C model have higher amplitude in high frequency area. Gravity anomaly calculated with both models shows a little difference in North Korea and some coast area of the Yellow sea. Through power spectrum analysis, residual anomaly that can be used in large scale structure or underground resources survey was calculated.

• Spatial Information Research
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• v.16 no.4
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• pp.401-410
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• 2008

In this contribution, the renovation of the Korean geodetic control points is introduced. The renovation is described in terms of structure and accuracy. Mainly the establishment of the three-dimensional control points is the renovation on structure which leads many changes in accuracy, contents and hierarchy of the control points. The accuracy of the information is being improved based on the satellite positioning technology and precision geoid being developed. Diverse spatial information such as gravity and environmental elements are considered to be measured at the position of the control points so that the related research is enhanced through the analysis of combined information. In addition, an access to the information of control points and service to the public with spatial information will be faster and more efficient through RFID and CDMA communication. With all these efforts being made currently, the Korean geodetic network will provide the most accurate and diverse spatial information in an efficient way. We hope that these activities lead the trends, roles, and future direction of the geodetic control points.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.24 no.2
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• pp.183-191
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• 2006

The accuracy of the upward continuation is assessed through the gravity modeling using an ultra-high degree spherical harmonic expansion. The difficulties in the numerical calculation of Legendre function with ultra-high degree, underflow and/or overflow, is successfully resolved in 128 bit calculation scheme. Using the generated Legendre function, the gravity anomaly with spatial resolution of $1'{\times}1'$ on the geoid is calculated. The generated gravity anomaly is degraded and extracted with various noise levels and data intervals, then upward continuation is applied to each data sets. The comparison between the upward continued gravity disturbances and the directly calculated from the spherical harmonics showed that the accuracy on the direct method was significantly better than that of Poisson method. In addition, it is verified that the denser and less noised gravity data on the geoid generates better gravity disturbance vectors at an altitude. Especially, it is found that the gravity noise level less than 5mGal, and the data interval less than 2arcmin is necessary for next generation precision INS navigation which requires the accuracy of 5mGal or better at an altitude.