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Gravity Surveying

Gravity Surveying
  • Geodetic origin point
  • Triangulation
  • Measurement of Unified Control Point
  • Leveling
  • Gravity Surveying
  • Geomagnetism Surveying
  • Very Long Baseline Interferometry (VLBI)
Gravity Surveying
Gravity surveying measures the value of the acceleration of gravity, precisely observing how it changes when the distribution of gravity values or time varies.
The value of gravity can vary depending on different locations or heights on the earth. The earth’s internal structures, including minerals or underground faults, have different values. Gravity also changes over time, such as with an earthquake or volcanic activity.
The survey results are used for the preparation of a gravity map and serve as fundamental data for research into the shape of the earth (geoid) and tectonic activities, including forecasts for earthquakes and volcanic eruptions.
Methods of gravity survey
Gravity is measured by two different methods: absolute surveying and relative surveying.
Absolute surveying obtains independent gravity measurements at a given point by using an absolute gravimeter.
Relative surveying uses a relative gravimeter to obtain the difference between gravity measurements at a point of observation and a point at which the gravity value is already known.
Present status of equipment
The NGII possesses two absolute gravimeters and one relative gravimeter. The institute acquired the two FG-5 absolute gravimeters made by Micro-g LaCoste, one each in 2008 and 2010. The relative gravimeter was made by LaCoste & Romberg in 2001.
The performance specifications of the FG-5 are as follows:
Accuracy: ±2×10-8 m/s2 (2µGal)
Precision: ±1×10-8 m/s2 (1µGal)
Measuring duration: Less than 2 hours for 1µGal precision in a secure site
Measuring range: World-wide
Measuring temperature range
- 10°C to 30°C (AL-1 HeNe laser)
- 15°C to 25°C (Iodine stabilized HeNe laser)
Absolute Gravity Survey
The NGII began carrying out gravity surveying in 1975. It defined the normal gravity value at the gravity control point established in the premises of the former National Geography Institute (Hwigyeong-dong, Dongdaemun-gu, Seoul) after obtaining relative gravity measurements and comparing them with the gravity control point at the Geospatial Information Authority of Japan in November 1974. In December 1999, the NGII installed an absolute gravimeter (FG-5) on its premises (Woncheon-dong, Suwon-si, Gyeonggi-do) to measure the value of the acceleration of absolute gravity and set the Korean geodetic absolute gravity datum.

After adopting a new absolute gravimeter in 2008, the NGII expanded its surveying area of absolute gravity and has conducted surveys at regular intervals. Accurate value of gravity can be measured at each absolute gravity point nationwide, correcting evaluated data by a relative gravimeter.
Present status of absolute gravity measurement
Present status of absolute gravity measurement
No Absolute Gravity Point Longitude Latitude Location Value of Gravity(mGal) Altitude(m) Standard Atmospheric Pressure(hPa) Establishment Year Measurement Year
1 Suwon GA1 127-03-14.4 37-16-31.8 Absolute Gravity Observation Station, NGII, Suwon 979918.775 56.536 1000.00 1999.11 2009.08
2 Gangneung GA1 128-52-08.9 37-46-19.6 Engineering Bldg. #2, Gangneung-Wonju Univ., Gangneug, Gangwon-do 979982.612 21.473 1000.00 2009.09 2010.12
3 Daegu GA1 128-48-04.9 35-54-37.0 Engineering Bldg. #4, Kyungil Univ., Gyeongsan, Gyeongsangbuk-do 979808.018 82.350 1000.00 2009.09 2009.09
4 Gwangju GA1 126-54-39.2 35-10-43.8 Engineering Bldg. #5, Jeonnam Univ., Gwangju 979746.827 42.450 1000.00 2009.09 2009.09
5 Daejeon GA1 127-22-14.9 36-23-16.4 Absolute Gravity Observation Room, KRISS, Daejeon 979831.351 82.087 1000.00 2010.05 2010.05
6 Changwon GA1 128-41-54.9 35-14-28.5 Bldg. #54, Changwon Univ., Changwon, Gyeongsangnam-do 979756.618 53.413 1000.00 2010.06 2010.06
7 Jeju GA1 126-33-50.4 33-27-25.6 Marine Science Bldg. #2, Jeju Univ., Jeju, Jeju-do 979562.665 293.425 1000.00 2010.06 2010.06
8 Pohang GA1 129-24-32.8 36-05-13.2 Geunmyeon Bldg., Pohang Univ., Pohang, Gyeongsangbuk-do 979856.066 29.141 1000.00 2010.10 2010.10
9 Taebaek GA1 128-58-27.0 37-09-40.2 Bldg. annexed to tennis court, Kangwon Tourism Univ., Taebaek, Gangwon-do 979767.977 732.786 928.26 2010.10 2010.10
The NGII will complete the nationwide survey of absolute gravity at 20 points by 2012, after which it will repeat surveys to monitor gravity changes.
Terrestrial Gravity Survey
The NGII collected 3,866 pieces of terrestrial gravity data from 1973 to 2009. However, most of the data were hand written, so it was difficult to organize data and the statistics were not verified. In addition, the data were hard to use because of their disproportionate concentration on certain areas. The establishment of Geoid model required a large number of gravity values which were surveyed proportionately nationwide. Therefore the institute carried out the comprehensive terrestrial gravity surveys at 6,283 benchmark sites nationwide from 2009 to 2010.
  • Gravity control point Gravity control point
  • Gravity surveying Gravity surveying
Geoid Model
Bessel Geoid Model (released in 1996)
This is an astronomical geoid model generated by combining the 40 components of the astronomical surveying result of the NGI and the 354 components of the astronomical surveying result of the GSI in Japan, which were collected from 1947 to 1979.
This model was set up by using deflection of vertical between Bessel ellipsoid and geoid.
This model, designed to provide a basis for the shape of the reference ellipsoid (Bessel) of Korea, can be utilized to compute geometry transformation parameters of the World Geodetic System (WGS).
KGEOID99 (released in 1998)
As a gravimetric geoid model, KGEOID99 was set up according to the geopotential model, EGM96.
Each long-wavelength effect calculated by geopotential model, middle-wavelength effect calculated by gravity data and short-wavelength effect calculated by DTM data was applied through the “Remove and Restore” technique (plus or minus error 70 cm).
KGD2002 Geoid (released in 2002)
The geocentric, hybric geoid model was set up by using GPS-leveling data collected from nationwide benchmark points and triangulation points according to the geopotential model, EGM96.
This can be applied for the evaluation of geoid heights of NGI_Pro Ver.1.0, the software of the NGII to convert the coordinate system.
GMK09 (released in 2009)
The latest hybric geoid model, based on the geopotential model EGM96, was set up using the 15,076 components of the GPS/Leveling result and the 2,187 components of the gravity data collected from unified control points and benchmark points established until 2008.
The software for the evaluation of geoid heights that can be applied to calculate orthogonal heights by using ellipsoidal heights can be found at <gps.ngii.go.kr>.