Geoid is an equipotential field of earth's gravity field which coincides with global average sea level, which is used as a reference field for determining the vertical or high position of a point on the surface of the earth. Based on the Head of BIG Regulation No. 15 of 2013 concerning the 2013 Indonesian Geospatial Reference System (SRGI 2013) article 10 states that the geoid is a national vertical reference system.
Based on this, all national development activities that use altitude data must refer to the geoid. Geoid itself is derived through gravity surveys that are tied to the National Gravity Control Net (JKGN).
JKGN is realized through major gravity points (GBU) located in selected locations that are distributed throughout Indonesia. The absolute gravity values available will be used as a reference for survey work and measurements of terrestrial gravity and airborne gravity.
Undulasi geoid Indonesia per Desember 2018Geoid is obtained from gravity measurement data that is bound to JKGN. Gravity measurements carried out include terrestrial gravity measurements, absolute gravity measurements and gravity measurements using aircraft (airborne) covering large islands in Indonesia as shown in the following figure.
Pulau yang telah dilakukan pengukuran gayaberat airborne per Desember 2018
Meanwhile, for the index of availability of the main gravity values that have been measured can be seen in the following figure.
The development of meticulous geoid in Indonesia is very important, urgent, and crucial in the holding of a national IG because of several important things below:
a. Indonesia is an archipelago so there must be a high datum pool between islands and more importantly is the integration of land and sea IG;
b. In large-scale IG development involving infrastructure development, for example high accurate information is needed both for the purpose of determining the height above, at and below the earth's surface. Large-scale IG development is needed to:
If geoid is not available, then the following things need to be considered:
a. One Map Policy (KSP) or One Map Policy to go to a national reference system cannot be achieved because of differences in altitude reference values, in this case the value of 'zero' between islands in Indonesia.
b. Geoid is used in forming a crucial Digital Elevation Model in determining three types of coastline in accordance with the law on geospatial information. The three types of coastline are the coastline value of average sea level (MSL), the coastline at the lowest tide (LAT), and the coastline at the time of the highest tide (HAT). Each of these three shorelines has their own important applications for example for the construction of the Indonesian Rupabumi, Chart Datum, island determination, and so on. The coastline is also used as an administrative boundary line and this is often used by the Provincial and Regency / City Governments to calculate the area of their respective regions in utilizing the General Allocation Fund (DAU).
c.Indonesia as an archipelagic country is vulnerable to the threat of sea level rise that continues due to global warming. Sea water floods have also occurred in major cities in Indonesia due to land subsidence and sea level rise itself. In mitigating this phenomenon, the availability of geoids is very important.
d. The Government has launched a National Peat Restoration Program to restore peat function and reduce land fires nationally. In its implementation, Geoid measured based on gravity measurement is very necessary because it is very dependent on the phenomenon of the direction of water flow, where a careful equipotential field (geoid meticulous) is required.
Geoid information can be accessed http://srgi.big.go.id/srgi2/. Geoid-related data services that can be provided include:
1.Global geoid undulations at certain locations in Indonesia
2.Regional geoid undulation of certain locations in Indonesia
3.Calculation of undulation difference between global geoid and regional geoid in a location in Indonesia
4.Absolute gravity values in the updated GBU pillars
to visit real time ups and downs to the page http://srgi.big.go.id