According to Law No. 4/2011 on Geospatial Information, the National Gravity Control Network (JKGN) is a part of Basic Geospatial Information (IGD) that used as a reference in the preparation of the Thematic Geospatial Information (IGT). The Regulation of Head of the Geospatial Information Agency (BIG) No. 15/2013 about Indonesia Geospatial Reference System 2013 (SRGI-2013) mentions that Indonesian Geoid is used as a national vertical geospatial reference system. A geoid is an equipotential field of earth gravity field that mathematically coincides with the global mean sea level. A geoid is formed as a result of the earth’s gravity variations, so that required adequate gravity data and covers all regions of the world in order to produce the ideal geoid.
The Indonesian geoid is modeled from several data such as gravity data, global geoid model, and elevation data. The gravity data is a primary data resulted from the gravity survey campaign by various methods like terrestrial and airborne. The Earth Gravity Model 2008 (EGM2008) with 360 degrees is chosen due to its quality more fitted to the Indonesian area. The elevation data is represented by Digital Elevation Model is Shuttle Radar Topographic Mission (SRTM) 30 meters. The Remove-Restore Technique and the Fast Fourier Transformation (FFT) approach is used for modeling. Figure 1 explains the flowchart of Indonesian geoid modeling.
Image 1. The flowchart of Indonesian geoid modeling
Indonesian geoid model 2020 uses gravity data up to 2019.. The accuracy of the Indonesian geoid model is evaluated by comparing values between the gravimetric and geometric undulation. The geometric undulation is obtained by subtracting the ellipsoidal heigth resulting from Global Navigation Satellite System (GNSS) observation with the orthometric heigth resulting from leveling measurements corrected with gravity data. The gravimetric undulation is obtained by interpolating the geoid undulation value generated in each control point based on its coordinates.
The accuracy of the Indonesian geoid model is defined in five island regions. This is due to the limited number of validation control points in Indonesia. The accuracy of the Indonesian Geoid model is shown inTable 1.
Table 1. The accuracy of the Indonesian geoid model
Table 1 shows the highest accuracy (standard deviation) of the Indonesian geoid model in Java Island for 5.1 cm. The range of the accuracy starts from 24.7 cm up to 5.1 cm. Based on the Regulation of Head of the BIG No. 6/2018 about the Amendment of the Regulation of Head of the BIG No. 15 of 2014 about Technical Guidelines for Accuracy of Base Map, the accuracies have only complied with the requirement for the base map with scale 1 : 5,000. In brief, the Indonesian geoid model will be updated annually to achieve a higher accuracy supporting One Map Policy and large scale map project. It is through by periodic observation of the gravity control points, densification of terrestrial gravity data , and the renewal of data acquisition.
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