<p class="JUDUL">pyGABEUR-ITB (Python<em> GayaBEUrat Relatif – Institut Teknologi Bandung)</em> is a free and interactive software for adjustment of relative gravimeter data, developed based on Python programming language. pyGABEUR-ITB can adjust relative gravity measurements and provide reliable estimates for correcting instrument’s systematic errors, such as gravimeter drift. Furthermore, pyGABEUR-ITB can also detect possible outliers in the observations using the t-criterion method. Since pyGABEUR-ITB is using the weighted constraint adjustment, at least one fixed station is required accordingly. Relative gravimeter data around Palu-Donggala area (Central Sulawesi) observed by Center for Geodesy Control Networks and Geodynamics, Geospatial Information Agency, were used to test the performance of pyGABEUR-ITB. The processing results were then compared against those calculated using GRAVNET software. The comparisons show that both pyGABEUR-ITB and GRAVNET softwares statistically provide simillar results, with the total RMS value of about 5 mGal. In term of computer’s requirement, pyGABEUR-ITB can be excecuted under a computer with the following minimal requirements: x64 CPU, 1 GB memory and WINDOWS 7 OS. Finally, it is important to mention that pyGABEUR-ITB is recently suited to process the data from the gravimeter that adopts the principle of vertical spring balance. In the near future, pyGABEUR-ITB will be extended to be able to automatically adapt to various observation principles.</p><strong></strong>
Geoid merupakan referensi tinggi di Indonesia sesuai amanat Peraturan Kepala BIG (Perka BIG) nomor 15 Tahun 2013 tentang Sistem Referensi Geospasial Indonesia (SRGI). Melalui website http://srgi.big.go.id/srgi2, BIG secara bertahap memenuhi kebutuhan masyarakat terkait dengan sistem referensi geospasial termasuk di dalamnya informasi model geoid Indonesia. Model geoid Indonesia yang dihasilkan pada tahun 2013 merupakan model geoid Indonesia yang diolah berbasis pulau. pada tahun 2018, dilakukan updating model geoid Indonesia. Tujuannya untuk menghasilkan model geoid Indonesia secara keseluruhan atau terintegrasi di seluruh wilayah Indonesia. Data yang digunakan adalah; Data spherical harmonic beberapa model geoid global sebagai data gelombang panjang, data gelombang menengah menggunakan Data DTU-10, data gayaberat airborne wilayah Pulau Sulawesi, Kalimantan dan Papua. Sedangkan data gelombang pendek menggunakan Data SRTM-15 meter. Metode yang digunakan dalam pemodelan geoid adalah metode Fast Fourier Transform (FFT). Data-data tersebut diolah dengan menggunakan perangkat lunak gravsoft yang telah dimodifikasi di sesuaikan dengan kebutuhan Indonesia. Validasi model geoid dilakukan dengan membandingkan nilai geoid gravimetrik hasil pengolahan model geoid dari data gayaberat, dengan nilai geoid geometrik dari pengukuran GNSS di pilar Tanda Tinggi Geodesi (TTG). Dari pengolahan data, menghasilkan model geoid dari beberapa data komponen gelombang panjang yang berbeda. Model geoid dengan standar deviasi terkecil adalah model geoid yang diperoleh dari kombinasi komponen gelombang panjang EGM2008 - derajat 2190 dengan nilai standar deviasi 0.2283. Metode pemodelan geoid secara menyeluruh di seluruh wilayah Indonesia lebih relevan dilakukan di negara kepulauan seperti Indonesia, dikarenakan lebih memudahkan unifikasi model geoid antara darat dan laut.
In the past, geoid was computed from gravity anomaly data using Stokes or Molodensky approaches. Obtaining gravity anomaly data is difficult because it needs some reductions of gravity from surface of the earth to the geoid using orthometric height from spirit level measurement. In the modern era, gravity anomaly data may be replaced by gravity disturbance data. It only required gravity and GNSS (Global Navigation Satellite System) measurement. This research aimed to determine geoid using Hotine’s approach. Disturbance data were generated from archived free air anomaly of airborne gravimetry in Sulawesi area. South East Sulawesi province was selected as a case study area. In this study, gravity observation was calculated at an altitude of 4000 m above the reference ellipsoid. Gravity estimation at the same height aims to increase the precision of the downward continuation process to the geoid. Hotine integral is calculated above the geoid, so that the gravity disturbance data is downwarded to the geoid. The geoid undulation is graded from north to south. Geoid from airborne gravity around Pegunugan Mekongga in the northern part of Southeast Sulawesi Province has the largest geoid undulation which reaches 63 m, while the geoid in the southern part of Buton Island reaches 52 m. Geoid validation of airborne gravity at 13 test points produces a standard deviation of ± 0.050 m. The standard deviation is much smaller than the results of geoid testing from airborne data in North Sulawesi, Central Sulawesi and Southeast Sulawesi. This fact indicates that the Hotine approach has the potential to produce a precise geoid if used in geoid-based airborne gravity calculations.
Airborne gravity method for high-resolution geoid model in Indonesia held in Sumatera Island using Lacoste & Romberg Air-Sea Gravity Meter S-130 and Trimble R9S GNSS installed in the cessna grand caravan type C208B. Flight altitude ranging from 3000 to 4000 meters and the aircraft speed is 277 km/h. Processing GNSS are using differential processing which is tied to SRGI2013 with a standard deviation tolerance <7 cm in high position and 90% data fix. The gravity raw data (LCR file) is filtered using the lowpass filtering filter mode with the weight function blackman window, the filter window used is 150-seconds. From crossover analysis, some cross over misfit relatively large difference with more than ±20 mGal difference, the average and standard deviation of crossover misfit were -0.0023 mGal and 9.0214 mGal respectively. The result of spectral analysis, airborne gravity signal has minimum 10 km of wavelength while the EGM2008 degree 2190 has minimum 18 km of wavelength.
The Geospatial Information Agency of Indonesia (BIG) recently carried out an airborne gravity survey mission to support a reliable Indonesian geoid model. The gravity observations covered all the main islands of Indonesia. This paper presents a state-of-the-art for gravity anomalies derivation using airborne gravity mission in Java, Indonesia. The common gravity corrections for deriving the scalar free-air gravity anomalies along the flight trajectory had been estimated using GNSS-derived positions. The corrected data were then filtered using the FIR method in which the cut-off frequency had been predetermined by considering aircraft altitude, geological setting, and instrument's accuracy.To assess the airborne gravity results, we compared them with the upward continued terrestrial gravity measurements. In addition, we performed crossover analysis and adjusted the estimated biases to the airborne gravity measurements. The accuracy of adjusted airborne gravity anomaly was estimated to 3.37 mGal. In conclusion, the airborne gravity mission provided valuable data needed for further geodesy and geophysics applications.
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