| Summary: | Local gravimetric geoid determination requires data comprising three components: long-wavelength, medium-wavelength, and short-wavelength components. The long-wavelength component is sourced from Global Geopotential Model (GGM). The medium-wavelength is sourced from terrestrial gravity data. The short-wavelength is sourced from Ditial Terrain Model (DTM) data. Among three components, GGM data contributes the most significant value and error. The previous study shows that the use of maximum degree of GGM EGM2008 makes good contribution to the accuracy of the local geoid model for the area which has tenuous terestrial gravity data. This study has evaluated the influence of GGM on local geoid model for the area with a good and dense terestrial gravity data distribution. With case study in Yogyakarta Special Region (DIY). Evaluation was done to determine the most optimal degree value for local geoid. The data used in this study were free air anomaly data resulted from terrestrial gravity survey results by UGM Research & Technology team in 2009 and DIKTI in 2013 as the middle-wavelength component, EGM2008 as the long-wavelength component and SRTM30plus as the short-wavelength component. The gravimetric geoid computation was done using 2D FFT (Fast Fourier Transform) through sphere field approach. The variation of the degree values used were 360, 540, 720, 1080, 1440, 1800, 2160 and 2190. The evaluation of the variation of the GGM degree values was done by calculating the differences between geometric geoid and gravimetric geoid.
The results showed that using the lowest degree value (360 degree) produces the most optimal accuracy for local geoid on the case study in DIY province. The accuracy resulted from degree value of 360 was 0.82 meters. While the biggest degree value of 2190, the resulted accuracy was 0.84 meters. There is a correlation between the use of GGM degree values and local geoid accuracy, although the correlation is not linear. The use of the lower degree values will result the most optimal accuracy in the local geoid determination in DIY province
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