The determination of Thailand Geoid Model 2017 (TGM2017) from airborne and terrestrial gravimetry

  • TGM2017 was based on land and airborne gravity data with GECO and DTU13
  • All heterogeneous data were combined using the least-squares collocation
  • The least-squares collocation played an important role in downward continuation

The Royal Thai Survey Department and Chiangmai University developed the Thailand geoid model 2017 (TGM2017) with a 1¢x1¢ grid to support the transformation between Global Navigation satellite System (GNSS) ellipsoid heights and Kolak-1915 vertical datum orthometric heights. TGM2007 was based on Thailand gravimetric geoid model 2017 (THAIG17) and 299 GNSS ellipsoidal heights co-located with Kolak-1915 heights. All terrestrial gravity data used for geoid computation came from the new national gravity network, consisting of 87 absolute and 9,929 relative gravity stations at 10-25 km intervals, mostly along with existing roads. From 2016 to 2017, airborne gravity surveys were conducted at a 4,000m-flight altitude and 10km along-track spacing to acquire the gravity data over mountainous and inaccessible areas, including coastal and marine areas, at an estimated accuracy of 3.0 mGal. Long-wavelength geoid structure was controlled by the GOCE-EGM2008 combined model (GECO) and the Technical University of Denmark’s global marine gravity model 2013 (DTU13). All gravity data were combined and downward, using least-squares collocation with the residual terrain model reductions from a digital terrain elevation data level 2 (DTED2). THA17G was determined by multi-band spherical Fast Fourier Transform and converted to TGM2017 with the 38.2cm root-mean-square (rms) fit of 299 GNSS/leveling co-points and a mean offset of 37.0cm. This value represents the separation between Kolak-1915 and a global mean sea level. The evaluation of TGM2017 at 100 GNSS/leveling checkpoints shows the rms of 4.9cm, consequently leading to reliable orthometric heights at a 10-cm accuracy level or better.

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