This paper aims to present the computations of two geoid models for Thailand. The first model, THAI12G, is a gravimetric geoid, referenced to the geocentric WGS84 ellipsoid, computed through one-dimensional spherical Fast Fourier Transform. The other model is THAI12H, which is a hybrid geoid that encompasses all gravimetric information of THAI12G as well as the 200 GPS ellipsoid heights (in the national WGS84 geodetic datum) co-located with orthometric heights (in the national Kolak vertical datum of 1915 (Kolak-1915)) through least-squares collocation (LSC). The non-tidal EGM2008 global geopotential model from degree 2 to 2190 and 3,949 terrestrial gravity measurements were used to contribute long- and medium-scale information of geoid structure. In the mountainous terrains devoid of gravities, the topography-implied gravity anomalies were simulated using the high-resolution residual terrain model (RTM) data from a three-arcsecond digital elevation model. Fits of 200 GPS/leveling reference points to THAI12G showed a 60.6-cm root mean square (rms) with an estimated offset of +71.5 cm around a 0.126-ppm north-south tilted plane. After applying LSC conversion surface to finally obtain THAI12H, the rms of the fit between the model and the same reference points reduced to 5.7 cm (no tilts and zero average).The THAI12H model was assessed using 53 GPS/leveling check points, yielding an overall rms of 16.1-cm.
