The Turkish hybrid geoid: TG-20

The Turkish Geoid Model-2020 (TG-20) is currently the official geoid model for Turkey computed by the General Directorate of Mapping, the national mapping agency. It is a hybrid model extending from 35°N to 43°N and from 25°E to 45°E. It was determined using more than 265,000 point gravity data, as well as the GOCO06S global geopotential model and a digital elevation model of 7.2 arc-second resolution based on SRTM V4.1. Free-air gravity anomalies over marine areas and neighboring countries at 1’×1’ resolution were computed using the XGM2019e model up to degree and order 5399. The computation was based on the Least-Squares Modification of Stokes’ Formula with Additive Corrections (LSMSA) method using the LSMS-GEOLAB scientific software. The transformation of the gravimetric model was carried out via 4-parameter fit to 182 historical and homogenously distributed GPS/levelling data. Absolute validation of the resulting hybrid model with 278 recent GPS/levelling data yielded standard deviations ranging from 1.2 cm to 6.3 cm (below 2 cm at almost every test profile) which brought about considerable improvement over the previous official geoid models. TG-20 with 5 arc-min spatial resolution is freely available to the public. The geoid model is provided in ISG format 2.0 (ISG Format Specifications), while the file in its original data format is available at the model ISG webpage.