Global Positioning System Reference
In-Depth Information
2.2 Regional geoid models in Turkey
In Turkey, various regional geoid models have been computed with different methods, since
the 1970's (see e.g. Ayan, 1976; Ayhan, 1993; Ayhan et al., 2002; TNUGG, 2003; TNUGG, 2011),
along with the technologic advances and increasing use of GNSS techniques in 1990's,
modernization of national geodetic infrastructure, including the vertical datum definition, was
required. As a consequence of these developments the geodetic control network was re-
established in ITRF96 datum by Turkish Ministry of National Defence-General Command of
Mapping between 1997 and 2001, a geoid model (TG99A) as a height transformation surface
from GNSS to the regional vertical datum was released in 2000 (Ayhan et al., 2002). Turkey
regional geoid model TG99A was gravimetrically determined and fitted to the regional
vertical datum at homogeneously distributed GNSS/levelling benchmarks throughout the
country. The absolute accuracy of TG99A model is reported between ±12 cm and ±25 cm,
however the performance of the model decreases from the central territories through the
coastline and boundaries of the country (Ayhan et al., 2002). An updated version of TG99A
was released by General Command of Mapping in 2003 (TG03) (TNUGG, 2003). TG03 was
computed with R-R method and Least Squares Collocation using terrestrial gravity data in 3-5
km density over the country (at Potsdam gravity datum), marine gravity data (acquired with
shipborne and satellite altimetry), terrain based elevation model in 450 m x 450 m resolution
and reference global model EGM96, and fitted to the regional vertical datum at 197 high order
GNSS/levelling benchmarks (TNUGG, 2003). The accuracy of TG03 is reported as ±8.8 cm by
TNUGG (2003) this revealed good improvement when compared the previous TG model.
Release of the Earth Gravitational Model 2008 (EGM08), the collection of new surface
gravity observations (~266000), the advanced satellite altimetry-derived gravity over the sea
(DNSC08), the availability of the high resolution digital terrain model (90 m resolution) and
increased number of GNSS/levelling benchmarks (approximately 2700 benchmarks cover
the entire country) enabled the computation of a new regional geoid model for Turkey in
2009, hence TG09 was released by General Command of Mapping as successor of TG03
(TNUGG, 2011). In computations, the quasi geoid model was constructed first using R-R
procedure based on EGM08 and RTM reduction of surface gravity data and since the
Helmert orthometric heights are used for vertical control in Turkey, the quasi geoid model
was then converted to the geoid model. Ultimately, the hybrid geoid model TG09 was
derived with combining the gravimetric geoid model and GNSS/levelling heights to be
used in GNSS positioning applications. In the test results of TG09 with GNSS/levelling data,
the accuracy of the model is reported as ±8.3 cm by TNUGG (2011). This result does not
signify much improvement when comparing the TG03.
This section examines the published accuracies of TG03 and TG09 models at 28
GNSS/levelling benchmarks used in the validation of global geopotential models in the
previous section. With this purpose in mind, the derived geoid heights at the benchmarks
were compared with observations and in the results: TG03 model revealed ±10.5 cm
standard deviation with minimum -10.1 cm, maximum 28.9 cm and mean 7.3 cm geoid
height differences, whereas the TG09 model has ±9.2 cm standard deviation with minimum
-11.3 cm, maximum 36.7 cm and mean of 10.5 cm in geoid height differences. The
distribution of geoid height residuals versus the numbers of point are given in histograms in
Figure 4. The geoidal height differences for TG03 and TG09 models are illustrated in Figures
5 and 6, respectively.
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