Global Positioning System Reference
In-Depth Information
modernization of vertical control systems of countries. The current position is that, the most
developed countries are concentrating efforts on establishing a dynamic geoid based vertical
datum accessible via GNSS positioning (see e.g. Rangelova et al., 2010). Besides enabling the
accurate determination of most up-to-date geoidal heights under the effects of secular
dynamic changes of the earth for GNSS levelling purposes, it is envisioned that this new
datum concept, will also provide a compatible vertical datum with global height system,
which is crucial for studies related to large scale geodynamics and geo-hazards processes.
The accurate determination of orthometric heights via GNSS levelling requires a
centimetre(s) accuracy of the geoid model. The level of achievable accuracy of the models
varies depending on the computational methodology (assumptions used) and available data
within the region of interest (Featherstone et al., 1998; Fotopoulos, 2003; Fotopoulos et al.,
2001; Erol, 2007; Erol et al., 2008). The regional models provide better accuracies in
comparison to global models. However, for many parts of the globe a high precision
regional geoid model is not accessible usually due to lack of data. In these cases, depending
on the required accuracy level of the derived heights, one may resort to applying global
geopotential model values. An alternative way to determining discrete geoid undulation
values is the geometric approach. The approach, which works well in relatively small areas,
utilises the relationship between the GNSS ellipsoidal and regional orthometric heights at
the known points to interpolate new values. In determining orthometric height with GNSS
levelling, apart from consideration for the error budgets of each height data ( h , H , N ), it will
also be necessary to take into account the systematic shifts and datum differences among
these data sets, which also restrict the precision of determination. Since the regional vertical
datum is not necessarily coincident with the geoid surface, the discrepancies between the
regional vertical datum and the geoid surface are preferably accounted for using a special
technique allowing for an improved computation of the regional heights with GNSS
coordinates (Fotopoulos, 2003; Erol, 2007).
This chapter aims to review the geoid models for GNSS levelling purposes in Turkey and
mapping the progress of the global and regional geoid models in Turkish territory. In this
respect the study consists of two parts; the first part provides validation results of the
recently released eight global geopotential models from satellite gravity missions namely;
EGM96, EGM08 (of full expansion and up to 360 degree), EIGEN-51C, EIGEN-6C, EIGEN-
6S, GGM03C, GGM03S and GOCO02S, as well as two Turkish regional geoid models TG03
and TG09, based at 28 homogeneously distributed reference benchmarks with known
ITRF96 coordinates and regional orthometric heights. The validations consist of comparison
of the geoid undulations between the used models and the observed height data ( h , H ). It
should be noted that the results from the validations were evaluated against the reported
precisions of the models by the responsible associations.
The second part of this chapter focuses on the determination and testing of the geoid models
using geometrical approach in small areas and their assessments in GNSS levelling. In the
numerical evaluation, two geodetic networks were used. Each network had 1205 and 109
reference benchmarks with known ITRF96 positions and regional vertical heights,
established in these neighbour local areas. Since the topographical character, distribution
and density of the reference benchmarks at each area were totally different, these networks
provided an appropriate test bed for the local geoid evaluations. In the coverage of the
second part, each network was evaluated independently. A group of modelling algorithms
Search WWH ::




Custom Search